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/****************************************************************************
**
** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
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****************************************************************************/
/*!
\page gestures-overview.html
\startpage index.html Qt Reference Documentation
\title Gestures Programming
\ingroup frameworks-technologies
\brief An overview of the Qt support for Gesture programming.
Qt includes a framework for gesture programming that gives has the ability
to form gestures from a series of events, independently of the input methods
used. A gesture could be a particular movement of a mouse, a touch screen
action, or a series of events from some other source. The nature of the input,
the interpretation of the gesture and the action taken are the choice of the
developer.
\tableofcontents
\section1 Overview
QGesture is the central class in Qt's gesture framework, providing a container
for information about gestures performed by the user, such as panning, pinching
and swiping. QGesture exposes properties that give general information that is
common to all gestures, and these can be extended to provide additional
gesture-specific information.
Developers can also implement new gestures by subclassing and extending the
QGestureRecognizer class. Adding support for a new gesture involves implementing
code to recognize the gesture from input events. This is described in the
\l{Creating Your Own Gesture Recognizer} section.
\section1 Using Standard Gestures with Widgets
Gestures can be enabled for instances of QWidget and QGraphicsObject subclasses.
An object that accepts gesture input is referred to as a \e{target object}.
To enable a gesture for a target object, call its QWidget::grabGesture() or
QGraphicsObject::grabGesture() function with an argument describing the
required gesture type. The standard types are defined by the Qt::GestureType
enum and include many commonly used gestures.
\snippet examples/gestures/imageviewer/imagewidget.cpp enable gestures
In the above code, the gesture is set up in the constructor of the target object
itself.
When the user performs a gesture, QGestureEvent events will be delivered to the
target object, and these can be handled by reimplementing the QWidget::event()
handler function for widgets or QGraphicsItem::sceneEvent() for graphics objects.
For convenience, the \l{Image Gestures Example} reimplements the general
\l{QWidget::}{event()} handler function and delegates gesture events to a
specialized gestureEvent() function:
\snippet examples/gestures/imageviewer/imagewidget.cpp event handler
The gesture events delivered to the target object can be examined individually
and dealt with appropriately:
\snippet examples/gestures/imageviewer/imagewidget.cpp gesture event handler
Responding to a gesture is simply a matter of obtaining the QGesture object
delivered in the QGestureEvent sent to the target object and examining the
information it contains.
\snippet examples/gestures/imageviewer/imagewidget.cpp swipe function
Here, we examine the direction in which the user swiped the widget and modify
its contents accordingly.
\section1 Creating Your Own Gesture Recognizer
Adding support for a new gesture involves creating and registering a new gesture
recognizer. Depending on the recognition process for the gesture, it may also
involve creating a new gesture object.
To create a new recognizer, you need to subclass QGestureRecognizer to create a
custom recognizer class. There is one virtual function that you must reimplement
and two others that can be reimplemented as required.
\section2 Filtering Input Events
The \l{QGestureRecognizer::}{filterEvent()} function must be reimplemented.
This function handles and filters the incoming input events for the target objects
and determines whether or not they correspond to the gesture the recognizer is
looking for.
Although the logic for gesture recognition is implemented in this function,
possibly using a state machine based on the Qt::GestureState enums, you can store
persistent information about the state of the recognition process in the QGesture
object supplied.
Your \l{QGestureRecognizer::}{filterEvent()} function must return a value of
Qt::GestureState that indicates the state of recognition for a given gesture and
target object. This determines whether or not a gesture event will be delivered
to a target object.
\section2 Custom Gestures
If you choose to represent a gesture by a custom QGesture subclass, you will need to
reimplement the \l{QGestureRecognizer::}{createGesture()} function to construct
instances of your gesture class instead of standard QGesture instances. Alternatively,
you may want to use standard QGesture instances, but add additional dynamic properties
to them to express specific details of the gesture you want to handle.
\section2 Resetting Gestures
If you use custom gesture objects that need to be reset or otherwise specially
handled when a gesture is canceled, you need to reimplement the
\l{QGestureRecognizer::}{reset()} function to perform these special tasks.
Note that QGesture objects are only created once for each combination of target object
and gesture type, and they are reused every time the user attempts to perform the
same gesture type on the target object. As a result, it can be useful to reimplement
the \l{QGestureRecognizer::}{reset()} function to clean up after each previous attempt
at recognizing a gesture.
\section1 Using a New Gesture Recognizer
To use a gesture recognizer, construct an instance of your QGestureRecognizer
subclass, and register it with the application with
QApplication::registerGestureRecognizer(). A recognizer for a given type of
gesture can be removed with QApplication::unregisterGestureRecognizer().
\section1 Further Reading
The \l{Image Gestures Example} shows how to enable gestures for a widget in
a simple image viewer application.
*/
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