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• PointHandler QML Type

PointHandler

Handler for reacting to a single touchpoint. More...

• List of all members, including inherited members

Properties

• acceptedButtons : int
• acceptedDevices : int
• acceptedModifiers : int
• acceptedPointerTypes : int
• active : bool
• enabled : bool
• grabPermission : bool
• parent : Item
• point : HandlerPoint
• target : Item

Signals

• canceled(EventPoint point)
• grabChanged(EventPoint point)

Detailed Description

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 SinglePointHandler::acceptedButtons, PointerDeviceHandler::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 2.10
import QtQuick.Window 2.2
import Qt.labs.handlers 1.0

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 pointer 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 the trackpad by default. 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.

Property Documentation

Item {
    TapHandler {
        onTapped: console.log("left clicked")
    }
    TapHandler {
        acceptedButtons: Qt.RightButton
        onTapped: console.log("right clicked")
    }
}

Item {
   TapHandler {
       acceptedDevices: PointerDevice.Mouse | PointerDevice.Stylus
       onTapped: console.log("clicked")
   }
   TapHandler {
       acceptedDevices: PointerDevice.TouchScreen
       onTapped: console.log("tapped")
   }
}

Item {
   TapHandler {
       acceptedModifiers: Qt.ControlModifier
       onTapped: console.log("control-tapped")
   }
   TapHandler {
       acceptedModifiers: Qt.NoModifier
       onTapped: console.log("tapped")
   }
}

Rectangle {
   id: rect
   TapHandler {
       acceptedPointerTypes: PointerDevice.GenericPointer | PointerDevice.Finger | PointerDevice.Pen
       onTapped: console.log("clicked")
   }
   TapHandler {
       acceptedPointerTypes: PointerDevice.Eraser
       onTapped: rect.destroy()
   }
}

Signal Documentation