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/****************************************************************************
**
** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
** Contact: Nokia Corporation (qt-info@nokia.com)
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:LGPL$
** No Commercial Usage
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** additional rights. These rights are described in the Nokia Qt LGPL
** Exception version 1.0, included in the file LGPL_EXCEPTION.txt in this
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**
** GNU General Public License Usage
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****************************************************************************/
/*!
\page eventsandfilters.html
\title Events and Event Filters
\ingroup architecture
\brief A guide to event handling in Qt.
In Qt, events are objects, derived from the abstract QEvent class,
that represent things that have happened either within an application
or as a result of outside activity that the application needs to know
about. Events can be received and handled by any instance of a
QObject subclass, but they are especially relevant to widgets. This
document describes how events are delivered and handled in a typical
application.
\tableofcontents
\section1 How Events are Delivered
When an event occurs, Qt creates an event object to represent it by
constructing an instance of the appropriate QEvent subclass, and
delivers it to a particular instance of QObject (or one of its
subclasses) by calling its \l{QObject::}{event()} function.
This function does not handle the event itself; based on the type
of event delivered, it calls an event handler for that specific
type of event, and sends a response based on whether the event
was accepted or ignored.
\omit
Event delivery means that an
event has occurred, the QEvent indicates precisely what, and the
QObject needs to respond. Most events are specific to QWidget and its
subclasses, but there are important events that aren't related to
graphics (e.g., \l{QTimer}{timer events}).
\endomit
Some events, such as QMouseEvent and QKeyEvent, come from the
window system; some, such as QTimerEvent, come from other sources;
some come from the application itself.
\section1 Event Types
Most events types have special classes, notably QResizeEvent,
QPaintEvent, QMouseEvent, QKeyEvent, and QCloseEvent. Each class
subclasses QEvent and adds event-specific functions. For example,
QResizeEvent adds \l{QResizeEvent::}{size()} and
\l{QResizeEvent::}{oldSize()} to enable widgets to discover how
their dimensions have been changed.
Some classes support more than one actual event type. QMouseEvent
supports mouse button presses, double-clicks, moves, and other
related operations.
Each event has an associated type, defined in QEvent::Type, and this
can be used as a convenient source of run-time type information to
quickly determine which subclass a given event object was constructed
from.
Since programs need to react in varied and complex ways, Qt's
event delivery mechanisms are flexible. The documentation for
QCoreApplication::notify() concisely tells the whole story; the
\e{Qt Quarterly} article
\l{http://doc.trolltech.com/qq/qq11-events.html}{Another Look at Events}
rehashes it less concisely. Here we will explain enough for 95%
of applications.
\section1 Event Handlers
The normal way for an event to be delivered is by calling a virtual
function. For example, QPaintEvent is delivered by calling
QWidget::paintEvent(). This virtual function is responsible for
reacting appropriately, normally by repainting the widget. If you
do not perform all the necessary work in your implementation of the
virtual function, you may need to call the base class's implementation.
For example, the following code handles left mouse button clicks on
a custom checkbox widget while passing all other button clicks to the
base QCheckBox class:
\snippet doc/src/snippets/events/events.cpp 0
If you want to replace the base class's function, you must
implement everything yourself. However, if you only want to extend
the base class's functionality, then you implement what you want and
call the base class to obtain the default behavior for any cases you
do not want to handle.
Occasionally, there isn't such an event-specific function, or the
event-specific function isn't sufficient. The most common example
involves \key Tab key presses. Normally, QWidget intercepts these to
move the keyboard focus, but a few widgets need the \key{Tab} key for
themselves.
These objects can reimplement QObject::event(), the general event
handler, and either do their event handling before or after the usual
handling, or they can replace the function completely. A very unusual
widget that both interprets \key Tab and has an application-specific
custom event might contain the following \l{QObject::event()}{event()}
function:
\snippet doc/src/snippets/events/events.cpp 1
Note that QWidget::event() is still called for all of the cases not
handled, and that the return value indicates whether an event was
dealt with; a \c true value prevents the event from being sent on
to other objects.
\section1 Event Filters
Sometimes an object needs to look at, and possibly intercept, the
events that are delivered to another object. For example, dialogs
commonly want to filter key presses for some widgets; for example,
to modify \key{Return}-key handling.
The QObject::installEventFilter() function enables this by setting
up an \e{event filter}, causing a nominated filter object to receive
the events for a target object in its QObject::eventFilter()
function. An event filter gets to process events before the target
object does, allowing it to inspect and discard the events as
required. An existing event filter can be removed using the
QObject::removeEventFilter() function.
When the filter object's \l{QObject::}{eventFilter()} implementation
is called, it can accept or reject the event, and allow or deny
further processing of the event. If all the event filters allow
further processing of an event (by each returning \c false), the event
is sent to the target object itself. If one of them stops processing
(by returning \c true), the target and any later event filters do not
get to see the event at all.
\snippet doc/src/snippets/eventfilters/filterobject.cpp 0
The above code shows another way to intercept \key{Tab} key press
events sent to a particular target widget. In this case, the filter
handles the relevant events and returns \c true to stop them from
being processed any further. All other events are ignored, and the
filter returns \c false to allow them to be sent on to the target
widget, via any other event filters that are installed on it.
It is also possible to filter \e all events for the entire application,
by installing an event filter on the QApplication or QCoreApplication
object. Such global event filters are called before the object-specific
filters. This is very powerful, but it also slows down event delivery
of every single event in the entire application; the other techniques
discussed should generally be used instead.
\section1 Sending Events
Many applications want to create and send their own events. You can
send events in exactly the same ways as Qt's own event loop by
constructing suitable event objects and sending them with
QCoreApplication::sendEvent() and QCoreApplication::postEvent().
\l{QCoreApplication::}{sendEvent()} processes the event immediately.
When it returns, the event filters and/or the object itself have
already processed the event. For many event classes there is a function
called isAccepted() that tells you whether the event was accepted
or rejected by the last handler that was called.
\l{QCoreApplication::}{postEvent()} posts the event on a queue for
later dispatch. The next time Qt's main event loop runs, it dispatches
all posted events, with some optimization. For example, if there are
several resize events, they are are compressed into one. The same
applies to paint events: QWidget::update() calls
\l{QCoreApplication::}{postEvent()}, which eliminates flickering and
increases speed by avoiding multiple repaints.
\l{QCoreApplication::}{postEvent()} is also used during object
initialization, since the posted event will typically be dispatched
very soon after the initialization of the object is complete.
When implementing a widget, it is important to realise that events
can be delivered very early in its lifetime so, in its constructor,
be sure to initialize member variables early on, before there's any
chance that it might receive an event.
To create events of a custom type, you need to define an event
number, which must be greater than QEvent::User, and you may need to
subclass QEvent in order to pass specific information about your
custom event. See the QEvent documentation for further details.
*/
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