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/*!
\page qmlevents.html
\ingroup qml-features

\title QML Signal and Handler Event System
\brief the event sytem in QML

Application and user interface components communicate with each other. For
example, a button component needs to know that the user is clicking on it.
The button may change colors to indicate its state or perform some logic. As
well, application needs to know whether the user is clicking the button. The
application may need to relay this clicking event to other applications.

QML has a signal and handler mechanism, where the \e signal is the event
and the component responds to the event through the \e handler. The signal
is emitted and the handler is invoked. Placing logic such as scripts or other
operations in the handler allows the component to respond to the event.

\keyword qml-signals-and-handlers
\section1 Signals and Handlers

Signals provide a way to notify other objects when an event has occurred. For
example, the MouseArea \c clicked signal notifies other objects that the mouse
has been clicked within the area.

The syntax for defining a new signal is:

\tt{signal <name>[([<type> <parameter name>[, ...]])]}

Attempting to declare two signals or methods with the same name in the same type
block generates an error.  However, a new signal may reuse the name of an existing signal on the type. (This should be done with caution, as the existing signal may be hidden and become inaccessible.)

Here are various examples of signal declarations:
\snippet doc/src/snippets/qml/events.qml parent begin
\snippet doc/src/snippets/qml/events.qml signal declaration
\snippet doc/src/snippets/qml/events.qml parent end

If the signal has no parameters, the "\c{()}" brackets are optional. If
parameters are used, the parameter types must be declared, as for the \c string
and \c variant arguments of the \c perform signal.

Adding a signal to an item automatically adds a \e{signal handler} as well. The
signal hander is named \c on<SignalName>, with the first letter of the signal in
uppercase.  The previous signals have the following signal handlers:
\snippet doc/src/snippets/qml/events.qml signal handler declaration

Further, each QML properties have a \c{<property_name>Changed} signal and its
corresponding \c{on<property_name>Changed} signal handler. As a result, property
changes may notify other components for any changes.
\snippet doc/src/snippets/qml/events.qml automatic signals

To emit a signal, invoke it as a method. The signal handler binding is similar
to a property binding and it is invoked when the signal is emitted. Use the
defined argument names to access the respective arguments.
\snippet doc/src/snippets/qml/events.qml signal emit
Note that the \c Component.onCompleted is an
\l{attached-signalhandlers}{attached signal handler}; it is invoked when the
\l Component initialization is complete.

\keyword qml-connect-signals-to-method
\section2 Connecting Signals to Methods and Signals

Signal objects have a \c connect() method to a connect a signal either to a
method or another signal. When a signal is connected to a method, the method is
automatically invoked whenever the signal is emitted. This mechanism enables a
signal to be received by a method instead of a
\l {Signal Handlers}{signal handler}.

\snippet doc/src/snippets/qml/events.qml connect method
The \c {connect()} method is appropriate when connecting a JavaScript method to
a signal.

There is a corresponding \c disconnect() method for removing connected
signals.

\section3 Signal to Signal Connect

By connecting signals to other signals, the \c connect() method can form different
signal chains.
\snippet doc/src/snippets/qml/events.qml forward signal


Whenever the \l MouseArea \c clicked signal is emitted, the \c send
signal will automatically be emitted as well.

\code
output:
    MouseArea clicked
    Send clicked
\endcode

\section1 Events from the Declarative Runtime

There maybe cases where a signal comes from the \l{The QML Engine}{declarative
runtime}. For example, it is possible to receive events from \l{QML Plugins}{QML
plugins}. For more signal control, the \c connect() method and the \l
Connections element may connect a signal from the runtime to another signal or
method.

For complete information on events from the runtime or creating signals from the
runtime, read the \l{The QML Engine} and the \l{Creating QML Types} articles.

*/