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/****************************************************************************
**
** Copyright (C) 2021 The Qt Company Ltd.
** Contact: https://www.qt.io/licensing/
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
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**
** GNU Free Documentation License Usage
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** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of
** this file. Please review the following information to ensure
** the GNU Free Documentation License version 1.3 requirements
** will be met: https://www.gnu.org/licenses/fdl-1.3.html.
** $QT_END_LICENSE$
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****************************************************************************/
/*!
\page qtquickhandlers-index.html
\title Qt Quick Input Handlers
\brief A module with a set of QML elements that handle events from input devices in a user interface.
Qt Quick Input Handlers are a set of QML types used to handle
\l {QInputEvent}{events} from keyboard, touch, mouse, and stylus
\l {QInputDevice}{devices} in a UI. In contrast to event-handling
items, such as \l MouseArea and \l Flickable, input handlers are explicitly
non-visual, require less memory and are intended to be used in greater
numbers: one handler instance per aspect of interaction. Each input handler
instance handles certain events on behalf of its
\l {QQuickPointerHandler::parent()}{parent} Item. Thus the visual and
behavioral concerns are better separated, and the behavior is built up by
finer-grained composition.
The pre-existing \l Keys attached property is similar in concept, so we
refer to the pointing-device-oriented handlers plus \c Keys together as the
set of Input Handlers. We expect to offer more attached-property use cases
in future versions of Qt.
\section1 Input Handlers
\annotatedlist qtquick-input-handlers
\section1 Key Features
Some of the key features are:
\list
\li Handle keystrokes within the focused Item
\li Handle gestures such as tapping or dragging regardless which device it comes from
\li Handle gestures from different classes of devices in different ways
\li Each Item can have unlimited Handlers
\endlist
\section1 Handlers Manipulating Items
Some Handlers add interactivity simply by being declared inside an Item:
\snippet pointerHandlers/dragHandler.qml 0
\section1 Handler Properties and Signals
All Handlers have properties that can be used in bindings, and signals that
can be handled to react to input:
\snippet pointerHandlers/hoverTapKeyButton.qml 0
\section1 Pointer Grab
An important concept with Pointer Handlers is the type of grabs that they
perform. The only kind of grab an Item can take is the exclusive grab: for
example if you call \l QPointerEvent::setExclusiveGrabber(), the following
mouse moves and mouse release event will be sent only to that object. (As a
workaround to this exclusivity, see \l QQuickItem::setFiltersChildMouseEvents()
and \l QQuickItem::childMouseEventFilter().) However Pointer Handlers have
an additional mechanism available: the
\l {QPointerEvent::addPassiveGrabber()} {passive grab}. Mouse and touch
\l {QEventPoint::state()}{press} events are delivered by visiting all the
Items in top-down Z order: first each Item's child Handlers, and then the
\l {QQuickItem::event()}{Item} itself. At the time a press event is
delivered, a Handler can take either a passive or an exclusive grab
depending on its needs. If it takes a passive grab, it is guaranteed to
receive the updates and the release, even if other Items or Handlers in the
scene take any kind of grab, passive or exclusve. Some Handlers (such as
PointHandler) can work only with passive grabs; others require exclusive
grabs; and others can "lurk" with passive grabs until they detect that a
gesture is being performed, and then make the transition from passive to
exclusive grab.
When a grab transition is requested, \l PointerHandler::grabPermissions,
\l QQuickItem::keepMouseGrab() and \l QQuickItem::keepTouchGrab() control
whether the transition will be allowed.
\section1 Related Information
\list
\li \l{Qt Quick}
\endlist
*/
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