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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.

    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



*/