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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 howto
\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 the API
used by classes that represent specific gestures, such as QPanGesture,
QPinchGesture, and QSwipeGesture. These standard classes are ready to use,
and each exposes functions and properties that give gesture-specific
information about the user's input. This is described in the
\l{#Using Standard Gestures}{Using Standard Gestures} section.
QGesture is also designed to be subclassed and extended so that support for
new gestures can be implemented by developers. Adding support for a new
gesture involves implementing code to recognize the gesture from incoming
events. This is described in the
\l{#Creating Your Own Gesture Recognizer}{Creating Your Own Gesture Recognizer}
section.
\section1 Using Standard Gestures with Widgets
Gesture objects are applied directly to widgets and other controls that accept
user input \mdash these are the \e{target objects}. When a gesture object is
constructed, the target object is typically passed to the constructor, though
it can also be passed as the argument to the \l{QGesture::}{setGestureTarget()}
function.
\snippet examples/gestures/imageviewer/imagewidget.cpp construct swipe gesture
In the above code, the gesture is set up in the constructor of the target object
itself, so the argument to the QSwipeGesture constructor is \e this.
When the user performs a gesture, various signals may be emitted by the
gesture object. To monitor the user's actions, you need to connect signals
from the gesture object to slots in your code.
\snippet examples/gestures/imageviewer/imagewidget.cpp connect swipe gesture
Here, the \l{QGesture::}{triggered()} signal is used to inform the application
that a gesture was used. More precise monitoring of a gesture can be implemented
by connecting its \l{QGesture::}{started()}, \l{QGesture::}{canceled()} and
\l{QGesture::}{finished()} signals to slots.
Responding to a signal is simply a matter of obtaining the gesture that sent
it and examining the information it contains.
\snippet examples/gestures/imageviewer/imagewidget.cpp swipe slot start
\snippet examples/gestures/imageviewer/imagewidget.cpp swipe slot finish
Here, we examine the direction in which the user swiped the widget and modify
its contents accordingly.
\section1 Using Standard Gestures with Graphics Items
The approach used for applying gestures to widgets can also be used with
graphics items. However, instead of passing a target object to the
gesture object's constructor, you set a target graphics item with the
\l{QGesture::}{setGraphicsItem()} function.
\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()} to filter out events that are not relevant
to your gesture.
Once you have implemented the \l{QGesture::}{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()} 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 the QGesture documentation.
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()}, 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()}
\row \o Qt::GestureStarted \o A continuous gesture has started \o emit \l{QGesture::}{started()} and emit \l{QGesture::}{triggered()}
\row \o Qt::GestureUpdated \o A gesture continues \o emit \l{QGesture::}{triggered()}
\row \o Qt::GestureFinished \o A gesture has finished. \o emit \l{QGesture::}{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 The ImageViewer Example
To illustrate how to use QGesture we will look at the ImageViewer
example. This example uses QPanGesture, a 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 canceled 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::canceled()}{canceled()}
signal being emitted by the recognizer.
ImageWidget handles the signals by connecting the slots to the signals,
although \c canceled() 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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