source: trunk/doc/src/declarative/extending-tutorial.qdoc@ 980

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/*!
\page qml-extending-tutorial-index.html
\title Tutorial: Writing QML extensions with C++

The Qt Declarative module provides a set of APIs for extending QML through
C++ extensions. You can write extensions to add your own QML types, extend existing
Qt types, or call C/C++ functions that are not accessible from ordinary QML code.

This tutorial shows how to write a QML extension using C++ that includes 
core QML features, including properties, signals and bindings. It also shows how
extensions can be deployed through plugins.

You can find the source code for this tutorial in \c Qt's
examples/declarative/tutorials/extending directory.

Tutorial chapters:

\list 1
\o \l{declarative/tutorials/extending/chapter1-basics}{Creating a New Type}
\o \l{declarative/tutorials/extending/chapter2-methods}{Connecting to C++ Methods and Signals}
\o \l{declarative/tutorials/extending/chapter3-bindings}{Adding Property Bindings}
\o \l{declarative/tutorials/extending/chapter4-customPropertyTypes}{Using Custom Property Types}
\o \l{declarative/tutorials/extending/chapter5-listproperties}{Using List Property Types}
\o \l{declarative/tutorials/extending/chapter6-plugins}{Writing an Extension Plugin}
\o \l{qml-extending-tutorial7.html}{In Summary}
\endlist

*/

/*!
\title Chapter 1: Creating a New Type

\example declarative/tutorials/extending/chapter1-basics

A common task when extending QML is to provide a new QML type that supports some
 custom functionality beyond what is provided by the built-in \l {QML Elements}.
For example, this could be done to implement particular data models, or provide
elements with custom painting and drawing capabilities, or access system features
like network programming that are not accessible through built-in QML features.

In this tutorial, we will show how to use the C++ classes in the Qt Declarative
module to extend QML. The end result will be a simple Pie Chart display implemented by
several custom QML types connected together through QML features like bindings and 
signals, and made available to the QML runtime through a plugin.

To begin with, let's create a new QML type called "PieChart" that has two properties: a name
and a color.  We will make it available in a \l {Modules}{module} called "Charts", with
a module version of 1.0. 

We want this \c PieChart type to be usable from QML like this:

\code
    import Charts 1.0
    
    PieChart {
        width: 100; height: 100
        name: "A simple pie chart"
        color: "red"
    }
\endcode

To do this, we need a C++ class that encapsulates this \c PieChart type and its two
properties. Since QML makes extensive use of Qt's \l{Meta-Object System}{meta object system},
this new class must:

\list
\o Inherit from QObject
\o Declare its properties using the Q_PROPERTY macro
\endlist

Here is our \c PieChart class, defined in \c piechart.h:

\snippet declarative/tutorials/extending/chapter1-basics/piechart.h 0

The class inherits from QDeclarativeItem because we want to override 
QDeclarativeItem::paint() in order to draw. If the class just represented some
data type and was not an item that actually needed to be displayed, it could simply inherit
from QObject. Or, if we want to extend the functionality of an existing QObject-based 
class, it could inherit from that class instead.

The \c PieChart class defines the two properties, \c name and \c color, with the Q_PROPERTY macro,
and overrides QDeclarativeItem::paint(). The class implementation in \c piechart.cpp 
simply sets and returns the \c m_name and \c m_color values as appropriate, and 
implements \c paint() to draw a simple pie chart. It also turns off the 
QGraphicsItem::ItemHasNoContents flag to enable painting:

\snippet declarative/tutorials/extending/chapter1-basics/piechart.cpp 0
\dots 0
\snippet declarative/tutorials/extending/chapter1-basics/piechart.cpp 1

Now that we have defined the \c PieChart type, we will use it from QML. The \c app.qml
file creates a \c PieChart item and display the pie chart's details
using a standard QML \l Text item:

\snippet declarative/tutorials/extending/chapter1-basics/app.qml 0

Notice that although the color is specified as a string in QML, it is automatically
converted to a QColor object for the PieChart \c color property. Automatic conversions are
provided for various other \l {QML Basic Types}{basic types}; for example, a string
like "640x480" can be automatically converted to a QSize value.

We'll also create a C++ application that uses a QDeclarativeView to run and
display \c app.qml. The application must register the \c PieChart type
using the qmlRegisterType() function, to allow it to be used from QML. If
you don't register the type, \c app.qml won't be able to create a \c PieChart.

Here is the application \c main.cpp:

\snippet declarative/tutorials/extending/chapter1-basics/main.cpp 0

This call to qmlRegisterType() registers the \c PieChart type as a type called "PieChart", in a module named "Charts",
with a module version of 1.0.

Lastly, we write a \c .pro project file that includes the files and the \c declarative library:

\quotefile declarative/tutorials/extending/chapter1-basics/chapter1-basics.pro

Now we can build and run the application:

\image extending-tutorial-chapter1.png

Try it yourself with the code in Qt's \c examples/tutorials/extending/chapter1-basics directory.

At the moment, the \c app.qml is run from within a C++ application.
This may seem odd if you're used to running QML files with the \l {QML Viewer}.
Later on, we'll show how to create a plugin so that you can run \c app.qml using the 
\l {QML Viewer} instead.

*/


/*!
\title Chapter 2: Connecting to C++ Methods and Signals 

\example declarative/tutorials/extending/chapter2-methods

Suppose we want \c PieChart to have a "clearChart()" method that erases the
chart and then emits a "chartCleared" signal. Our \c app.qml would be able 
to call \c clearChart() and receive \c chartCleared() signals like this:

\snippet declarative/tutorials/extending/chapter2-methods/app.qml 0

\image extending-tutorial-chapter2.png

To do this, we add a \c clearChart() method and a \c chartCleared() signal
to our C++ class:

\snippet declarative/tutorials/extending/chapter2-methods/piechart.h 0
\dots
\snippet declarative/tutorials/extending/chapter2-methods/piechart.h 1
\dots
\snippet declarative/tutorials/extending/chapter2-methods/piechart.h 2
\dots
\snippet declarative/tutorials/extending/chapter2-methods/piechart.h 3

The use of Q_INVOKABLE makes the \c clearChart() method available to the
Qt Meta-Object system, and in turn, to QML. Note that it could have
been declared as as a Qt slot instead of using Q_INVOKABLE, as
slots are also callable from QML. Both of these approaches are valid.

The \c clearChart() method simply changes the color to Qt::transparent,
repaints the chart, then emits the \c chartCleared() signal:

\snippet declarative/tutorials/extending/chapter2-methods/piechart.cpp 0

Now when we run the application and click the window, the pie chart
disappears, and the application outputs:

\code
    The chart has been cleared
\endcode

Try out the example yourself with the updated code in Qt's \c examples/tutorials/extending/chapter2-methods directory.

*/

/*!
\title Chapter 3: Adding Property Bindings

\example declarative/tutorials/extending/chapter3-bindings

Property bindings is a powerful feature of QML that allows values of different
elements to be synchronized automatically. It uses signals to notify and update
other elements' values when property values are changed.

Let's enable property bindings for the \c color property. That means 
if we have code like this:

\snippet declarative/tutorials/extending/chapter3-bindings/app.qml 0

\image extending-tutorial-chapter3.png

The "color: chartA.color" statement binds the \c color value of
\c chartB to the \c color of \c chartA.
Whenever \c chartA's \c color value changes, \c chartB's \c color value
updates to the same value. When the window is clicked, the \c onClicked
handler in the MouseArea changes the color of \c chartA, thereby changing
both charts to the color blue.

It's easy to enable property binding for the \c color property. 
We add a \l{Qt's Property System}{NOTIFY} feature to its Q_PROPERTY() declaration to indicate that a "colorChanged" signal
is emitted whenever the value changes.

\snippet declarative/tutorials/extending/chapter3-bindings/piechart.h 0
\dots
\snippet declarative/tutorials/extending/chapter3-bindings/piechart.h 1
\dots
\snippet declarative/tutorials/extending/chapter3-bindings/piechart.h 2
\dots
\snippet declarative/tutorials/extending/chapter3-bindings/piechart.h 3

Then, we emit this signal in \c setPieSlice():

\snippet declarative/tutorials/extending/chapter3-bindings/piechart.cpp 0

It's important for \c setColor() to check that the color value has actually changed
before emitting \c colorChanged(). This ensures the signal is not emitted unnecessarily and
also prevents loops when other elements respond to the value change.

The use of bindings is essential to QML. You should always add NOTIFY 
signals for properties if they are able to be implemented, so that your
properties can be used in bindings. Properties that cannot be bound cannot be
automatically updated and cannot be used as flexibly in QML. Also, since
bindings are invoked so often and relied upon in QML usage, users of your
custom QML types may see unexpected behavior if bindings are not implemented.

*/

/*!
\title Chapter 4: Using Custom Property Types

\example declarative/tutorials/extending/chapter4-customPropertyTypes

The \c PieChart type currently has a string-type property and a color-type property.
It could have many other types of properties. For example, it could have an
int-type property to store an identifier for each chart:

\code
    // C++
    class PieChart : public QDeclarativeItem
    {
        Q_PROPERTY(int chartId READ chartId WRITE setChartId NOTIFY chartIdChanged)
        ...

    public:
        void setChartId(int chartId);
        int chartId() const;
        ...

    signals:
        void chartIdChanged();
    };

    // QML
    PieChart {
        ...
        chartId: 100
    }
\endcode

We can also use various other property types. QML has built-in support for the types
listed in the \l{Adding Properties} documentation, which includes the following:

\list
\o bool, unsigned int, int, float, double, qreal
\o QString, QUrl, QColor
\o QDate, QTime, QDateTime
\o QPoint, QPointF, QSize, QSizeF, QRect, QRectF
\o QVariant
\endlist

If we want to create a property whose type is not supported by QML by default,
we need to register the type with QML.

For example, let's replace the use of the \c property with a type called 
"PieSlice" that has a \c color property. Instead of assigning a color,
we assign an \c PieSlice value which itself contains a \c color:

\snippet declarative/tutorials/extending/chapter4-customPropertyTypes/app.qml 0

Like \c PieChart, this new \c PieSlice type inherits from QDeclarativeItem and declares
its properties with Q_PROPERTY():

\snippet declarative/tutorials/extending/chapter4-customPropertyTypes/pieslice.h 0

To use it in \c PieChart, we modify the \c color property declaration
and associated method signatures:

\snippet declarative/tutorials/extending/chapter4-customPropertyTypes/piechart.h 0
\dots
\snippet declarative/tutorials/extending/chapter4-customPropertyTypes/piechart.h 1
\dots
\snippet declarative/tutorials/extending/chapter4-customPropertyTypes/piechart.h 2
\dots
\snippet declarative/tutorials/extending/chapter4-customPropertyTypes/piechart.h 3

There is one thing to be aware of when implementing \c setPieSlice(). The \c PieSlice
is a visual item, so it must be set as a child of the \c PieChart using
QDeclarativeItem::setParentItem() so that the \c PieChart knows to paint this child
item when its contents are drawn:

\snippet declarative/tutorials/extending/chapter4-customPropertyTypes/piechart.cpp 0


Like the \c PieChart type, the \c PieSlice type has to be registered
using qmlRegisterType() to be used from QML. As with \c PieChart, we'll add the
type to the "Charts" module, version 1.0:

\snippet declarative/tutorials/extending/chapter4-customPropertyTypes/main.cpp 0
\dots
\snippet declarative/tutorials/extending/chapter4-customPropertyTypes/main.cpp 1
\dots
\snippet declarative/tutorials/extending/chapter4-customPropertyTypes/main.cpp 2

Try it out with the code in Qt's \c examples/tutorials/extending/chapter4-customPropertyTypes directory.

*/


/*!
\title Chapter 5: Using List Property Types

\example declarative/tutorials/extending/chapter5-listproperties

Right now, a \c PieChart can only have one \c PieSlice. Ideally a chart would
have multiple slices, with different colors and sizes. To do this, we could
have a \c slices property that accepts a list of \c PieSlice items:

\snippet declarative/tutorials/extending/chapter5-listproperties/app.qml 0

\image extending-tutorial-chapter5.png

To do this, we replace the \c pieSlice property in \c PieChart with a \c slices property,
declared as a QDeclarativeListProperty type. The QDeclarativeListProperty class enables the 
creation of list properties in QML extensions. We replace the \c pieSlice()
function with a \c slices() function that returns a list of slices, and add 
an internal \c append_slice() function (discussed below). We also use a QList to 
store the internal list of slices as \c m_slices:

\snippet declarative/tutorials/extending/chapter5-listproperties/piechart.h 0
\dots
\snippet declarative/tutorials/extending/chapter5-listproperties/piechart.h 1
\dots
\snippet declarative/tutorials/extending/chapter5-listproperties/piechart.h 2

Although the \c slices property does not have an associated \c WRITE function,
it is still modifiable because of the way QDeclarativeListProperty works.
In the \c PieChart implementation, we implement \c PieChart::slices() to
return a QDeclarativeListProperty value and indicate that the internal
\c PieChart::append_slice() function is to be called whenever a request is made from QML
to add items to the list:

\snippet declarative/tutorials/extending/chapter5-listproperties/piechart.cpp 0

The \c append_slice() function simply sets the parent item as before,
and adds the new item to the \c m_slices list. As you can see, the append function for a
QDeclarativeListProperty is called with two arguments: the list property, and
the item that is to be appended.

The \c PieSlice class has also been modified to include \c fromAngle and \c angleSpan
properties and to draw the slice according to these values. This is a straightforward
modification if you have read the previous pages in this tutorial, so the code is not shown here.

The complete code can be seen in the updated \c examples/tutorials/extending/chapter5-listproperties directory.

*/


/*!
\title Chapter 6: Writing an Extension Plugin

\example declarative/tutorials/extending/chapter6-plugins

Currently the \c PieChart and \c PieSlice types are used by \c app.qml,
which is displayed using a QDeclarativeView in a C++ application. An alternative
way to use our QML extension is to create a plugin library to make it available
to the QML engine. This allows \c app.qml to be loaded with the \l {QML Viewer}
(or some other QML \l{Qt Declarative UI Runtime}{runtime} application) instead of writing a \c main.cpp file and
loading our own C++ application.

To create a plugin library, we need:

\list
\o A plugin class that registers our QML types
\o A project file that describes the plugin 
\o A \l{Writing a qmldir file}{qmldir} file that tells the QML engine to load the plugin
\endlist

First, we create a plugin class named \c ChartsPlugin. It subclasses QDeclarativeExtensionPlugin
and registers our QML types in the inherited \l{QDeclarativeExtensionPlugin::}{registerTypes()} method. It also calls
Q_EXPORT_PLUGIN2 for Qt's \l{How to Create Qt Plugins}{plugin system}.

Here is the \c ChartsPlugin definition in \c chartsplugin.h:

\snippet declarative/tutorials/extending/chapter6-plugins/chartsplugin.h 0

And its implementation in \c chartsplugin.cpp:

\snippet declarative/tutorials/extending/chapter6-plugins/chartsplugin.cpp 0

Then, we write a \c .pro project file that defines the project as a plugin library
and specifies with DESTDIR that library files should be built into a "lib" subdirectory:

\quotefile declarative/tutorials/extending/chapter6-plugins/chapter6-plugins.pro

Finally, we add a \l{Writing a qmldir file}{qmldir} file that is automatically parsed by the QML engine.
In this file, we specify that a plugin named "chapter6-plugin" (the name
of the example project) can be found in the "lib" subdirectory:

\quotefile declarative/tutorials/extending/chapter6-plugins/qmldir

Now we have a plugin, and instead of having a main.cpp and an executable, we can build
the project and then load the QML file in the \l {QML Viewer}:

\code
    qmlviewer app.qml
\endcode

(On Mac OS X, you can launch the "QMLViewer" application instead.)

Notice the "import Charts 1.0" statement has disappeared from \c app.qml. This is
because the \c qmldir file is in the same directory as \c app.qml: this is equivalent to
having PieChart.qml and PieSlice.qml files inside the project directory, which could both
be used by \c app.qml without import statements.
*/


/*!
\page qml-extending-tutorial7.html
\title Chapter 7: In Summary

In this tutorial, we've shown the basic steps for creating a QML extension:

\list
\o Define new QML types by subclassing QObject and registering them with qmlRegisterType()
\o Add callable methods using Q_INVOKABLE or Qt slots, and connect to Qt signals with an \c onSignal syntax
\o Add property bindings by defining \l{Qt's Property System}{NOTIFY} signals
\o Define custom property types if the built-in types are not sufficient
\o Define list property types using QDeclarativeListProperty
\o Create a plugin library by defining a Qt plugin and writing a \c qmldir file
\endlist


The \l {Extending QML in C++} reference documentation shows other useful features that can be added to
QML extensions. For example, we could use \l{Default Property}{default properties} to allow 
slices to be added without using the \c slices property:

\code
    PieChart {
        PieSlice { ... }
        PieSlice { ... }
        PieSlice { ... }
    }
\endcode

Or randomly add and remove slices from time to time using \l{Property Value Sources}{property value sources}:

\code
    PieChart {
        PieSliceRandomizer on slices {}
    }
\endcode


See the \l{Extending QML in C++}{reference documentation} for more information.

*/