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/****************************************************************************
**
** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
** Contact: Nokia Corporation (qt-info@nokia.com)
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****************************************************************************/
/*!
\page gestures-overview.html
\startpage index.html Qt Reference Documentation
\title Gestures Programming
\ingroup howto
\brief An overview of the Qt support for Gesture programming.
The QGesture class provides the ability to form gestures from a series
of events independent of the input method. 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 implementing
developer.
\tableofcontents
\section1 Creating Your Own Gesture Recognizer
QGesture is a base class for a user defined gesture recognizer class. In
order to implement the recognizer you will need to subclass the
QGesture class and implement the pure virtual function \l{QGesture::filterEvent()}{filterEvent()}. Once
you have implemented the \l{QGesture::filterEvent()}{filterEvent()} function to
make your own recognizer you can process events. A sequence of events may,
according to your own rules, represent a gesture. The events can be singly
passed to the recognizer via the \l{QGesture::filterEvent()}{filterEvent()} function or as a stream of
events by specifying a parent source of events. The events can be from any
source and could result in any action as defined by the user. The source
and action need not be graphical though that would be the most likely
scenario. To find how to connect a source of events to automatically feed into the recognizer see QGesture.
Recognizers based on QGesture can emit any of the following signals:
\snippet doc/src/snippets/gestures/qgesture.h qgesture-signals
These signals are emitted when the state changes with the call to
\l{QGesture::updateState()}{updateState()}, more than one signal may
be emitted when a change of state occurs. There are four GestureStates
\table
\header \o New State \o Description \o QGesture Actions on Entering this State
\row \o Qt::NoGesture \o Initial value \o emit \l {QGesture::cancelled()}{cancelled()}
\row \o Qt::GestureStarted \o A continuous gesture has started \o emit \l{QGesture::started()}{started()} and emit \l{QGesture::triggered()}{triggered()}
\row \o Qt::GestureUpdated \o A gesture continues \o emit \l{QGesture::triggered()}{triggered()}
\row \o Qt::GestureFinished \o A gesture has finished. \o emit \l{QGesture::finished()}{finished()}
\endtable
\note \l{QGesture::started()}{started()} can be emitted if entering any
state greater than NoGesture if NoGesture was the previous state. This
means that your state machine does not need to explicitly use the
Qt::GestureStarted state, you can simply proceed from NoGesture to
Qt::GestureUpdated to emit a \l{QGesture::started()}{started()} signal
and a \l{QGesture::triggered()}{triggered()} signal.
You may use some or all of these states when implementing the pure
virtual function \l{QGesture::filterEvent()}{filterEvent()}.
\l{QGesture::filterEvent()}{filterEvent()} will usually implement a
state machine using the GestureState enums, but the details of which
states are used is up to the developer.
You may also need to reimplement the virtual function \l{QGesture::reset()}{reset()}
if internal data or objects need to be re-initialized. The function must
conclude with a call to \l{QGesture::updateState()}{updateState()} to
change the current state to Qt::NoGesture.
\section1 An Example, ImageViewer
To illustrate how to use QGesture we will look at the ImageViewer
example. This example uses QPanGesture, standard gesture, and an
implementation of TapAndHoldGesture. Note that TapAndHoldGesture is
platform dependent.
\snippet doc/src/snippets/gestures/imageviewer/tapandholdgesture.cpp tapandhold-reset
In ImageViewer we see that the ImageWidget class uses two gestures:
\l QPanGesture and TapAndHoldGesture. The
QPanGesture is a standard gesture which is part of Qt.
TapAndHoldGesture is defined and implemented as part of the example.
The ImageWidget listens for signals from the gestures, but is not
interested in the \l{QGesture::started()}{started()} signal.
\snippet doc/src/snippets/gestures/imageviewer/imagewidget.h imagewidget-slots
TapAndHoldGesture uses QTouchEvent events and mouse events to detect
start, update and end events that can be mapped onto the GestureState
changes. The implementation in this case uses a timer as well. If the
timeout event occurs a given number of times after the start of the gesture
then the gesture is considered to have finished whether or not the
appropriate touch or mouse event has occurred. Also if a large jump in
the position of the event occurs, as calculated by the \l {QPoint::manhattanLength()}{manhattanLength()}
call, then the gesture is cancelled by calling \l{QGesture::reset()}{reset()}
which tidies up and uses \l{QGesture::updateState()}{updateState()} to
change state to NoGesture which will result in the \l{QGesture::cancelled()}{cancelled()}
signal being emitted by the recognizer.
ImageWidget handles the signals by connecting the slots to the signals,
although \c cancelled() is not connected here.
\snippet doc/src/snippets/gestures/imageviewer/imagewidget.cpp imagewidget-connect
These functions in turn will have to be aware of which gesture
object was the source of the signal since we have more than one source
per slot. This is easily done by using the QObject::sender() function
as shown here
\snippet doc/src/snippets/gestures/imageviewer/imagewidget.cpp imagewidget-triggered-1
As \l{QGesture::triggered()}{triggered()} signals are handled by
gestureTriggered() there may be position updates invoking calls to,
for example, goNextImage(), this will cause a change in the image
handling logic of ImageWidget and a call to updateImage() to display
the changed state.
Following the logic of how the QEvent is processed we can summmarize
it as follows:
\list
\o filterEvent() becomes the event filter of the parent ImageWidget object for a QPanGesture object and a
TapAndHoldGesture object.
\o filterEvent() then calls updateState() to change states
\o updateState() emits the appropriate signal(s) for the state change.
\o The signals are caught by the defined slots in ImageWidget
\o The widget logic changes and an update() results in a paint event.
\endlist
*/
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