QTouchEvent

The QTouchEvent class contains parameters that describe a touch event. More

Inheritance diagram of PySide2.QtGui.QTouchEvent

New in version 4.6.

Synopsis

Functions

Detailed Description

Enabling Touch Events

Touch events occur when pressing, releasing, or moving one or more touch points on a touch device (such as a touch-screen or track-pad). To receive touch events, widgets have to have the WA_AcceptTouchEvents attribute set and graphics items need to have the acceptTouchEvents attribute set to true.

When using QAbstractScrollArea based widgets, you should enable the WA_AcceptTouchEvents attribute on the scroll area’s viewport .

Similarly to QMouseEvent , Qt automatically grabs each touch point on the first press inside a widget, and the widget will receive all updates for the touch point until it is released. Note that it is possible for a widget to receive events for numerous touch points, and that multiple widgets may be receiving touch events at the same time.

Event Handling

All touch events are of type TouchBegin , TouchUpdate , TouchEnd or TouchCancel . Reimplement event() or viewportEvent() for widgets and sceneEvent() for items in a graphics view to receive touch events.

Unlike widgets, QWindows receive touch events always, there is no need to opt in. When working directly with a QWindow , it is enough to reimplement touchEvent() .

The TouchUpdate and TouchEnd events are sent to the widget or item that accepted the TouchBegin event. If the TouchBegin event is not accepted and not filtered by an event filter, then no further touch events are sent until the next TouchBegin .

Some systems may send an event of type TouchCancel . Upon receiving this event applications are requested to ignore the entire active touch sequence. For example in a composited system the compositor may decide to treat certain gestures as system-wide gestures. Whenever such a decision is made (the gesture is recognized), the clients will be notified with a TouchCancel event so they can update their state accordingly.

The touchPoints() function returns a list of all touch points contained in the event. Note that this list may be empty, for example in case of a TouchCancel event. Information about each touch point can be retrieved using the TouchPoint class. The TouchPointState enum describes the different states that a touch point may have.

Note

The list of touchPoints() will never be partial: A touch event will always contain a touch point for each existing physical touch contacts targetting the window or widget to which the event is sent. For instance, assuming that all touches target the same window or widget, an event with a condition of touchPoints() .count()==2 is guaranteed to imply that the number of fingers touching the touchscreen or touchpad is exactly two.

Event Delivery and Propagation

By default, QGuiApplication translates the first touch point in a QTouchEvent into a QMouseEvent . This makes it possible to enable touch events on existing widgets that do not normally handle QTouchEvent . See below for information on some special considerations needed when doing this.

TouchBegin is the first touch event sent to a widget. The TouchBegin event contains a special accept flag that indicates whether the receiver wants the event. By default, the event is accepted. You should call ignore() if the touch event is not handled by your widget. The TouchBegin event is propagated up the parent widget chain until a widget accepts it with accept() , or an event filter consumes it. For QGraphicsItems, the TouchBegin event is propagated to items under the mouse (similar to mouse event propagation for QGraphicsItems).

Touch Point Grouping

As mentioned above, it is possible that several widgets can be receiving QTouchEvents at the same time. However, Qt makes sure to never send duplicate TouchBegin events to the same widget, which could theoretically happen during propagation if, for example, the user touched 2 separate widgets in a QGroupBox and both widgets ignored the TouchBegin event.

To avoid this, Qt will group new touch points together using the following rules:

  • When the first touch point is detected, the destination widget is determined firstly by the location on screen and secondly by the propagation rules.

  • When additional touch points are detected, Qt first looks to see if there are any active touch points on any ancestor or descendent of the widget under the new touch point. If there are, the new touch point is grouped with the first, and the new touch point will be sent in a single QTouchEvent to the widget that handled the first touch point. (The widget under the new touch point will not receive an event).

This makes it possible for sibling widgets to handle touch events independently while making sure that the sequence of QTouchEvents is always correct.

Mouse Events and Touch Event Synthesizing

QTouchEvent delivery is independent from that of QMouseEvent . The application flags AA_SynthesizeTouchForUnhandledMouseEvents and AA_SynthesizeMouseForUnhandledTouchEvents can be used to enable or disable automatic synthesizing of touch events to mouse events and mouse events to touch events.

Caveats

  • As mentioned above, enabling touch events means multiple widgets can be receiving touch events simultaneously. Combined with the default event() handling for QTouchEvents, this gives you great flexibility in designing touch user interfaces. Be aware of the implications. For example, it is possible that the user is moving a QSlider with one finger and pressing a QPushButton with another. The signals emitted by these widgets will be interleaved.

  • Recursion into the event loop using one of the exec() methods (e.g., exec() or exec() ) in a QTouchEvent event handler is not supported. Since there are multiple event recipients, recursion may cause problems, including but not limited to lost events and unexpected infinite recursion.

  • QTouchEvents are not affected by a mouse grab or an active pop-up widget . The behavior of QTouchEvents is undefined when opening a pop-up or grabbing the mouse while there are more than one active touch points.

See also

TouchPoint TouchPointState WA_AcceptTouchEvents acceptTouchEvents()

class QTouchEvent(eventType[, device=None[, modifiers=Qt.NoModifier[, touchPointStates=Qt.TouchPointStates()[, touchPoints=list()]]]])
param touchPointStates

TouchPointStates

param modifiers

KeyboardModifiers

param eventType

Type

param device

QTouchDevice

param touchPoints

Constructs a QTouchEvent with the given eventType , device , and touchPoints . The touchPointStates and modifiers are the current touch point states and keyboard modifiers at the time of the event.

PySide2.QtGui.QTouchEvent._target
PySide2.QtGui.QTouchEvent.device()
Return type

QTouchDevice

Returns the touch device from which this touch event originates.

PySide2.QtGui.QTouchEvent.setDevice(adevice)
Parameters

adeviceQTouchDevice

See also

device()

PySide2.QtGui.QTouchEvent.setTarget(atarget)
Parameters

atargetQObject

See also

target()

PySide2.QtGui.QTouchEvent.setTouchPointStates(aTouchPointStates)
Parameters

aTouchPointStatesTouchPointStates

PySide2.QtGui.QTouchEvent.setTouchPoints(atouchPoints)
Parameters

atouchPoints

See also

touchPoints()

PySide2.QtGui.QTouchEvent.setWindow(awindow)
Parameters

awindowQWindow

See also

window()

PySide2.QtGui.QTouchEvent.target()
Return type

QObject

Returns the target object within the window on which the event occurred. This is typically a QWidget or a QQuickItem . May be 0 when no specific target is available.

PySide2.QtGui.QTouchEvent.touchPointStates()
Return type

TouchPointStates

Returns a bitwise OR of all the touch point states for this event.

PySide2.QtGui.QTouchEvent.touchPoints()
Return type

Returns the list of touch points contained in the touch event.

PySide2.QtGui.QTouchEvent.window()
Return type

QWindow

Returns the window on which the event occurred. Useful for doing global-local mapping on data like rawScreenPositions() which, for performance reasons, only stores the global positions in the touch event.