PointHandler QML Type
Handler for reacting to a single touchpoint. More...
Properties
- acceptedButtons : flags
- acceptedDevices : flags
- acceptedModifiers : flags
- acceptedPointerTypes : flags
- active : bool
- dragThreshold : flags
- margin : real
- target : real
Detailed Description
\inherits
SinglePointHandler \inqmlmodule
QtQuick \ingroup
qtquick-input-handlers
PointHandler can be used to show feedback about a touchpoint or the mouse position, or to otherwise react to pointer events.
When a press event occurs, each instance of PointHandler chooses a single point which is not yet "taken" at that moment: if the press occurs within the bounds of the PointerHandler::parent, and no sibling PointHandler within the same PointerHandler::parent has yet acquired a passive grab on that point, and if the other constraints such as acceptedButtons, acceptedDevices etc. are satisfied, it's eligible, and the PointHandler then acquires a passive grab. In this way, the PointerHandler::parent acts like an exclusive group: there can be multiple instances of PointHandler, and the set of pressed touchpoints will be distributed among them. Each PointHandler which has chosen a point to track has its active property true
. It then continues to track its chosen point until release: the properties of the point will be kept up-to-date. Any Item can bind to these properties, and thereby follow the point's movements.
By being only a passive grabber, it has the ability to keep independent oversight of all movements. The passive grab cannot be stolen or overridden even when other gestures are detected and exclusive grabs occur.
If your goal is orthogonal surveillance of eventpoints, an older alternative was QObject::installEventFilter(), but that has never been a built-in QtQuick feature: it requires some C++ code, such as a QQuickItem subclass. PointHandler is more efficient than that, because only pointer events will be delivered to it, during the course of normal event delivery in QQuickWindow; whereas an event filter needs to filter all QEvents of all types, and thus sets itself up as a potential event delivery bottleneck.
One possible use case is to add this handler to a transparent Item which is on top of the rest of the scene (by having a high z value), so that when a point is freshly pressed, it will be delivered to that Item and its handlers first, providing the opportunity to take the passive grab as early as possible. Such an item (like a pane of glass over the whole UI) can be a convenient parent for other Items which visualize the kind of reactive feedback which must always be on top; and likewise it can be the parent for popups, popovers, dialogs and so on. If it will be used in that way, it can be helpful for your main.cpp to use QQmlContext::setContextProperty() to make the "glass pane" accessible by ID to the entire UI, so that other Items and PointHandlers can be reparented to it.
import QtQuick Window { width: 480 height: 320 visible: true Item { id: glassPane z: 10000 anchors.fill: parent PointHandler { id: handler acceptedDevices: PointerDevice.TouchScreen | PointerDevice.TouchPad target: Rectangle { parent: glassPane color: "red" visible: handler.active x: handler.point.position.x - width / 2 y: handler.point.position.y - height / 2 width: 20; height: width; radius: width / 2 } } } }
Like all input handlers, a PointHandler has a target property, which may be used as a convenient place to put a point-tracking Item; but PointHandler will not automatically manipulate the target
item in any way. You need to use bindings to make it react to the point.
Note: On macOS, PointHandler does not react to multiple fingers on the trackpad by default, although it does react to a pressed point (mouse position). That is because macOS can provide either native gesture recognition, or raw touchpoints, but not both. We prefer to use the native gesture event in PinchHandler, so we do not want to disable it by enabling touch. However MultiPointTouchArea does enable touch, thus disabling native gesture recognition within the entire window; so it's an alternative if you only want to react to all the touchpoints but do not require the smooth native-gesture experience.
See also MultiPointTouchArea, HoverHandler, and Qt Quick Examples - Pointer Handlers.
Property Documentation
acceptedButtons : flags |
The mouse buttons that can activate this PointHandler.
By default, this property is set to Qt.LeftButton. It can be set to an OR combination of mouse buttons, and will ignore events in which other buttons are pressed or held.
import QtQuick Item { width: 480; height: 320 Rectangle { color: handler.active ? "tomato" : "wheat" x: handler.point.position.x - width / 2 y: handler.point.position.y - height / 2 width: 20; height: width; radius: width / 2 } PointHandler { id: handler acceptedButtons: Qt.MiddleButton | Qt.RightButton } }
Note: On a touchscreen, there are no buttons, so this property does not prevent PointHandler from reacting to touchpoints.
acceptedDevices : flags |
The types of pointing devices that can activate this PointHandler.
By default, this property is set to PointerDevice.AllDevices. If you set it to an OR combination of device types, it will ignore events from non-matching devices:
PointHandler { id: handler acceptedDevices: PointerDevice.TouchScreen | PointerDevice.TouchPad target: Rectangle { parent: glassPane color: "red" visible: handler.active x: handler.point.position.x - width / 2 y: handler.point.position.y - height / 2 width: 20; height: width; radius: width / 2 } }
acceptedModifiers : flags |
If this property is set, PointHandler requires the given keyboard modifiers to be pressed in order to react to PointerEvents, and otherwise ignores them.
If this property is set to Qt.KeyboardModifierMask
(the default value), then PointHandler ignores the modifier keys.
For example, an Item could have two handlers, one of which is enabled only if the required keyboard modifier is pressed:
import QtQuick Item { id: feedbackPane width: 480; height: 320 PointHandler { id: control acceptedModifiers: Qt.ControlModifier cursorShape: Qt.PointingHandCursor target: Rectangle { parent: feedbackPane color: control.active ? "indianred" : "khaki" x: control.point.position.x - width / 2 y: control.point.position.y - height / 2 width: 20; height: width; radius: width / 2 } } PointHandler { id: shift acceptedModifiers: Qt.ShiftModifier | Qt.MetaModifier cursorShape: Qt.CrossCursor target: Rectangle { parent: feedbackPane color: shift.active ? "darkslateblue" : "lightseagreen" x: shift.point.position.x - width / 2 y: shift.point.position.y - height / 2 width: 30; height: width; radius: width / 2 } } }
If you set acceptedModifiers
to an OR combination of modifier keys, it means all of those modifiers must be pressed to activate the handler.
The available modifiers are as follows:
Constant | Description |
---|---|
NoModifier | No modifier key is allowed. |
ShiftModifier | A Shift key on the keyboard must be pressed. |
ControlModifier | A Ctrl key on the keyboard must be pressed. |
AltModifier | An Alt key on the keyboard must be pressed. |
MetaModifier | A Meta key on the keyboard must be pressed. |
KeypadModifier | A keypad button must be pressed. |
GroupSwitchModifier | X11 only (unless activated on Windows by a command line argument). A Mode_switch key on the keyboard must be pressed. |
KeyboardModifierMask | The handler does not care which modifiers are pressed. |
See also Qt::KeyboardModifier.
acceptedPointerTypes : flags |
The types of pointing instruments (finger, stylus, eraser, etc.) that can activate this PointHandler.
By default, this property is set to PointerDevice.AllPointerTypes. If you set it to an OR combination of device types, it will ignore events from non-matching devices:
import QtQuick Canvas { id: canvas width: 800 height: 600 antialiasing: true renderTarget: Canvas.FramebufferObject property var points: [] onPaint: { if (points.length < 2) return var ctx = canvas.getContext('2d'); ctx.save() ctx.strokeStyle = stylusHandler.active ? "blue" : "white" ctx.lineCap = "round" ctx.beginPath() ctx.moveTo(points[0].x, points[0].y) for (var i = 1; i < points.length; i++) ctx.lineTo(points[i].x, points[i].y) ctx.lineWidth = 3 ctx.stroke() points = points.slice(points.length - 2, 1) ctx.restore() } PointHandler { id: stylusHandler acceptedPointerTypes: PointerDevice.Pen onPointChanged: { canvas.points.push(point.position) canvas.requestPaint() } } PointHandler { id: eraserHandler acceptedPointerTypes: PointerDevice.Eraser onPointChanged: { canvas.points.push(point.position) canvas.requestPaint() } } Rectangle { width: 10; height: 10 color: stylusHandler.active ? "green" : eraserHandler.active ? "red" : "beige" } }
The Qt Quick Examples - Pointer Handlers includes a more complex example for drawing on a Canvas with a graphics tablet.
[read-only] active : bool |
\readonly
This holds true
whenever the constraints are satisfied and this PointHandler is reacting. This means that it is keeping its properties up-to-date according to the movements of the eventPoints that satisfy the constraints.
dragThreshold : flags |
\internal
This property is not used in PointHandler.
margin : real |
The margin beyond the bounds of the parent item within which an eventPoint can activate this handler.
The default value is 0
.
import QtQuick Item { width: 480; height: 320 Rectangle { anchors.fill: handlingContainer anchors.margins: -handler.margin color: "beige" } Rectangle { id: handlingContainer width: 200; height: 200 anchors.centerIn: parent border.color: "green" color: handler.active ? "lightsteelblue" : "khaki" Text { text: "X" x: handler.point.position.x - width / 2 y: handler.point.position.y - height / 2 visible: handler.active } PointHandler { id: handler margin: 30 } } }
target : real |
A property that can conveniently hold an Item to be manipulated or to show feedback. Unlike other Pointer Handlers, PointHandler does not do anything with the target
on its own: you usually need to create reactive bindings to properties such as SinglePointHandler::point and PointHandler::active. If you declare an Item instance here, you need to explicitly set its parent, because PointHandler is not an Item.
By default, it is the same as the parent, the Item within which the handler is declared.