gestures.qdoc

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// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
/*!
    \page gestures-overview.html
    \title Gestures in Widgets and Graphics View
    \startpage index.html Qt Reference Documentation
    \ingroup qt-gui-concepts
 
    \brief An overview of Qt support for Gesture programming
 
    Qt includes a framework for gesture programming that 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.
 
    \section1 Overview
 
    QGesture is the central class in Qt's gesture framework, providing a container
    for information about gestures performed by the user. QGesture exposes
    properties that give general information that is common to all gestures, and
    these can be extended to provide additional gesture-specific information.
    Common panning, pinching and swiping gestures are represented by specialized
    classes: QPanGesture, QPinchGesture and QSwipeGesture.
 
    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 throughout the documentation
    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 gestures/imagegestures/imagewidget.cpp enable gestures
 
    In the above code, the gestures are set up in the constructor of the target object
    itself.
 
    \section1 Handling Events
 
    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.
 
    As one target object can subscribe to more than one gesture type, the QGestureEvent
    can contain more than one QGesture, indicating several possible gestures are active
    at the same time. It is then up to the widget to determine how to handle those
    multiple gestures and choose if some should be canceled in favor of others.
 
    Each QGesture contained within a QGestureEvent object can be accepted() or ignored()
    individually, or all together. Additionally, you can query the individual QGesture
    data objects (the state) using several getters.
 
    \section2 Standard Procedure for Event Handling
 
    A QGesture is by default accepted when it arrives at your widget. However, it is good
    practice to always explicitly accept or reject a gesture. The general rule is that, if
    you accept a gesture, you are using it. If you are ignoring it you are not interested
    in it. Ignoring a gesture may mean it gets offered to another target object, or it will
    get canceled.
 
    Each QGesture has several states it goes through; there is a well defined way to change
    the state, typically the user input is the cause of state changes (by starting and
    stopping interaction, for instance) but the widget can also cause state changes.
 
    The first time a particular QGesture is delivered to a widget or graphics item, it will
    be in the Qt::GestureStarted state. The way you handle the gesture at this point
    influences whether you can interact with it later.
 
    \list
    \li Accepting the gesture means the widget acts on the gesture and there will follow
       gestures with the Qt::GestureUpdatedstate.
    \li Ignoring the gesture will mean the gesture will never be offered to you again.
       It will be offered to a parent widget or item as well.
    \li Calling setGestureCancelPolicy() on the gesture when it is in its starting state,
       and is also accepted can cause other gestures to be canceled.
    \endlist
 
    Using QGesture::CancelAllInContext to cancel a gesture will cause all gestures, in any
    state, to be canceled unless they are explicitly accepted. This means that active
    gestures on children will get canceled. It also means that gestures delivered in the
    same QGestureEvent will get canceled if the widget ignores them. This can be a useful
    way to filter out all gestures except the one you are interested in.
 
    \section2 Example Event Handling
 
    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 gestures/imagegestures/imagewidget.cpp event handler
 
    The gesture events delivered to the target object can be examined individually
    and dealt with appropriately:
 
    \snippet gestures/imagegestures/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 gestures/imagegestures/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::}{recognize()} 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::}{recognize()} function must return a value of
    QGestureRecognizer::Result 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::}{create()} 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 might be 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
    QGestureRecognizer::registerRecognizer(). A recognizer for a given type of
    gesture can be removed with QGestureRecognizer::unregisterRecognizer().
 
 
    \section1 Further Reading
 
    The \l{gestures/imagegestures}{Image Gestures Example} shows how to enable
    gestures for a widget in a simple image viewer application.
 
    \section2 Gestures in Qt Quick
 
    Qt Quick does not have a generic global gesture recognizer; rather, individual
    components can respond to touch events in their own ways.  For example
    the \l PinchArea handles two-finger gestures, \l Flickable is for flicking
    content with a single finger, and \l MultiPointTouchArea can handle an
    arbitrary number of touch points and allow the application developer to
    write custom gesture recognition code.
*/
 
// TODO mention Sensor Gestures when qtsensors becomes a maintained module