path: root/doc/src/signalsandslots.qdoc

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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
    implemented in the \c .cpp file. They can never have return types
    (i.e. use \c void).

    A note about arguments: Our experience shows that signals and slots
    are more reusable if they do not use special types. If
    QScrollBar::valueChanged() were to use a special type such as the
    hypothetical QScrollBar::Range, it could only be connected to
    slots designed specifically for QScrollBar. Connecting different
    input widgets together would be impossible.

    \section1 Slots

    A slot is called when a signal connected to it is emitted. Slots are
    normal C++ functions and can be called normally; their only special
    feature is that signals can be connected to them.

    Since slots are normal member functions, they follow the normal C++
    rules when called directly. However, as slots, they can be invoked
    by any component, regardless of its access level, via a signal-slot
    connection. This means that a signal emitted from an instance of an
    arbitrary class can cause a private slot to be invoked in an instance
    of an unrelated class.

    You can also define slots to be virtual, which we have found quite
    useful in practice.

    Compared to callbacks, signals and slots are slightly slower
    because of the increased flexibility they provide, although the
    difference for real applications is insignificant. In general,
    emitting a signal that is connected to some slots, is
    approximately ten times slower than calling the receivers
    directly, with non-virtual function calls. This is the overhead
    required to locate the connection object, to safely iterate over
    all connections (i.e. checking that subsequent receivers have not
    been destroyed during the emission), and to marshall any
    parameters in a generic fashion. While ten non-virtual function
    calls may sound like a lot, it's much less overhead than any \c
    new or \c delete operation, for example. As soon as you perform a
    string, vector or list operation that behind the scene requires
    \c new or \c delete, the signals and slots overhead is only
    responsible for a very small proportion of the complete function
    call costs.

    The same is true whenever you do a system call in a slot; or
    indirectly call more than ten functions. On an i586-500, you can
    emit around 2,000,000 signals per second connected to one
    receiver, or around 1,200,000 per second connected to two
    receivers. The simplicity and flexibility of the signals and
    slots mechanism is well worth the overhead, which your users
    won't even notice.

    Note that other libraries that define variables called \c signals
    or \c slots may cause compiler warnings and errors when compiled
    alongside a Qt-based application. To solve this problem, \c
    #undef the offending preprocessor symbol.

    \section1 Meta-Object Information

    The meta-object compiler (\l moc) parses the class declaration in
    a C++ file and generates C++ code that initializes the
    meta-object. The meta-object contains the names of all the signal
    and slot members, as well as pointers to these functions.

    The meta-object contains additional information such as the
    object's \link QObject::className() class name\endlink. You can
    also check if an object \link QObject::inherits()
    inherits\endlink a specific class, for example:

    \snippet doc/src/snippets/signalsandslots/signalsandslots.cpp 5
    \snippet doc/src/snippets/signalsandslots/signalsandslots.cpp 6

    The meta-object information is also used by qobject_cast<T>(), which
    is similar to QObject::inherits() but is less error-prone:

    \snippet doc/src/snippets/signalsandslots/signalsandslots.cpp 7

    See \l{Meta-Object System} for more information.

    \section1 A Real Example

    Here is a simple commented example of a widget.

    \snippet doc/src/snippets/signalsandslots/lcdnumber.h 0
    \snippet doc/src/snippets/signalsandslots/lcdnumber.h 1
    \codeline
    \snippet doc/src/snippets/signalsandslots/lcdnumber.h 2
    \codeline
    \snippet doc/src/snippets/signalsandslots/lcdnumber.h 3
    \snippet doc/src/snippets/signalsandslots/lcdnumber.h 4
    \snippet doc/src/snippets/signalsandslots/lcdnumber.h 5

    \c LcdNumber inherits QObject, which has most of the signal-slot
    knowledge, via QFrame and QWidget. It is somewhat similar to the
    built-in QLCDNumber widget.

    The Q_OBJECT macro is expanded by the preprocessor to declare
    several member functions that are implemented by the \c{moc}; if
    you get compiler errors along the lines of "undefined reference
    to vtable for \c{LcdNumber}", you have probably forgotten to
    \l{moc}{run the moc} or to include the moc output in the link
    command.

    \snippet doc/src/snippets/signalsandslots/lcdnumber.h 6
    \snippet doc/src/snippets/signalsandslots/lcdnumber.h 7

    It's not obviously relevant to the moc, but if you inherit
    QWidget you almost certainly want to have the \c parent argument
    in your constructor and pass it to the base class's constructor.

    Some destructors and member functions are omitted here; the \c
    moc ignores member functions.

    \snippet doc/src/snippets/signalsandslots/lcdnumber.h 8
    \snippet doc/src/snippets/signalsandslots/lcdnumber.h 9

    \c LcdNumber emits a signal when it is asked to show an impossible
    value.

    If you don't care about overflow, or you know that overflow
    cannot occur, you can ignore the \c overflow() signal, i.e. don't
    connect it to any slot.

    If on the other hand you want to call two different error
    functions when the number overflows, simply connect the signal to
    two different slots. Qt will call both (in arbitrary order).

    \snippet doc/src/snippets/signalsandslots/lcdnumber.h 10
    \snippet doc/src/snippets/signalsandslots/lcdnumber.h 11
    \snippet doc/src/snippets/signalsandslots/lcdnumber.h 12
    \codeline
    \snippet doc/src/snippets/signalsandslots/lcdnumber.h 13

    A slot is a receiving function used to get information about
    state changes in other widgets. \c LcdNumber uses it, as the code
    above indicates, to set the displayed number. Since \c{display()}
    is part of the class's interface with the rest of the program,
    the slot is public.

    Several of the example programs connect the
    \l{QScrollBar::valueChanged()}{valueChanged()} signal of a
    QScrollBar to the \c display() slot, so the LCD number
    continuously shows the value of the scroll bar.

    Note that \c display() is overloaded; Qt will select the
    appropriate version when you connect a signal to the slot. With
    callbacks, you'd have to find five different names and keep track
    of the types yourself.

    Some irrelevant member functions have been omitted from this
    example.

    \section1 Advanced Signals and Slots Usage

    For cases where you may require information on the sender of the
    signal, Qt provides the QObject::sender() function, which returns
    a pointer to the object that sent the signal.

    The QSignalMapper class is provided for situations where many
    signals are connected to the same slot and the slot needs to
    handle each signal differently.

    Suppose you have three push buttons that determine which file you
    will open: "Tax File", "Accounts File", or "Report File". 

    In order to open the correct file, you use QSignalMapper::setMapping() to
    map all the clicked() signals to a QSignalMapper object. Then you connect
    the file's QPushButton::clicked() signal to the QSignalMapper::map() slot.

    \snippet doc/src/snippets/signalmapper/filereader.cpp 0

    Then, you connect the \l{QSignalMapper::}{mapped()} signal to
    \c{readFile()} where a different file will be opened, depending on
    which push button is pressed.

    \snippet doc/src/snippets/signalmapper/filereader.cpp 1

    \sa {Meta-Object System}, {Qt's Property System}

    \target 3rd Party Signals and Slots
    \section2 Using Qt with 3rd Party Signals and Slots

    It is possible to use Qt with a 3rd party signal/slot mechanism.
    You can even use both mechanisms in the same project. Just add the
    following line to your qmake project (.pro) file.

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

    It tells Qt not to define the moc keywords \c{signals}, \c{slots},
    and \c{emit}, because these names will be used by a 3rd party
    library, e.g. Boost. Then to continue using Qt signals and slots
    with the \c{no_keywords} flag, simply replace all uses of the Qt
    moc keywords in your sources with the corresponding Qt macros
    Q_SIGNALS (or Q_SIGNAL), Q_SLOTS (or Q_SLOT), and Q_EMIT.
*/