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/*!
    \page signalsandslots.html
    \title Signals and Slots
    \ingroup architecture
    \brief An overview of Qt's signals and slots inter-object
    communication mechanism.

    Signals and slots are used for communication between objects. The
    signals and slots mechanism is a central feature of Qt and
    probably the part that differs most from the features provided by
    other frameworks.

    \tableofcontents

    \section1 Introduction

    In GUI programming, when we change one widget, we often want
    another widget to be notified. More generally, we want objects of
    any kind to be able to communicate with one another. For example,
    if a user clicks a \gui{Close} button, we probably want the
    window's \l{QWidget::close()}{close()} function to be called.

    Older toolkits achieve this kind of communication using
    callbacks. A callback is a pointer to a function, so if you want
    a processing function to notify you about some event you pass a
    pointer to another function (the callback) to the processing
    function. The processing function then calls the callback when
    appropriate. Callbacks have two fundamental flaws: Firstly, they
    are not type-safe. We can never be certain that the processing
    function will call the callback with the correct arguments.
    Secondly, the callback is strongly coupled to the processing
    function since the processing function must know which callback
    to call.

    \section1 Signals and Slots

    In Qt, we have an alternative to the callback technique: We use
    signals and slots. A signal is emitted when a particular event
    occurs. Qt's widgets have many predefined signals, but we can
    always subclass widgets to add our own signals to them. A slot
    is a function that is called in response to a particular signal.
    Qt's widgets have many pre-defined slots, but it is common
    practice to subclass widgets and add your own slots so that you
    can handle the signals that you are interested in.

    \img abstract-connections.png
    \omit
    \caption An abstract view of some signals and slots connections
    \endomit

    The signals and slots mechanism is type safe: The signature of a
    signal must match the signature of the receiving slot. (In fact a
    slot may have a shorter signature than the signal it receives
    because it can ignore extra arguments.) Since the signatures are
    compatible, the compiler can help us detect type mismatches.
    Signals and slots are loosely coupled: A class which emits a
    signal neither knows nor cares which slots receive the signal.
    Qt's signals and slots mechanism ensures that if you connect a
    signal to a slot, the slot will be called with the signal's
    parameters at the right time. Signals and slots can take any
    number of arguments of any type. They are completely type safe.

    All classes that inherit from QObject or one of its subclasses
    (e.g., QWidget) can contain signals and slots. Signals are emitted by
    objects when they change their state in a way that may be interesting
    to other objects. This is all the object does to communicate. It
    does not know or care whether anything is receiving the signals it
    emits. This is true information encapsulation, and ensures that the
    object can be used as a software component.

    Slots can be used for receiving signals, but they are also normal
    member functions. Just as an object does not know if anything receives
    its signals, a slot does not know if it has any signals connected to
    it. This ensures that truly independent components can be created with
    Qt.

    You can connect as many signals as you want to a single slot, and a
    signal can be connected to as many slots as you need. It is even
    possible to connect a signal directly to another signal. (This will
    emit the second signal immediately whenever the first is emitted.)

    Together, signals and slots make up a powerful component programming
    mechanism.

    \section1 A Small Example

    A minimal C++ class declaration might read:

    \snippet doc/src/snippets/signalsandslots/signalsandslots.h 0

    A small QObject-based class might read:

    \snippet doc/src/snippets/signalsandslots/signalsandslots.h 1
    \codeline
    \snippet doc/src/snippets/signalsandslots/signalsandslots.h 2
    \snippet doc/src/snippets/signalsandslots/signalsandslots.h 3

    The QObject-based version has the same internal state, and provides
    public methods to access the state, but in addition it has support
    for component programming using signals and slots. This class can
    tell the outside world that its state has changed by emitting a
    signal, \c{valueChanged()}, and it has a slot which other objects
    can send signals to.

    All classes that contain signals or slots must mention
    Q_OBJECT at the top of their declaration. They must also derive
    (directly or indirectly) from QObject.

    Slots are implemented by the application programmer.
    Here is a possible implementation of the \c{Counter::setValue()}
    slot:

    \snippet doc/src/snippets/signalsandslots/signalsandslots.cpp 0

    The \c{emit} line emits the signal \c valueChanged() from the
    object, with the new value as argument.

    In the following code snippet, we create two \c Counter objects
    and connect the first object's \c valueChanged() signal to the
    second object's \c setValue() slot using QObject::connect():

    \snippet doc/src/snippets/signalsandslots/signalsandslots.cpp 1
    \snippet doc/src/snippets/signalsandslots/signalsandslots.cpp 2
    \codeline
    \snippet doc/src/snippets/signalsandslots/signalsandslots.cpp 3
    \snippet doc/src/snippets/signalsandslots/signalsandslots.cpp 4

    Calling \c{a.setValue(12)} makes \c{a} emit a
    \c{valueChanged(12)} signal, which \c{b} will receive in its
    \c{setValue()} slot, i.e. \c{b.setValue(12)} is called. Then
    \c{b} emits the same \c{valueChanged()} signal, but since no slot
    has been connected to \c{b}'s \c{valueChanged()} signal, the
    signal is ignored.

    Note that the \c{setValue()} function sets the value and emits
    the signal only if \c{value != m_value}. This prevents infinite
    looping in the case of cyclic connections (e.g., if
    \c{b.valueChanged()} were connected to \c{a.setValue()}).

    A signal is emitted for every connection you make; if you
    duplicate a connection, two signals will be emitted. You can
    always break a connection using QObject::disconnect().

    This example illustrates that objects can work together without needing to
    know any information about each other. To enable this, the objects only
    need to be connected together, and this can be achieved with some simple
    QObject::connect() function calls, or with \c{uic}'s
    \l{Using a Designer .ui File in Your Application#Automatic Connections}
    {automatic connections} feature.

    \section1 Building the Example

    The C++ preprocessor changes or removes the \c{signals},
    \c{slots}, and \c{emit} keywords so that the compiler is
    presented with standard C++.

    By running the \l moc on class definitions that contain signals
    or slots, a C++ source file is produced which should be compiled
    and linked with the other object files for the application. If
    you use \l qmake, the makefile rules to automatically invoke \c
    moc will be added to your project's makefile.

    \section1 Signals

    Signals are emitted by an object when its internal state has changed
    in some way that might be interesting to the object's client or owner.
    Only the class that defines a signal and its subclasses can emit the
    signal.

    When a signal is emitted, the slots connected to it are usually
    executed immediately, just like a normal function call. When this
    happens, the signals and slots mechanism is totally independent of
    any GUI event loop. Execution of the code following the \c emit
    statement will occur once all slots have returned. The situation is
    slightly different when using \l{Qt::ConnectionType}{queued
    connections}; in such a case, the code following the \c emit keyword
    will continue immediately, and the slots will be executed later.

    If several slots are connected to one signal, the slots will be
    executed one after the other, in an arbitrary order, when the signal
    is emitted.

    Signals are automatically generated by the \l moc and must not be