source: trunk/doc/src/scripting/scripting.qdoc

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/*!
    \group script
    \title Scripting Classes and Overviews

    \brief Classes that add scripting capabilities to Qt applications.
*/

/*!
  \page scripting.html
  \title Making Applications Scriptable
  \ingroup frameworks-technologies

  Qt 4.3 and later provides support for application scripting with ECMAScript.
  The following guides and references cover aspects of programming with
  ECMAScript and Qt.

  \tableofcontents

  \section1 Scripting Classes

  The following classes add scripting capabilities to Qt applications.

  \annotatedlist script

  \section1 Language Overview

  Qt Script is based on the ECMAScript scripting language, as defined
  in standard \l{ECMA-262}. Microsoft's JScript, and Netscape's
  JavaScript are also based on the ECMAScript standard. For an
  overview of ECMAScript, see the
  \l{ECMAScript Reference}{ECMAScript reference}.
  If you are not familiar with the ECMAScript language, there are
  several existing tutorials and books that cover this subject, such
  as \l{JavaScript: The Definitive Guide}.

  Existing users of \l{Qt Script for Applications (QSA)} may find the
  \l{Moving from QSA to Qt Script} document useful when porting
  QSA scripts to Qt Script.

  \section1 Basic Usage

  To evaluate script code, you create a QScriptEngine and call its
  evaluate() function, passing the script code (text) to evaluate
  as argument.

  \snippet doc/src/snippets/qtscript/evaluation/main.cpp 0

  The return value will be the result of the evaluation (represented
  as a QScriptValue object); this can be converted to standard C++
  and Qt types.

  Custom properties can be made available to scripts by registering
  them with the script engine. This is most easily done by setting
  properties of the script engine's \e{Global Object}:

  \snippet doc/src/snippets/qtscript/registeringvalues/main.cpp 0

  This places the properties in the script environment, thus making them
  available to script code.

  \section1 Making a QObject Available to the Script Engine

  Any QObject-based instance can be made available for use with scripts.

  When a QObject is passed to the QScriptEngine::newQObject() function,
  a Qt Script wrapper object is created that can be used to make the
  QObject's signals, slots, properties, and child objects available
  to scripts.

  Here's an example of making an instance of a QObject subclass
  available to script code under the name \c{"myObject"}:

  \snippet doc/src/snippets/qtscript/registeringobjects/main.cpp 0

  This will create a global variable called \c{myObject} in the
  script environment. The variable serves as a proxy to the
  underlying C++ object. Note that the name of the script variable
  can be anything; i.e., it is not dependent upon QObject::objectName().

  The \l{QScriptEngine::}{newQObject()} function accepts two additional
  optional arguments: one is the ownership mode, and the other is a
  collection of options that allow you to control certain aspects of how
  the QScriptValue that wraps the QObject should behave. We will come
  back to the usage of these arguments later.

  \section2 Using Signals and Slots

  Qt Script adapts Qt's central \l{Signals and Slots} feature for
  scripting. There are three principal ways to use signals and slots
  with Qt Script:

  \list
  \i \bold{Hybrid C++/script}: C++ application code connects a
  signal to a script function. The script function can, for example, be
  a function that the user has typed in, or one that you have read from a
  file. This approach is useful if you have a QObject but don't want
  to expose the object itself to the scripting environment; you just
  want a script to be able to define how a signal should be reacted
  to, and leave it up to the C++ side of your application to establish
  the connection.

  \i \bold{Hybrid script/C++}: A script can connect signals and slots
  to establish connections between pre-defined objects that the
  application exposes to the scripting environment. In this scenario,
  the slots themselves are still written in C++, but the definition of
  the connections is fully dynamic (script-defined).

  \i \bold{Purely script-defined}: A script can both define signal
  handler functions (effectively "slots written in Qt Script"),
  \e{and} set up the connections that utilize those handlers. For
  example, a script can define a function that will handle the
  QLineEdit::returnPressed() signal, and then connect that signal to the
  script function.
  \endlist

  Use the qScriptConnect() function to connect a C++ signal to a
  script function. In the following example a script signal handler is
  defined that will handle the QLineEdit::textChanged() signal:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 47

  The first two arguments to qScriptConnect() are the same
  as you would pass to QObject::connect() to establish a normal C++
  connection. The third argument is the script object that will act
  as the \c this object when the signal handler is invoked; in the above
  example we pass an invalid script value, so the \c this object will
  be the Global Object. The fourth argument is the script function
  ("slot") itself. The following example shows how the \c this argument
  can be put to use:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 48

  We create two QLineEdit objects and define a single signal handler
  function. The connections use the same handler function, but the
  function will be invoked with a different \c this object depending on
  which object's signal was triggered, so the output of the print()
  statement will be different for each.

  In script code, Qt Script uses a different syntax for connecting to
  and disconnecting from signals than the familiar C++ syntax; i.e.,
  QObject::connect().
  To connect to a signal, you reference the relevant signal as a property
  of the sender object, and invoke its \c{connect()} function. There
  are three overloads of \c{connect()}, each with a corresponding
  \c{disconnect()} overload. The following subsections describe these
  three forms.

  \section3 Signal to Function Connections

  \c{connect(function)}

  In this form of connection, the argument to \c{connect()} is the
  function to connect to the signal.

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 2

  The argument can be a Qt Script function, as in the above
  example, or it can be a QObject slot, as in
  the following example:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 3

  When the argument is a QObject slot, the argument types of the
  signal and slot do not necessarily have to be compatible;
  QtScript will, if necessary, perform conversion of the signal
  arguments to match the argument types of the slot.

  To disconnect from a signal, you invoke the signal's
  \c{disconnect()} function, passing the function to disconnect
  as argument:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 4

  When a script function is invoked in response to a signal, the
  \c this object will be the Global Object.

  \section3 Signal to Member Function Connections

  \c{connect(thisObject, function)}

  In this form of the \c{connect()} function, the first argument
  is the object that will be bound to the variable, \c this, when
  the function specified using the second argument is invoked.

  If you have a push button in a form, you typically want to do
  something involving the form in response to the button's
  \c{clicked} signal; passing the form as the \c this object
  makes sense in such a case.

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 5

  To disconnect from the signal, pass the same arguments to \c{disconnect()}:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 6

  \section3 Signal to Named Member Function Connections

  \c{connect(thisObject, functionName)}

  In this form of the \c{connect()} function, the first argument is
  the object that will be bound to the variable, \c this, when
  a function is invoked in response to the signal. The second argument
  specifies the name of a function that is connected to the signal,
  and this refers to a member function of the object passed as the
  first argument (\c thisObject in the above scheme).

  Note that the function is resolved when the connection is made, not
  when the signal is emitted.

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 7

  To disconnect from the signal, pass the same arguments to \c{disconnect()}:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 8

  \section3 Error Handling

  When \c{connect()} or \c{disconnect()} succeeds, the function will
  return \c{undefined}; otherwise, it will throw a script exception.
  You can obtain an error message from the resulting \c{Error} object.
  Example:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 9

  \section3 Emitting Signals from Scripts

  To emit a signal from script code, you simply invoke the signal
  function, passing the relevant arguments:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 10

  It is currently not possible to define a new signal in a script;
  i.e., all signals must be defined by C++ classes.

  \section3 Overloaded Signals and Slots

  When a signal or slot is overloaded, QtScript will attempt to
  pick the right overload based on the actual types of the QScriptValue arguments
  involved in the function invocation. For example, if your class has slots
  \c{myOverloadedSlot(int)} and \c{myOverloadedSlot(QString)}, the following
  script code will behave reasonably:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 11

  You can specify a particular overload by using array-style property access
  with the \l{QMetaObject::normalizedSignature()}{normalized signature} of
  the C++ function as the property name:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 12

  If the overloads have different number of arguments, QtScript will
  pick the overload with the argument count that best matches the
  actual number of arguments passed to the slot.

  For overloaded signals, Qt Script will throw an error if you try to connect
  to the signal by name; you have to refer to the signal with the full
  normalized signature of the particular overload you want to connect to.

  \section2 Accessing Properties

  The properties of the QObject are available as properties
  of the corresponding QtScript object. When you manipulate
  a property in script code, the C++ get/set method for that
  property will automatically be invoked. For example, if your
  C++ class has a property declared as follows:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 13

  then script code can do things like the following:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 14

  \section2 Accessing Child QObjects

  Every named child of the QObject (that is, for which
  QObject::objectName() is not an empty string) is by default available as
  a property of the QtScript wrapper object. For example,
  if you have a QDialog with a child widget whose \c{objectName} property is
  \c{"okButton"}, you can access this object in script code through
  the expression

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 15

  Since \c{objectName} is itself a Q_PROPERTY, you can manipulate
  the name in script code to, for example, rename an object:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 16

  You can also use the functions \c{findChild()} and \c{findChildren()}
  to find children. These two functions behave identically to
  QObject::findChild() and QObject::findChildren(), respectively.

  For example, we can use these functions to find objects using strings
  and regular expressions:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 17

  You typically want to use \c{findChild()} when manipulating a form
  that uses nested layouts; that way the script is isolated from the
  details about which particular layout a widget is located in.

  \section2 Controlling QObject Ownership

  Qt Script uses garbage collection to reclaim memory used by script
  objects when they are no longer needed; an object's memory can be
  automatically reclaimed when it is no longer referenced anywhere in
  the scripting environment. Qt Script lets you control what happens
  to the underlying C++ QObject when the wrapper object is reclaimed
  (i.e., whether the QObject is deleted or not); you do this when you
  create an object by passing an ownership mode as the second argument
  to QScriptEngine::newQObject().

  Knowing how Qt Script deals with ownership is important, since it can
  help you avoid situations where a C++ object isn't deleted when it
  should be (causing memory leaks), or where a C++ object \e{is}
  deleted when it shouldn't be (typically causing a crash if C++ code
  later tries to access that object).

  \section3 Qt Ownership

  By default, the script engine does not take ownership of the
  QObject that is passed to QScriptEngine::newQObject(); the object
  is managed according to Qt's object ownership (see
  \l{Object Trees & Ownership}). This mode is appropriate
  when, for example, you are wrapping C++ objects that are part of
  your application's core; that is, they should persist regardless of
  what happens in the scripting environment. Another way of stating
  this is that the C++ objects should outlive the script engine.

  \section3 Script Ownership

  Specifying QScriptEngine::ScriptOwnership as the ownership mode
  will cause the script engine to take full ownership of the QObject
  and delete it when it determines that it is safe to do so
  (i.e., when there are no more references to it in script code).
  This ownership mode is appropriate if the QObject does not have a
  parent object, and/or the QObject is created in the context of the
  script engine and is not intended to outlive the script engine.

  For example, a constructor function that constructs QObjects
  only to be used in the script environment is a good candidate:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 18

  \section3 Auto-Ownership

  With QScriptEngine::AutoOwnership the ownership is based on whether
  the QObject has a parent or not.
  If the QtScript garbage collector finds that the QObject is no
  longer referenced within the script environment, the QObject will
  be deleted \e{only} if it does not have a parent.

  \section3 What Happens When Someone Else Deletes the QObject?

  It is possible that a wrapped QObject is deleted outside of
  Qt Script's control; i.e., without regard to the ownership mode
  specified. In this case, the wrapper object will still
  be an object (unlike the C++ pointer it wraps, the script object
  won't become null). Any attempt to access properties of the script
  object will, however, result in a script exception being thrown.

  Note that QScriptValue::isQObject() will still return true for a
  deleted QObject, since it tests the type of the script object, not
  whether the internal pointer is non-null. In other words, if
  QScriptValue::isQObject() returns true but QScriptValue::toQObject()
  returns a null pointer, this indicates that the QObject has been
  deleted outside of Qt Script (perhaps accidentally).

  \section2 Customizing Access to the QObject

  QScriptEngine::newQObject() can take a third argument which allows
  you to control various aspects of the access to the QObject through
  the QtScript wrapper object it returns.

  QScriptEngine::ExcludeChildObjects specifies that child objects of
  the QObject should not appear as properties of the wrapper object.

  QScriptEngine::ExcludeSuperClassProperties and
  QScriptEngine::ExcludeSuperClassMethods can be used to avoid
  exposing members that are inherited from the QObject's superclass.
  This is useful for defining a "pure" interface where inherited members
  don't make sense from a scripting perspective; e.g., you don't want
  script authors to be able to change the \c{objectName} property of
  the object or invoke the \c{deleteLater()} slot.

  QScriptEngine::AutoCreateDynamicProperties specifies that properties
  that don't already exist in the QObject should be created as dynamic
  properties of the QObject, rather than as properties of the QtScript
  wrapper object. If you want new properties to truly become persistent
  properties of the QObject, rather than properties that are destroyed
  along with the wrapper object (and that aren't shared if the QObject
  is wrapped multiple times with QScriptEngine::newQObject()), you
  should use this option.

  QScriptEngine::SkipMethodsInEnumeration specifies that signals and
  slots should be skipped when enumerating the properties of the QObject
  wrapper in a for-in script statement. This is useful when defining
  prototype objects, since by convention function properties of
  prototypes should not be enumerable.

  \section2 Making a QObject-based Class New-able from a Script

  The QScriptEngine::newQObject() function is used to wrap an
  existing QObject instance, so that it can be made available to
  scripts. A different scenario is that you want scripts to be
  able to construct new objects, not just access existing ones.

  The Qt meta-type system currently does not provide dynamic
  binding of constructors for QObject-based classes. If you want to
  make such a class new-able from scripts, Qt Script can generate
  a reasonable script constructor for you; see
  QScriptEngine::scriptValueFromQMetaObject().

  You can also use QScriptEngine::newFunction() to wrap your own
  factory function, and add it to the script environment; see
  QScriptEngine::newQMetaObject() for an example.

  \section2 Enum Values

  Values for enums declared with Q_ENUMS are not available as
  properties of individual wrapper objects; rather, they are
  properties of the QMetaObject wrapper object that can be created
  with QScriptEngine::newQMetaObject().

  \section1 Conversion Between QtScript and C++ Types

  QtScript will perform type conversion when a value needs to be
  converted from the script side to the C++ side or vice versa; for
  instance, when a C++ signal triggers a script function, when
  you access a QObject property in script code, or when
  you call QScriptEngine::toScriptValue() or
  QScriptEngine::fromScriptValue() in C++. QtScript provides default
  conversion operations for many of the built-in Qt types. You can
  change the conversion operation for a type (including your custom
  C++ types) by registering your own conversion functions with
  qScriptRegisterMetaType().

  \section2 Default Conversion from Qt Script to C++

  The following table describes the default conversion from a
  QScriptValue to a C++ type.

    \table 80%
    \header \o C++ Type \o Default Conversion
    \row    \o bool \o QScriptValue::toBool()
    \row    \o int \o QScriptValue::toInt32()
    \row    \o uint \o QScriptValue::toUInt32()
    \row    \o float \o float(QScriptValue::toNumber())
    \row    \o double \o QScriptValue::toNumber()
    \row    \o short \o short(QScriptValue::toInt32())
    \row    \o ushort \o QScriptValue::toUInt16()
    \row    \o char \o char(QScriptValue::toInt32())
    \row    \o uchar \o unsigned char(QScriptValue::toInt32())
    \row    \o qlonglong \o qlonglong(QScriptValue::toInteger())
    \row    \o qulonglong \o qulonglong(QScriptValue::toInteger())
    \row    \o QString \o An empty string if the QScriptValue is null
               or undefined; QScriptValue::toString() otherwise.
    \row    \o QDateTime \o QScriptValue::toDateTime()
    \row    \o QDate \o QScriptValue::toDateTime().date()
    \row    \o QRegExp \o QScriptValue::toRegExp()
    \row    \o QObject* \o QScriptValue::toQObject()
    \row    \o QWidget* \o QScriptValue::toQObject()
    \row    \o QVariant \o QScriptValue::toVariant()
    \row    \o QChar \o If the QScriptValue is a string, the result
               is the first character of the string, or a null QChar
               if the string is empty; otherwise, the result is a QChar
               constructed from the unicode obtained by converting the
               QScriptValue to a \c{ushort}.
    \row    \o QStringList \o If the QScriptValue is an array, the
               result is a QStringList constructed from the result of
               QScriptValue::toString() for each array element; otherwise,
               the result is an empty QStringList.
    \row    \o QVariantList \o If the QScriptValue is an array, the result
               is a QVariantList constructed from the result of
               QScriptValue::toVariant() for each array element; otherwise,
               the result is an empty QVariantList.
    \row    \o QVariantMap \o If the QScriptValue is an object, the result
               is a QVariantMap with a (key, value) pair of the form
               (propertyName, propertyValue.toVariant()) for each property,
               using QScriptValueIterator to iterate over the object's
               properties.
    \row    \o QObjectList \o If the QScriptValue is an array, the result
               is a QObjectList constructed from the result of
               QScriptValue::toQObject() for each array element; otherwise,
               the result is an empty QObjectList.
    \row    \o QList<int> \o If the QScriptValue is an array, the result is
               a QList<int> constructed from the result of
               QScriptValue::toInt32() for each array element; otherwise,
               the result is an empty QList<int>.
    \endtable

  Additionally, QtScript will handle the following cases:

  \list
  \i If the QScriptValue is a QObject and the target type name ends with
     \c * (i.e., it is a pointer), the QObject pointer will be cast to the
     target type with qobject_cast().
  \i If the QScriptValue is a QVariant and the target type name ends with
     \c * (i.e., it is a pointer), and the \l{QVariant::userType()}{userType()}
     of the QVariant is the type that the target type points to, the result
     is a pointer to the QVariant's data.
  \i If the QScriptValue is a QVariant and it can be converted to the
     target type (according to QVariant::canConvert()), the QVariant will
     be cast to the target type with qvariant_cast().
  \endlist

  \section2 Default Conversion from C++ to Qt Script

  The following table describes the default behavior when a QScriptValue is
  constructed from a C++ type:

    \table 80%
    \header \o C++ Type \o Default Construction
    \row    \o void \o QScriptEngine::undefinedValue()
    \row    \o bool \o QScriptValue(engine, value)
    \row    \o int \o QScriptValue(engine, value)
    \row    \o uint \o QScriptValue(engine, value)
    \row    \o float \o QScriptValue(engine, value)
    \row    \o double \o QScriptValue(engine, value)
    \row    \o short \o QScriptValue(engine, value)
    \row    \o ushort \o QScriptValue(engine, value)
    \row    \o char \o QScriptValue(engine, value)
    \row    \o uchar \o QScriptValue(engine, value)
    \row    \o QString \o QScriptValue(engine, value)
    \row    \o qlonglong \o QScriptValue(engine, qsreal(value)). Note that
               the conversion may lead to loss of precision, since not all
               64-bit integers can be represented using the qsreal type.
    \row    \o qulonglong \o QScriptValue(engine, qsreal(value)). Note that
               the conversion may lead to loss of precision, since not all
               64-bit unsigned integers can be represented using the qsreal
               type.
    \row    \o QChar \o QScriptValue(this, value.unicode())
    \row    \o QDateTime \o \l{QScriptEngine::newDate()}{QScriptEngine::newDate}(value)
    \row    \o QDate \o \l{QScriptEngine::newDate()}{QScriptEngine::newDate}(value)
    \row    \o QRegExp \o \l{QScriptEngine::newRegExp()}{QScriptEngine::newRegExp}(value)
    \row    \o QObject* \o \l{QScriptEngine::newQObject()}{QScriptEngine::newQObject}(value)
    \row    \o QWidget* \o \l{QScriptEngine::newQObject()}{QScriptEngine::newQObject}(value)
    \row    \o QVariant \o \l{QScriptEngine::newVariant()}{QScriptEngine::newVariant}(value)
    \row    \o QStringList \o A new script array (created with
               QScriptEngine::newArray()), whose elements are created using
               the QScriptValue(QScriptEngine *, QString) constructor for
               each element of the list.
    \row    \o QVariantList \o A new script array (created with
               QScriptEngine::newArray()), whose elements are created using
               QScriptEngine::newVariant() for each element of the list.
    \row    \o QVariantMap \o A new script object (created with
               QScriptEngine::newObject()), whose properties are initialized
               according to the (key, value) pairs of the map.
    \row    \o QObjectList \o A new script array (created with
               QScriptEngine::newArray()), whose elements are created using
               QScriptEngine::newQObject() for each element of the list.
    \row    \o QList<int> \o A new script array (created with
               QScriptEngine::newArray()), whose elements are created using
               the QScriptValue(QScriptEngine *, int) constructor for each
               element of the list.
    \endtable

  Other types (including custom types) will be wrapped using
  QScriptEngine::newVariant(). For null pointers of any type, the
  result is QScriptEngine::nullValue().

  \section1 How to Design and Implement Application Objects

  This section explains how to implement application objects and
  provides the necessary technical background material.

  \section2 Making a C++ object available to Scripts Written in QtScript

  Making C++ classes and objects available to a scripting language is
  not trivial because scripting languages tend to be more dynamic than
  C++, and it must be possible to introspect objects (query information
  such as function names, function signatures, properties, etc., at
  run-time). Standard C++ does not provide features for this.

  We can achieve the functionality we want by extending C++, using
  C++'s own facilities so our code is still standard C++. The Qt
  meta-object system provides the necessary additional functionality.
  It allows us to write using an extended C++ syntax, but converts this
  into standard C++ using a small utility program called \l{moc}
  (Meta-Object Compiler). Classes that wish to take advantage of the
  meta-object facilities are either subclasses of QObject, or use the
  \c{Q_OBJECT} macro. Qt has used this approach for many years and it has
  proven to be solid and reliable. QtScript uses this meta-object
  technology to provide scripters with dynamic access to C++ classes
  and objects.

  To completely understand how to make C++ objects available to Qt
  Script, some basic knowledge of the Qt meta-object system is very
  helpful. We recommend that you read about the Qt \l{Object Model}
  and \l{The Meta-Object System}, which are useful for understanding
  how to implement application objects.

  However, this knowledge is not essential in the simplest cases.
  To make an object available in QtScript, it must derive from
  QObject. All classes which derive from QObject can be introspected
  and can provide the information needed by the scripting engine at
  run-time; e.g., class name, functions, signatures. Because we obtain
  the information we need about classes dynamically at run-time, there
  is no need to write wrappers for QObject derived classes.

  \section2 Making C++ Class Member Functions Available in QtScript

  The meta-object system also makes information about signals and slots
  dynamically available at run-time. By default, for QObject subclasses,
  only the signals and slots are automatically made available to scripts.
  This is very convenient because, in practice, we normally only want to
  make specially chosen functions available to scripters. When you create
  a QObject subclass, make sure that the functions you want to expose to
  QtScript are public slots.

  For example, the following class definition enables scripting only for
  certain functions:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 19

  In the example above, aNonScriptableFunction() is not declared as a
  slot, so it will not be available in QtScript. The other three
  functions will automatically be made available in QtScript because
  they are declared in the \c{public slots} section of the class
  definition.

  It is possible to make any function script-invokable by specifying
  the \c{Q_INVOKABLE} modifier when declaring the function:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 20

  Once declared with \c{Q_INVOKABLE}, the method can be invoked from
  QtScript code just as if it were a slot. Although such a method is
  not a slot, you can still specify it as the target function in a
  call to \c{connect()} in script code; \c{connect()} accepts both
  native and non-native functions as targets.

  \section2 Making C++ Class Properties Available in QtScript

  In the previous example, if we wanted to get or set a property using
  QtScript we would have to write code like the following:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 21

  Scripting languages often provide a property syntax to modify and
  retrieve properties (in our case the enabled state) of an
  object. Many script programmers would want to write the above code
  like this:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 22

  To make this possible, you must define properties in the C++ QObject
  subclass. For example, the following \c MyObject class declaration
  declares a boolean property called \c enabled, which uses the function
  \c{setEnabled(bool)} as its setter function and \c{isEnabled()} as its
  getter function:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 23

  The only difference from the original code is the use of the macro
  \c{Q_PROPERTY}, which takes the type and name of the property, and
  the names of the setter and getter functions as arguments.

  If you don't want a property of your class to be accessible in
  QtScript, you set the \c{SCRIPTABLE} attribute to \c false when
  declaring the property; by default, the \c{SCRIPTABLE} attribute is
  \c true. For example:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 24

  \section2 Reacting to C++ Objects Signals in Scripts

  In the Qt object model, signals are used as a notification mechanism
  between QObjects. This means one object can connect a signal to
  another object's slot and, every time the signal is emitted, the slot
  is called. This connection is established using the QObject::connect()
  function.

  The signals and slots mechanism is also available to QtScript
  programmers. The code to declare a signal in C++ is the same,
  regardless of whether the signal will be connected to a slot in C++
  or in QtScript.

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 25

  The only change we have made to the code in the previous section is
  to declare a signals section with the relevant signal. Now, the
  script writer can define a function and connect to the object like
  this:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 26

  \section2 Design of Application Objects

  The previous section described how to implement C++ objects which
  can be used in QtScript. Application objects are the same kind of
  objects, and they make your application's functionality available to
  QtScript scripters.  Since the C++ application is already written
  in Qt, many objects are already QObjects. The easiest approach would
  be to simply add all these QObjects as application objects to the
  scripting engine. For small applications this might be sufficient,
  but for larger applications this is probably not the right
  approach. The problem is that this method reveals too much of the
  internal API and gives script programmers access to application
  internals which should not be exposed.

  Generally, the best way of making application functionality available
  to scripters is to code some QObjects which define the applications
  public API using signals, slots, and properties. This gives you
  complete control of the functionality made available by the
  application. The implementations of these objects simply call the
  functions in the application which do the real work. So, instead of
  making all your QObjects available to the scripting engine, just add
  the wrapper QObjects.

  \section3 Returning QObject Pointers

  If you have a slot that returns a QObject pointer, you should note
  that, by default, Qt Script only handles conversion of the types
  QObject* and QWidget*. This means that if your slot is declared
  with a signature like "MyObject* getMyObject()", QtScript doesn't
  automatically know that MyObject* should be handled in the same way
  as QObject* and QWidget*. The simplest way to solve this is to only
  use QObject* and QWidget* in the method signatures of your scripting
  interface.

  Alternatively, you can register conversion functions for your custom
  type with the qScriptRegisterMetaType() function. In this way, you
  can preserve the precise typing in your C++ declarations, while
  still allowing pointers to your custom objects to flow seamlessly
  between C++ and scripts. Example:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 43

  \section1 Function Objects and Native Functions

  In Qt Script, functions are first-class values; they are objects that
  can have properties of their own, just like any other type of
  object. They can be stored in variables and passed as arguments to
  other functions. Knowing how function calls in Qt Script behave is
  useful when you want to define and use your own script functions.
  This section discusses this matter, and also explains how you can
  implement native functions; that is, Qt Script functions written in
  C++, as opposed to functions written in the scripting language
  itself. Even if you will be relying mostly on the dynamic QObject
  binding that Qt Script provides, knowing about these powerful
  concepts and techniques is important to understand what's actually
  going on when script functions are executed.

  \section2 Calling a Qt Script Function from C++

  Calling a Qt Script function from C++ is achieved with the
  QScriptValue::call() function. A typical scenario is that you evaluate a
  script that defines a function, and at some point you want to call that
  function from C++, perhaps passing it some arguments, and then handle the
  result. The following script defines a Qt Script object that has a
  toKelvin() function:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 90

  The toKelvin() function takes a temperature in Kelvin as argument, and
  returns the temperature converted to Celsius. The following snippet shows
  how the toKelvin() function might be obtained and called from C++:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 91

  If a script defines a global function, you can access the function as a
  property of QScriptEngine::globalObject(). For example, the following script
  defines a global function add():

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 56

  C++ code might call the add() function as follows:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 92

  As already mentioned, functions are just values in Qt Script; a function by
  itself is not "tied to" a particular object. This is why you have to specify
  a \c{this} object (the first argument to QScriptValue::call()) that the
  function should be applied to.

 If the function is supposed to act as a method (i.e. it can only be applied
  to a certain class of objects), it is up to the function itself to check
  that it is being called with a compatible \c{this} object.

 Passing an invalid QScriptValue as the \c{this} argument to
  QScriptValue::call() indicates that the Global Object should be used as the
  \c{this} object; in other words, that the function should be invoked as a
  global function.

  \section2 The \c this Object

  When a Qt Script function is invoked from a script, the \e{way} in which it
  is invoked determines the \c this object when the function body is executed,
  as the following script example illustrates:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 49

  An important thing to note is that in Qt Script, unlike C++ and Java, the
  \c this object is not part of the execution scope. This means that
  member functions (i.e., functions that operate on \c this) must always
  use the \c this keyword to access the object's properties. For example,
  the following script probably doesn't do what you want:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 50

  You will get a reference error saying that 'a is not defined' or, worse,
  two totally unrelated global variables \c a and \c b will be used to
  perform the computation, if they exist. Instead, the script should look
  like this:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 51

  Accidentally omitting the \c this keyword is a typical source of
  error for programmers who are used to the scoping rules of C++ and Java.

  \section2 Wrapping a Native Function

  Qt Script provides QScriptEngine::newFunction() as a way of wrapping a
  C++ function pointer; this enables you to implement a function in
  C++ and add it to the script environment, so that scripts can invoke
  your function as if it were a "normal" script function. Here is how the
  previous \c{getProperty()} function can be written in C++:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 52

  Call QScriptEngine::newFunction() to wrap the function. This will
  produce a special type of function object that carries a pointer to
  the C++ function internally. Once the resulting wrapper has been
  added to the scripting environment (e.g., by setting it as a property
  of the Global Object), scripts can call the function without having
  to know nor care that it is, in fact, a native function.

  Note that the name of the C++ function doesn't matter in the
  scripting sense; the name by which the function is invoked by
  scripts depends only on what you call the script object property
  in which you store the function wrapper.

  It is currently not possible to wrap member functions; i.e., methods
  of a C++ class that require a \c this object.

  \section2 The QScriptContext Object

  A QScriptContext holds all the state associated with a particular
  invocation of your function. Through the QScriptContext, you can:
  \list
  \i Get the arguments that were passed to the function.
  \i Get the \c this object.
  \i Find out whether the function was called with the \c new operator
     (the significance of this will be explained later).
  \i Throw a script error.
  \i Get the function object that's being invoked.
  \i Get the activation object (the object used to hold local variables).
  \endlist

  The following sections explain how to make use of this
  functionality.

  \section2 Processing Function Arguments

  Two things are worth noting about function arguments:

  \list 1
  \o Any script function \mdash including native functions \mdash can
  be invoked with any number of arguments. This means that it is up to
  the function itself to check the argument count if necessary, and act
  accordingly (e.g., throw an error if the number of arguments is
  too large, or prepare a default value if the number is too small).
  \o A value of any type can be supplied as an argument to any
  function. This means that it is up to you to check the type of the
  arguments if necessary, and act accordingly (e.g., throw an error
  if an argument is not an object of a certain type).
  \endlist

  In summary: Qt Script does not automatically enforce any constraints on the
  number or type of arguments involved in a function call.

  \section3 Formal Parameters and the Arguments Object

  A native Qt Script function is analogous to a script function that defines no
  formal parameters and only uses the built-in \c arguments variable to
  process its arguments. To see this, let's first consider how a
  script would normally define an \c{add()} function that takes two
  arguments, adds them together and returns the result:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 56

  When a script function is defined with formal parameters, their
  names can be viewed as mere aliases of properties of the \c
  arguments object; for example, in the \c{add(a, b)} definition's
  function body, \c a and \c arguments[0] refer to the same
  variable. This means that the \c{add()} function can equivalently be
  written like this:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 57

  This latter form closely matches what a native implementation
  typically looks like:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 58

  \section3 Checking the Number of Arguments

  Again, remember that the presence (or lack) of formal parameter
  names in a function definition does not affect how the function
  may be invoked; \c{add(1, 2, 3)} is allowed by the engine, as is
  \c{add(42)}. In the case of the \c {add()} function, the function
  really needs two arguments in order to do something useful. This
  can be expressed by the script definition as follows:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 59

  This would result in an error being thrown if a script invokes
  \c{add()} with anything other than two arguments. The native
  function can be modified to perform the same check:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 62

  \section3 Checking the Types of Arguments

  In addition to expecting a certain number of arguments, a function might
  expect that those arguments are of certain types (e.g., that the first
  argument is a number and that the second is a string). Such a function
  should explicitly check the type of arguments and/or perform a conversion,
  or throw an error if the type of an argument is incompatible.

  As it is, the native implementation of \c{add()} shown above doesn't
  have the exact same semantics as the script counterpart; this is
  because the behavior of the Qt Script \c{+} operator depends on the
  types of its operands (for example, if one of the operands is a string,
  string concatenation is performed). To give the script function
  stricter semantics (namely, that it should only add numeric
  operands), the argument types can be tested:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 60

  Then an invocation like \c{add("foo", new Array())} will
  cause an error to be thrown.

  The C++ version can call QScriptValue::isNumber() to perform similar
  tests:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 63

  A less strict script implementation might settle for performing an
  explicit to-number conversion before applying the \c{+} operator:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 61

  In a native implementation, this is equivalent to calling
  QScriptValue::toNumber() without performing any type test first,
  since QScriptValue::toNumber() will automatically perform a type
  conversion if necessary.

  To check if an argument is of a certain object type (class),
  scripts can use the \c instanceof operator (e.g., \c{"arguments[0]
  instanceof Array"} evaluates to true if the first argument is an
  Array object); native functions can call QScriptValue::instanceOf().

  To check if an argument is of a custom C++ type, you typically use
  qscriptvalue_cast() and check if the result is valid. For object types,
  this means casting to a pointer and checking if it is non-zero; for
  value types, the class should have an \c{isNull()}, \c{isValid()}
  or similar method. Alternatively, since most custom types are
  transported in \l{QVariant}s, you can check if the script value is a
  QVariant using QScriptValue::isVariant(), and then check if the
  QVariant can be converted to your type using QVariant::canConvert().

  \section3 Functions with Variable Numbers of Arguments

  Because of the presence of the built-in \c arguments object,
  implementing functions that take a variable number of arguments
  is simple. In fact, as we have seen, in the technical sense \e{all}
  Qt Script functions can be seen as variable-argument functions.
  As an example, consider a concat() function that takes an arbitrary
  number of arguments, converts the arguments to their string
  representation and concatenates the results; for example,
  \c{concat("Qt", " ", "Script ", 101)} would return "Qt Script 101".
  A script definition of \c{concat()} might look like this:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 64

  Here is an equivalent native implementation:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 65

  A second use case for a variable number of arguments is to implement
  optional arguments. Here's how a script definition typically does
  it:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 66

  And here's the native equivalent:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 67

  A third use case for a variable number of arguments is to simulate
  C++ overloads. This involves checking the number of arguments and/or
  their type at the beginning of the function body (as already shown),
  and acting accordingly. It might be worth thinking twice before
  doing this, and instead favor unique function names; e.g., having
  separate \c{processNumber(number)} and \c{processString(string)}
  functions rather than a generic \c{process(anything)} function.
  On the caller side, this makes it harder for scripts to accidentally
  call the wrong overload (since they don't know or don't comprehend
  your custom sophisticated overloading resolution rules), and on the
  callee side, you avoid the need for potentially complex (read:
  error-prone) checks to resolve ambiguity.

  \section3 Accessing the Arguments Object

  Most native functions use the QScriptContext::argument() function to
  access function arguments. However, it is also possible to access
  the built-in \c arguments object itself (the one referred to by the
  \c arguments variable in script code), by calling the
  QScriptContext::argumentsObject() function. This has three principal
  applications:

  \list
  \o The \c arguments object can be used to easily forward a function
     call to another function. In script code, this is what it
     typically looks like:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 68

     For example, \c{foo(10, 20, 30)} would result in the \c{foo()} function
     executing the equivalent of \c{bar(10, 20, 30)}. This is useful if
     you want to perform some special pre- or post-processing when
     calling a function (e.g., to log the call to \c{bar()} without having
     to modify the \c{bar()} function itself, like the above example), or if
     you want to call a "base implementation" from a prototype
     function that has the exact same "signature". In C++, the forwarding
     function might look like this:

     \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 69

  \o The arguments object can serve as input to a QScriptValueIterator,
     providing a generic way to iterate over the arguments. A debugger
     might use this to display the arguments object in a general purpose
     "Qt Script Object Explorer", for example.

  \o The arguments object can be serialized (e.g., with JSON) and transferred
     to another entity (e.g., a script engine running in another thread),
     where the object can be deserialized and passed as argument to
     another script function.
  \endlist

  \section2 Constructor Functions

  Some script functions are constructors; they are expected to initialize
  new objects. The following snippet is a small example:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 75

  There is nothing special about constructor functions. In fact, any
  script function can act as a constructor function (i.e., any function
  can serve as the operand to \c{new}). Some functions behave differently
  depending on whether they are called as part of a \c{new} expression
  or not; for example, the expression \c{new Number(1)} will create a
  Number object, whereas \c{Number("123")} will perform a type
  conversion. Other functions, like \c{Array()}, will always create
  and initialize a new object (e.g., \c{new Array()} and \c{Array()} have
  the same effect).

  A native Qt Script function can call the
  QScriptContext::isCalledAsConstructor() function to determine if it
  is being called as a constructor or as a regular function. When a
  function is called as a constructor (i.e., it is the operand in a
  \c{new} expression), this has two important implications:

  \list
  \i The \c this object, QScriptContext::thisObject(), contains
     the new object to be initialized; the engine creates this
     new object automatically before invoking your function. This means
     that your native constructor function normally doesn't have to (and
     shouldn't) create a new object when it is called as a
     constructor, since the engine has already prepared a new
     object. Instead your function should operate on the supplied
     \c this object.
  \i The constructor function should return an undefined value,
     QScriptEngine::undefinedValue(), to tell the engine that the
     \c this object should be the final result of the \c new
     operator. Alternatively, the function can return the \c this
     object itself.
  \endlist

  When QScriptContext::isCalledAsConstructor() returns false, how your
  constructor handles this case depends on what behavior you desire.
  If, like the built-in \c{Number()} function, a plain function call should
  perform a type conversion of its argument, then you perform the conversion
  and return the result. If, on the other hand, you want your constructor
  to behave \e{as if it was called as a constructor} (with
  \c{new}), you have to explicitly create a new object (that is,
  ignore the \c this object), initialize that object, and return it.

  The following example implements a constructor function that always
  creates and initializes a new object:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 76

  Given this constructor, scripts would be able to use either the
  expression \c{new Person("Bob")} or \c{Person("Bob")} to create a
  new \c{Person} object; both behave in the same way.

  There is no equivalent way for a function defined in script
  code to determine whether or not it was invoked as a constructor.

  Note that, even though it is not considered good practice, there is
  nothing that stops you from choosing to ignore the default
  constructed (\c this) object when your function is called as a
  constructor and creating your own object anyway; simply have the
  constructor return that object. The object will "override" the
  default object that the engine constructed (i.e., the default
  object will simply be discarded internally).

  \section2 Associating Data with a Function

  Even if a function is global \mdash i.e., not associated with any particular
  (type of) object \mdash you might still want to associate some data with it,
  so that it becomes self-contained; for example, the function could have
  a pointer to some C++ resource that it needs to access. If your application
  only uses a single script engine, or the same C++ resource can/should be
  shared among all script engines, you can simply use a static C++ variable
  and access it from within the native Qt Script function.

  In the case where a static C++ variable or singleton class is
  not appropriate, you can call QScriptValue::setProperty() on the
  function object, but be aware that those properties will also be
  accessible to script code. The alternative is to use QScriptValue::setData();
  this data is not script-accessible. The implementation can access this
  internal data through the QScriptContext::callee() function, which
  returns the function object being invoked. The following example
  shows how this might be used:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 55

  \section2 Native Functions as Arguments to Functions

  As previously mentioned, a function object can be passed as argument
  to another function; this is also true for native functions,
  naturally. As an example, here's a native comparison function
  that compares its two arguments numerically:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 53

  The above function can be passed as argument to the standard
  \c{Array.prototype.sort} function to sort an array numerically,
  as the following C++ code illustrates:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 54

  Note that, in this case, we are truly treating the native function
  object as a value \mdash i.e., we don't store it as a property of the
  scripting environment \mdash we simply pass it on as an "anonymous"
  argument to another script function and then forget about it.

  \section2 The Activation Object

  Every Qt Script function invocation has an \e{activation object}
  associated with it; this object is accessible through the
  QScriptContext::activationObject() function. The activation object
  is a script object whose properties are the local variables
  associated with the invocation (including the arguments for which
  the script function has a corresponding formal parameter name).
  Thus, getting, modifying, creating and deleting local variables
  from C++ is done using the regular QScriptValue::property() and
  QScriptValue::setProperty() functions. The activation object itself
  is not directly accessible from script code (but it is implicitly
  accessed whenever a local variable is read from or written to).

  For C++ code, there are two principal applications of the
  activation object:

  \list
  \i The activation object provides a standard way to traverse the
  variables associated with a function call, by using it as the input
  to QScriptValueIterator. This is useful for debugging purposes.

  \i The activation object can be used to prepare local variables
  that should be available when a script is evaluated inline; this
  can be viewed as a way of passing arguments to the script
  itself. This technique is typically used in conjunction with
  QScriptEngine::pushContext(), as in the following example:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 77

  We create a temporary execution context, create a local variable
  for it, evaluate the script, and finally restore the old context.
  \endlist

  \section2 Property Getters and Setters

  A script object property can be defined in terms of a getter/setter
  function, similar to how a Qt C++ property has read and write
  functions associated with it. This makes it possible for a script to
  use expressions like \c{object.x} instead of \c{object.getX()}; the
  getter/setter function for \c{x} will implicitly be invoked
  whenever the property is accessed. To scripts, the property looks
  and behaves just like a regular object property.

  A single Qt Script function can act as both getter and setter for
  a property. When it is called as a getter, the argument count is 0.
  When it is called as a setter, the argument count is 1; the argument
  is the new value of the property. In the following example, we
  define a native combined getter/setter that transforms the value
  slightly:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 78

  The example uses the internal data of the object to store and
  retrieve the transformed value. Alternatively, the property
  could be stored in another, "hidden" property of the object itself
  (e.g., \c{__x__}). A native function is free to implement whatever
  storage scheme it wants, as long as the external behavior of the
  property itself is consistent (e.g., that scripts should not be able
  to distinguish it from a regular property).

  The following C++ code shows how an object property can be defined
  in terms of the native getter/setter:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 79

  When the property is accessed, like in the following script, the
  getter/setter does its job behind the scenes:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 80

  \note It is important that the setter function, not just the getter,
  returns the value of the property; i.e., the setter should \e{not}
  return QScriptValue::UndefinedValue. This is because the result of
  the property assignment is the value returned by the setter, and
  not the right-hand side expression. Also note that you normally
  should not attempt to read the same property that the getter modifies
  within the getter itself, since this will cause the getter to be
  called recursively.

  You can remove a property getter/setter by calling
  QScriptValue::setProperty(), passing an invalid QScriptValue
  as the getter/setter. Remember to specify the
  QScriptValue::PropertyGetter/QScriptValue::PropertySetter flag(s),
  otherwise the only thing that will happen is that the setter will be
  invoked with an invalid QScriptValue as its argument!

  Property getters and setters can be defined and installed by script
  code as well, as in the following example:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 81

  Getters and setters can only be used to implement "a priori
  properties"; i.e., the technique can't be used to react to an access
  to a property that the object doesn't already have. To gain total
  control of property access in this way, you need to subclass
  QScriptClass.

  \section1 Making Use of Prototype-Based Inheritance

  In ECMAScript, inheritance is based on the concept of \e{shared
  prototype objects}; this is quite different from the class-based
  inheritance familiar to C++ programmers. With QtScript, you can
  associate a custom prototype object with a C++ type using
  QScriptEngine::setDefaultPrototype(); this is the key to providing
  a script interface to that type. Since the QtScript module is built
  on top of Qt's meta-type system, this can be done for any C++ type.

  You might be wondering when exactly you would need to use this
  functionality in your application; isn't the automatic binding
  provided by QScriptEngine::newQObject() enough? No, not under all
  circumstances.
  Firstly, not every C++ type is derived from QObject; types that
  are not QObjects cannot be introspected through Qt's meta-object
  system (they do not have properties, signals and slots). Secondly,
  even if a type is QObject-derived, the functionality you want to
  expose to scripts might not all be available, since it is unusual to
  define every function to be a slot (and it's not always
  possible/desirable to change the C++ API to make it so).

  It is perfectly possible to solve this problem by using "conventional"
  C++ techniques. For instance, the QRect class could effectively be
  made scriptable by creating a QObject-based C++ wrapper class with
  \c{x}, \c{y}, \c{width} properties and so on, which forwarded property
  access and function calls to the wrapped value. However, as we shall
  see, by taking advantage of the ECMAScript object model and combining
  it with Qt's meta-object system, we can arrive at a solution that is
  more elegant, consistent and lightweight, supported by a small API.

  This section explains the underlying concepts of prototype-based
  inheritance. Once these concepts are understood, the associated
  practices can be applied throughout the QtScript API in order to
  create well-behaved, consistent bindings to C++ that will fit nicely
  into the ECMAScript universe.

  When experimenting with QtScript objects and inheritance, it can be
  helpful to use the interactive interpreter included with the
  \l{Qt Script Examples}, located in \c{examples/script/qscript}.

  \section2 Prototype Objects and Shared Properties

  The purpose of a QtScript \e{prototype object} is to define
  behavior that should be shared by a set of other QtScript
  objects. We say that objects which share the same prototype object
  belong to the same \e{class} (again, on the technical side this
  should not to be confused with the class constructs of languages
  like C++ and Java; ECMAScript has no such construct).

  The basic prototype-based inheritance mechanism works as follows: Each
  QtScript object has an internal link to another object, its
  \e{prototype}. When a property is looked up in an object, and the
  object itself does not have the property, the property is looked up
  in the prototype object instead; if the prototype has the property,
  then that property is returned. Otherwise, the property is looked up
  in the prototype of the prototype object, and so on; this chain of
  objects constitutes a \e{prototype chain}. The chain of prototype
  objects is followed until the property is found or the end of the
  chain is reached.

  For example, when you create a new object by the expression \c{new
  Object()}, the resulting object will have as its prototype the
  standard \c{Object} prototype, \c{Object.prototype}; through this
  prototype relation, the new object inherits a set of properties,
  including the \c{hasOwnProperty()} function and \c{toString()}
  function:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 27

  The \c{toString()} function itself is not defined in \c{o} (since we
  did not assign anything to \c{o.toString}), so instead the
  \c{toString()} function in the standard \c{Object} prototype is
  called, which returns a highly generic string representation of
  \c{o} ("[object Object]").

  Note that the properties of the prototype object are not \e{copied} to
  the new object; only a \e{link} from the new object to the prototype
  object is maintained. This means that changes done to the prototype
  object will immediately be reflected in the behavior of all objects
  that have the modified object as their prototype.

  \section2 Defining Classes in a Prototype-Based Universe

  In QtScript, a class is not defined explicitly; there is no
  \c{class} keyword. Instead, you define a new class in two steps:

  \list 1
  \i Define a \e{constructor function} that will initialize new objects.
  \i Set up a \e{prototype object} that defines the class interface, and
     assign this object to the public \c{prototype} property of the
     constructor function.
  \endlist

  With this arrangement, the constructor's public \c{prototype}
  property will automatically be set as the prototype of objects created
  by applying the \c{new} operator to your constructor function;
  e.g., the prototype of an object created by \c{new Foo()} will be the
  value of \c{Foo.prototype}.

  Functions that don't operate on the \c this object ("static" methods)
  are typically stored as properties of the constructor function, not
  as properties of the prototype object. The same is true for
  constants, such as enum values.

  The following code defines a simple constructor function for a class
  called \c{Person}:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 28

  Next, you want to set up \c{Person.prototype} as your prototype
  object; i.e., define the interface that should be common to all
  \c{Person} objects. QtScript automatically creates a default
  prototype object (by the expression \c{new Object()}) for every
  script function; you can add properties to this object, or you can
  assign your own custom object. (Generally speaking, any QtScript
  object can act as prototype for any other object.)

  Here's an example of how you might want to override the
  \c{toString()} function that \c{Person.prototype} inherits from
  \c{Object.prototype}, to give your \c{Person} objects a more
  appropriate string representation:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 29

  This resembles the process of reimplementing a virtual function
  in C++. Henceforth, when the property named \c{toString} is
  looked up in a \c{Person} object, it will be resolved in
  \c{Person.prototype}, not in \c{Object.prototype} as before:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 30

  There are also some other interesting things we can learn about a
  \c{Person} object:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 31

  The \c{hasOwnProperty()} function is not inherited from
  \c{Person.prototype}, but rather from \c{Object.prototype}, which is
  the prototype of \c{Person.prototype} itself; i.e., the prototype
  chain of \c{Person} objects is \c{Person.prototype} followed by
  \c{Object.prototype}. This prototype chain establishes a \e{class
  hierarchy}, as demonstrated by applying the \c{instanceof} operator;
  \c{instanceof} checks if the value of the public \c{prototype}
  property of the constructor function on the right-hand side is
  reached by following the prototype chain of the object on the
  left-hand side.

  When defining subclasses, there's a general pattern you can use. The
  following example shows how one can create a subclass of \c{Person}
  called \c{Employee}:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 32

  Again, you can use the \c{instanceof} to verify that the
  class relationship between \c{Employee} and \c{Person} has been
  correctly established:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 33

  This shows that the prototype chain of \c{Employee} objects is the
  same as that of \c{Person} objects, but with \c{Employee.prototype}
  added to the front of the chain.

  \section2 Prototype-Based Programming with the QtScript C++ API

  You can use QScriptEngine::newFunction() to wrap
  native functions. When implementing a constructor function,
  you also pass the prototype object as an argument to
  QScriptEngine::newFunction().
  You can call QScriptValue::construct() to call a constructor
  function, and you can use QScriptValue::call() from within a
  native constructor function if you need to call a base class
  constructor.

  The QScriptable class provides a convenient way to implement a
  prototype object in terms of C++ slots and properties. Take a look
  at the \l{Default Prototypes Example} to see how this is done.
  Alternatively, the prototype functionality can be implemented in
  terms of standalone native functions that you wrap with
  QScriptEngine::newFunction() and set as properties of your prototype
  object by calling QScriptValue::setProperty().

  In the implementation of your prototype functions, you use
  QScriptable::thisObject() (or QScriptContext::thisObject()) to
  obtain a reference to the QScriptValue being operated upon; then you
  call qscriptvalue_cast() to cast it to your C++ type, and perform
  the relevant operations using the usual C++ API for the type.

  You associate a prototype object with a C++ type by calling
  QScriptEngine::setDefaultPrototype(). Once this mapping is
  established, QtScript will automatically assign the correct
  prototype when a value of such a type is wrapped in a QScriptValue;
  either when you explicitly call QScriptEngine::toScriptValue(), or
  when a value of such a type is returned from a C++ slot and
  internally passed back to script code by the engine. This means you
  \e{don't} have to implement wrapper classes if you use this
  approach.

  As an example, let's consider how the \c{Person} class from the
  preceding section can be implemented in terms of the Qt Script API.
  We begin with the native constructor function:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 34

  Here's the native equivalent of the \c{Person.prototype.toString}
  function we saw before:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 35

  The \c{Person} class can then be initialized as follows:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 36

  The implementation of the \c{Employee} subclass is similar. We
  use QScriptValue::call() to call the super-class (Person) constructor:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 37

  The \c{Employee} class can then be initialized as follows:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 38

  When implementing the prototype object of a class, you may want to use
  the QScriptable class, as it enables you to define the API of your
  script class in terms of Qt properties, signals and slots, and
  automatically handles value conversion between the Qt Script and C++
  side.

  \section2 Implementing Prototype Objects for Value-based Types

  When implementing a prototype object for a value-based type --
  e.g. QPointF -- the same general technique applies; you populate
  a prototype object with functionality that should be shared
  among instances. You then associate the prototype object with
  the type by calling QScriptEngine::setDefaultPrototype(). This
  ensures that when e.g. a value of the relevant type is returned
  from a slot back to the script, the prototype link of the script
  value will be initialized correctly.

  When values of the custom type are stored in QVariants -- which Qt
  Script does by default --, qscriptvalue_cast() enables you to safely
  cast the script value to a pointer to the C++ type. This makes it
  easy to do type-checking, and, for prototype functions that should
  modify the underlying C++ value, lets you modify the actual value
  contained in the script value (and not a copy of it).

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 39

  \section2 Implementing Constructors for Value-based Types

  You can implement a constructor function for a value-based type
  by wrapping a native factory function. For example, the following
  function implements a simple constructor for QPoint:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 44

  In the above code we simplified things a bit, e.g. we didn't check
  the argument count to decide which QPoint C++ constructor to use.
  In your own constructors you have to do this type of resolution
  yourself, i.e. by checking the number of arguments passed to the
  native function, and/or by checking the type of the arguments and
  converting the arguments to the desired type. If you detect a problem
  with the arguments you may want to signal this by throwing a script
  exception; see QScriptContext::throwError().

  \section2 Managing Non-QObject-based Objects

  For value-based types (e.g. QPoint), the C++ object will be destroyed when
  the Qt Script object is garbage-collected, so managing the memory of the C++
  object is not an issue. For QObjects, Qt Script provides several
  alternatives for managing the underlying C++ object's lifetime; see the
  \l{Controlling QObject Ownership} section. However, for polymorphic types
  that don't inherit from QObject, and when you can't (or won't) wrap the type
  in a QObject, you have to manage the lifetime of the C++ object yourself.

  A behavior that's often reasonable when a Qt Script object wraps a C++
  object, is that the C++ object is deleted when the Qt Script object is
  garbage-collected; this is typically the case when the objects can be
  constructed by scripts, as opposed to the application providing the scripts
  with pre-made "environment" objects. A way of making the lifetime of the C++
  object follow the lifetime of the Qt Script object is by using a shared
  pointer class, such as QSharedPointer, to hold a pointer to your object;
  when the Qt Script object containing the QSharedPointer is
  garbage-collected, the underlying C++ object will be deleted if there are no
  other references to the object.

  The following snippet shows a constructor function that constructs
  QXmlStreamReader objects that are stored using QSharedPointer:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 93

  Prototype functions can use qscriptvalue_cast() to cast the \c this object
  to the proper type:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 94

  The prototype and constructor objects are set up in the usual way:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 95

  Scripts can now construct QXmlStreamReader objects by calling the \c
  XmlStreamReader constructor, and when the Qt Script object is
  garbage-collected (or the script engine is destroyed), the QXmlStreamReader
  object is destroyed as well.

  \section1 Defining Custom Script Classes with QScriptClass

  There are cases where neither the dynamic QObject binding provided
  by QScriptEngine::newQObject() or the manual binding provided by
  QScriptEngine::newFunction() is sufficient. For example, you might
  want to implement a dynamic script proxy to an underlying object;
  or you might want to implement an array-like class (i.e. that gives
  special treatment to properties that are valid array indexes, and
  to the property "length"). In such cases, you can subclass
  QScriptClass to achieve the desired behavior.

  QScriptClass allows you to handle all property access for a
  (class of) script object through virtual get/set property functions.
  Iteration of custom properties is also supported through the
  QScriptClassPropertyIterator class; this means you can advertise
  properties to be reported by for-in script statements and
  QScriptValueIterator.

  \section1 Error Handling and Debugging Facilities

  Syntax errors in scripts will be reported as soon as a script is
  evaluated; QScriptEngine::evaluate() will return a SyntaxError object
  that you can convert to a string to get a description of the error.

  The QScriptEngine::uncaughtExceptionBacktrace() function gives you
  a human-readable backtrace of the last uncaught exception. In order
  to get useful filename information in backtraces, you should pass
  proper filenames to QScriptEngine::evaluate() when evaluating your
  scripts.

  Often an exception doesn't happen at the time the script is evaluated,
  but at a later time when a function defined by the script is actually
  executed. For C++ signal handlers, this is tricky; consider the case
  where the clicked() signal of a button is connected to a script function,
  and that script function causes a script exception when it is handling
  the signal. Where is that script exception propagated to?

  The solution is to connect to the QScriptEngine::signalHandlerException()
  signal; this will give you notification when a signal handler causes
  an exception, so that you can find out what happened and/or recover
  from it.

  In Qt 4.4 the QScriptEngineAgent class was introduced. QScriptEngineAgent
  provides an interface for reporting low-level "events" in a script engine,
  such as when a function is entered or when a new script statement is
  reached. By subclassing QScriptEngineAgent you can be notified of these
  events and perform some action, if you want. QScriptEngineAgent itself
  doesn't provide any debugging-specific functionality (e.g. setting
  breakpoints), but it is the basis of tools that do.

  The QScriptEngineDebugger class introduced in Qt 4.5 provides a 
  \l{Qt Script Debugger Manual}{Qt Script debugger} that can be embedded
  into your application.

  \section2 Redefining print()

  Qt Script provides a built-in print() function that can be useful for
  simple debugging purposes. The built-in print() function writes to
  standard output. You can redefine the print() function (or add your
  own function, e.g. debug() or log()) that redirects the text to
  somewhere else. The following code shows a custom print() that adds
  text to a QPlainTextEdit.

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 45

  The following code shows how the custom print() function may be
  initialized and used.

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 46

  A pointer to the QPlainTextEdit is stored as an internal property
  of the script function itself, so that it can be retrieved when
  the function is called.

  \section1 Using QtScript Extensions

  The QScriptEngine::importExtension() function can be used to load plugins
  into a script engine. Plugins typically add some extra functionality to
  the engine; for example, a plugin might add full bindings for the Qt
  Arthur painting API, so that those classes may be used from Qt Script
  scripts. There are currently no script plugins shipped with Qt.

  If you are implementing some Qt Script functionality that you want other
  Qt application developers to be able to use, \l{Creating QtScript Extensions}
  {developing an extension} (e.g. by subclassing QScriptExtensionPlugin) is
  worth looking into.

  \section1 Internationalization

  Since Qt 4.5, Qt Script supports internationalization of scripts by building
  on the C++ internationalization functionality (see \l{Internationalization
  with Qt}).

  \section2 Use qsTr() for All Literal Text

  Wherever your script uses "quoted text" for text that will be presented to
  the user, ensure that it is processed by the QCoreApplication::translate()
  function. Essentially all that is necessary to achieve this is to use
  the qsTr() script function. Example:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 82

  This accounts for 99% of the user-visible strings you're likely to write.

  The qsTr() function uses the basename of the script's filename (see
  QFileInfo::baseName()) as the translation context; if the filename is not
  unique in your project, you should use the qsTranslate() function and pass a
  suitable context as the first argument. Example:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 83

  If you need to have translatable text completely outside a function, there
  are two functions to help: QT_TR_NOOP() and QT_TRANSLATE_NOOP(). They merely
  mark the text for extraction by the \c lupdate utility described below.  At
  runtime, these functions simply return the text to translate unmodified.

  Example of QT_TR_NOOP():

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 84

  Example of QT_TRANSLATE_NOOP():

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 85

  \section2 Use String.prototype.arg() for Dynamic Text

  The String.prototype.arg() function (which is modeled after QString::arg())
  offers a simple means for substituting arguments:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 86

    \section2 Produce Translations

    Once you are using qsTr() and/or qsTranslate() throughout your scripts, you
    can start producing translations of the user-visible text in your program.

    The \l{Qt Linguist manual} provides further information about
    Qt's translation tools, \e{Qt Linguist}, \c lupdate and \c
    lrelease.

    Translation of Qt Script scripts is a three-step process:

    \list 1

    \o Run \c lupdate to extract translatable text from the script source code
    of the Qt application, resulting in a message file for translators (a TS
    file). The utility recognizes qsTr(), qsTranslate() and the
    \c{QT_TR*_NOOP()} functions described above and produces TS files
    (usually one per language).

    \o Provide translations for the source texts in the TS file, using
    \e{Qt Linguist}. Since TS files are in XML format, you can also
    edit them by hand.

    \o Run \c lrelease to obtain a light-weight message file (a QM
    file) from the TS file, suitable only for end use. Think of the TS
     files as "source files", and QM files as "object files". The
    translator edits the TS files, but the users of your application
    only need the QM files. Both kinds of files are platform and
    locale independent.

    \endlist

    Typically, you will repeat these steps for every release of your
    application. The \c lupdate utility does its best to reuse the
    translations from previous releases.

    When running \c lupdate, you must specify the location of the script(s),
    and the name of the TS file to produce. Examples:

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 87

    will extract translatable text from \c myscript.qs and create the
    translation file \c myscript_la.qs.

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 88

    will extract translatable text from all files ending with \c{.qs} in the
    \c scripts folder and create the translation file \c scripts_la.qs.

    Alternatively, you can create a separate qmake project file that sets up
    the \c SOURCES and \c TRANSLATIONS variables appropriately; then run
    \c lupdate with the project file as input.

    \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 89

    When running \c lrelease, you must specify the name of the TS input
    file; or, if you are using a qmake project file to manage script
    translations, you specify the name of that file. \c lrelease will create
    \c myscript_la.qm, the binary representation of the translation.

    \section2 Apply Translations

    In your application, you must use QTranslator::load() to load the
    translation files appropriate for the user's language, and install them
    using QCoreApplication::installTranslator(). Finally, you must call
    QScriptEngine::installTranslatorFunctions() to make the script translation
    functions (qsTr(), qsTranslate() and \c{QT_TR*_NOOP()}) available to scripts
    that are subsequently evaluated by QScriptEngine::evaluate(). For scripts
    that are using the qsTr() function, the proper filename must be passed as
    second argument to QScriptEngine::evaluate().

    \c linguist, \c lupdate and \c lrelease are installed in the \c bin
    subdirectory of the base directory Qt is installed into. Click Help|Manual
    in \e{Qt Linguist} to access the user's manual; it contains a tutorial
    to get you started.

    See also the \l{Hello Script Example}.

  \section1 ECMAScript Compatibility

  QtScript implements all the built-in objects and properties defined
  in the \l{ECMA-262} standard; see the
  \l{ECMAScript Reference}{ECMAScript reference} for an overview.

  \section1 QtScript Extensions to ECMAScript

  \list
  \i \c{__proto__} \br
    The prototype of an object (QScriptValue::prototype())
    can be accessed through its \c{__proto__} property in script code.
    This property has the QScriptValue::Undeletable flag set.
    For example:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 40

  \i \c{Object.prototype.__defineGetter__} \br
    This function installs a
    getter function for a property of an object. The first argument is
    the property name, and the second is the function to call to get
    the value of that property. When the function is invoked, the
    \c this object will be the object whose property is accessed.
    For example:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 41

  \i \c{Object.prototype.__defineSetter__} \br
    This function installs a
    setter function for a property of an object. The first argument is
    the property name, and the second is the function to call to set
    the value of that property.  When the function is invoked, the
    \c this object will be the object whose property is accessed.
    For example:

  \snippet doc/src/snippets/code/doc_src_qtscript.qdoc 42

  \i \c{Function.prototype.connect} \br
    This function connects
    a signal to a slot. Usage of this function is described in
    the section \l{Using Signals and Slots}.

  \i \c{Function.prototype.disconnect} \br
    This function disconnects
    a signal from a slot. Usage of this function is described in
    the section \l{Using Signals and Slots}.

  \i \c{QObject.prototype.findChild} \br
    This function is semantically equivalent to QObject::findChild().

  \i \c{QObject.prototype.findChildren} \br
    This function is semantically equivalent to QObject::findChildren().

  \i \c{QObject.prototype.toString} \br
    This function returns a default string representation of a QObject.

  \i \c{gc} \br
    This function invokes the garbage collector.

  \i \c{Error.prototype.backtrace} \br
    This function returns a human-readable backtrace, in the form of
    an array of strings.

  \i Error objects have the following additional properties:
    \list
    \i \c{lineNumber}: The line number where the error occurred.
    \i \c{fileName}: The file name where the error occurred (if a file name
    was passed to QScriptEngine::evaluate()).
    \i \c{stack}: An array of objects describing the stack. Each object has
      the following properties:
      \list
      \i \c{functionName}: The function name, if available.
      \i \c{fileName}: The file name, if available.
      \i \c{lineNumber}: The line number, if available.
      \endlist
    \endlist

  \endlist

 */