DragHandler QML Type

Handler for dragging. More...

Import Statement: import QtQuick
Inherits:

MultiPointHandler

Properties

Signals

  • canceled(eventPoint point)
  • grabChanged(PointerDevice::GrabTransition transition, eventPoint point)

Detailed Description

DragHandler is a handler that is used to interactively move an Item. Like other Input Handlers, by default it is fully functional, and manipulates its target.

import QtQuick

Rectangle {
    width: 100
    height: 100
    color: "lightsteelblue"
    DragHandler { }
}

It has properties to restrict the range of dragging.

If it is declared within one Item but is assigned a different target, then it handles events within the bounds of the parent Item but manipulates the target Item instead:

import QtQuick

Item {
    width: 640
    height: 480

    Rectangle {
        id: feedback
        border.color: "red"
        width: Math.max(10, handler.centroid.ellipseDiameters.width)
        height: Math.max(10, handler.centroid.ellipseDiameters.height)
        radius: Math.max(width, height) / 2
        visible: handler.active
    }

    DragHandler {
        id: handler
        target: feedback
    }
}

A third way to use it is to set target to null and react to property changes in some other way:

import QtQuick

Item {
    width: 640
    height: 480

    DragHandler {
        id: handler
        target: null
    }

    Text {
        color: handler.active ? "darkgreen" : "black"
        text: handler.centroid.position.x.toFixed(1) + "," + handler.centroid.position.y.toFixed(1)
        x: handler.centroid.position.x - width / 2
        y: handler.centroid.position.y - height
    }
}

If minimumPointCount and maximumPointCount are set to values larger than 1, the user will need to drag that many fingers in the same direction to start dragging. A multi-finger drag gesture can be detected independently of both a (default) single-finger DragHandler and a PinchHandler on the same Item, and thus can be used to adjust some other feature independently of the usual pinch behavior: for example adjust a tilt transformation, or adjust some other numeric value, if the target is set to null. But if the target is an Item, centroid is the point at which the drag begins and to which the target will be moved (subject to constraints).

DragHandler can be used together with the Drag attached property to implement drag-and-drop.

See also Drag, MouseArea, and Qt Quick Examples - Pointer Handlers.

Property Documentation

acceptedButtons : flags

The mouse buttons that can activate this DragHandler.

By default, this property is set to Qt.LeftButton. It can be set to an OR combination of mouse buttons, and will ignore events from other buttons.

For example, if a component (such as TextEdit) already handles left-button drags in its own way, it can be augmented with a DragHandler that does something different when dragged via the right button:

Rectangle {
    id: canvas
    width: 640
    height: 480
    color: "#333"
    property int highestZ: 0

    Repeater {
        model: FolderListModel { nameFilters: ["*.qml"] }

        delegate: Rectangle {
            required property string fileName
            required property url fileUrl
            required property int index

            id: frame
            x: index * 30; y: index * 30
            width: 320; height: 240
            property bool dragging: ldh.active || rdh.active
            onDraggingChanged: if (dragging) z = ++canvas.highestZ
            border { width: 2; color: dragging ? "red" : "steelblue" }
            color: "beige"
            clip: true

            TextEdit {
                // drag to select text
                id: textEdit
                textDocument.source: frame.fileUrl
                x: 3; y: 3

                BoundaryRule on y {
                    id: ybr
                    minimum: textEdit.parent.height - textEdit.height; maximum: 0
                    minimumOvershoot: 200; maximumOvershoot: 200
                    overshootFilter: BoundaryRule.Peak
                }
            }

            DragHandler {
                id: rdh
                // right-drag to position the "window"
                acceptedButtons: Qt.RightButton
            }

            WheelHandler {
                target: textEdit
                property: "y"
                onActiveChanged: if (!active) ybr.returnToBounds()
            }

            Rectangle {
                anchors.right: parent.right
                width: titleText.implicitWidth + 12
                height: titleText.implicitHeight + 6
                border { width: 2; color: parent.border.color }
                bottomLeftRadius: 6
                Text {
                    id: titleText
                    color: "saddlebrown"
                    anchors.centerIn: parent
                    text: frame.fileName
                    textFormat: Text.PlainText
                }
                DragHandler {
                    id: ldh
                    // left-drag to position the "window"
                    target: frame
                }
            }
        }
    }
}

acceptedDevices : flags

The types of pointing devices that can activate this DragHandler.

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.

Note: Not all platforms are yet able to distinguish mouse and touchpad; and on those that do, you often want to make mouse and touchpad behavior the same.


acceptedModifiers : flags

If this property is set, it will require the given keyboard modifiers to be pressed in order to react to pointer events, and otherwise ignore them.

For example, two DragHandlers can perform two different drag-and-drop operations, depending on whether the Control modifier is pressed:

GridView {
    id: root
    width: 320
    height: 480
    cellWidth: 80
    cellHeight: 80
    interactive: false

    displaced: Transition {
        NumberAnimation {
            properties: "x,y"
            easing.type: Easing.OutQuad
        }
    }

    model: DelegateModel {
        id: visualModel
        model: 24
        property var dropTarget: undefined
        property bool copy: false
        delegate: DropArea {
            id: delegateRoot

            width: 80
            height: 80

            onEntered: drag => {
                if (visualModel.copy) {
                    if (drag.source !== icon)
                        visualModel.dropTarget = icon
                } else {
                    visualModel.items.move(drag.source.DelegateModel.itemsIndex, icon.DelegateModel.itemsIndex)
                }
            }

            Rectangle {
                id: icon
                objectName: DelegateModel.itemsIndex

                property string text
                Component.onCompleted: {
                    color = Qt.rgba(0.2 + (48 - DelegateModel.itemsIndex) * Math.random() / 48,
                                    0.3 + DelegateModel.itemsIndex * Math.random() / 48,
                                    0.4 * Math.random(),
                                    1.0)
                    text = DelegateModel.itemsIndex
                }
                border.color: visualModel.dropTarget === this ? "black" : "transparent"
                border.width: 2
                radius: 3
                width: 72
                height: 72
                anchors {
                    horizontalCenter: parent.horizontalCenter
                    verticalCenter: parent.verticalCenter
                }

                states: [
                    State {
                        when: dragHandler.active || controlDragHandler.active
                        ParentChange {
                            target: icon
                            parent: root
                        }

                        AnchorChanges {
                            target: icon
                            anchors {
                                horizontalCenter: undefined
                                verticalCenter: undefined
                            }
                        }
                    }
                ]

                Text {
                    anchors.centerIn: parent
                    color: "white"
                    font.pointSize: 14
                    text: controlDragHandler.active ? "+" : icon.text
                }

                DragHandler {
                    id: dragHandler
                    acceptedModifiers: Qt.NoModifier
                    onActiveChanged: if (!active) visualModel.dropTarget = undefined
                }

                DragHandler {
                    id: controlDragHandler
                    acceptedModifiers: Qt.ControlModifier
                    onActiveChanged: {
                        visualModel.copy = active
                        if (!active) {
                            visualModel.dropTarget.text = icon.text
                            visualModel.dropTarget.color = icon.color
                            visualModel.dropTarget = undefined
                        }
                    }
                }

                Drag.active: dragHandler.active || controlDragHandler.active
                Drag.source: icon
                Drag.hotSpot.x: 36
                Drag.hotSpot.y: 36
            }
        }
    }
}

If this property is set to Qt.KeyboardModifierMask (the default value), then the DragHandler ignores the modifier keys.

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:

ConstantDescription
NoModifierNo modifier key is allowed.
ShiftModifierA Shift key on the keyboard must be pressed.
ControlModifierA Ctrl key on the keyboard must be pressed.
AltModifierAn Alt key on the keyboard must be pressed.
MetaModifierA Meta key on the keyboard must be pressed.
KeypadModifierA keypad button must be pressed.
GroupSwitchModifierX11 only (unless activated on Windows by a command line argument). A Mode_switch key on the keyboard must be pressed.
KeyboardModifierMaskThe 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 DragHandler.

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.


active : bool [read-only]

This holds true whenever this Input Handler has taken sole responsibility for handing one or more eventPoints, by successfully taking an exclusive grab of those points. This means that it is keeping its properties up-to-date according to the movements of those eventPoints and actively manipulating its target (if any).


activeTranslation : QVector2D [read-only]

The translation while the drag gesture is being performed. It is 0, 0 when the gesture begins, and increases as the event point(s) are dragged downward and to the right. After the gesture ends, it stays the same; and when the next drag gesture begins, it is reset to 0, 0 again.


cursorShape : Qt::CursorShape

This property holds the cursor shape that will appear whenever the mouse is hovering over the parent item while active is true.

The available cursor shapes are:

  • Qt.ArrowCursor
  • Qt.UpArrowCursor
  • Qt.CrossCursor
  • Qt.WaitCursor
  • Qt.IBeamCursor
  • Qt.SizeVerCursor
  • Qt.SizeHorCursor
  • Qt.SizeBDiagCursor
  • Qt.SizeFDiagCursor
  • Qt.SizeAllCursor
  • Qt.BlankCursor
  • Qt.SplitVCursor
  • Qt.SplitHCursor
  • Qt.PointingHandCursor
  • Qt.ForbiddenCursor
  • Qt.WhatsThisCursor
  • Qt.BusyCursor
  • Qt.OpenHandCursor
  • Qt.ClosedHandCursor
  • Qt.DragCopyCursor
  • Qt.DragMoveCursor
  • Qt.DragLinkCursor

The default value is not set, which allows the cursor of parent item to appear. This property can be reset to the same initial condition by setting it to undefined.

Note: When this property has not been set, or has been set to undefined, if you read the value it will return Qt.ArrowCursor.

See also Qt::CursorShape, QQuickItem::cursor(), and HoverHandler::cursorShape.


dragThreshold : int

The distance in pixels that the user must drag an eventPoint in order to have it treated as a drag gesture.

The default value depends on the platform and screen resolution. It can be reset back to the default value by setting it to undefined. The behavior when a drag gesture begins varies in different handlers.


enabled : bool

If a PointerHandler is disabled, it will reject all events and no signals will be emitted.


grabPermissions : flags

This property specifies the permissions when this handler's logic decides to take over the exclusive grab, or when it is asked to approve grab takeover or cancellation by another handler.

ConstantDescription
PointerHandler.TakeOverForbiddenThis handler neither takes from nor gives grab permission to any type of Item or Handler.
PointerHandler.CanTakeOverFromHandlersOfSameTypeThis handler can take the exclusive grab from another handler of the same class.
PointerHandler.CanTakeOverFromHandlersOfDifferentTypeThis handler can take the exclusive grab from any kind of handler.
PointerHandler.CanTakeOverFromItemsThis handler can take the exclusive grab from any type of Item.
PointerHandler.CanTakeOverFromAnythingThis handler can take the exclusive grab from any type of Item or Handler.
PointerHandler.ApprovesTakeOverByHandlersOfSameTypeThis handler gives permission for another handler of the same class to take the grab.
PointerHandler.ApprovesTakeOverByHandlersOfDifferentTypeThis handler gives permission for any kind of handler to take the grab.
PointerHandler.ApprovesTakeOverByItemsThis handler gives permission for any kind of Item to take the grab.
PointerHandler.ApprovesCancellationThis handler will allow its grab to be set to null.
PointerHandler.ApprovesTakeOverByAnythingThis handler gives permission for any type of Item or Handler to take the grab.

The default is PointerHandler.CanTakeOverFromItems | PointerHandler.CanTakeOverFromHandlersOfDifferentType | PointerHandler.ApprovesTakeOverByAnything which allows most takeover scenarios but avoids e.g. two PinchHandlers fighting over the same touchpoints.


margin : real

The margin beyond the bounds of the parent item within which an eventPoint can activate this handler. For example, you can make it easier to drag small items by allowing the user to drag from a position nearby:

Rectangle {
    width: 24
    height: 24
    border.color: "steelblue"
    Text {
        text: "it's\ntiny"
        font.pixelSize: 7
        rotation: -45
        anchors.centerIn: parent
    }

    DragHandler {
        margin: 12
    }
}

parent : Item

The Item which is the scope of the handler; the Item in which it was declared. The handler will handle events on behalf of this Item, which means a pointer event is relevant if at least one of its eventPoints occurs within the Item's interior. Initially target() is the same, but it can be reassigned.

See also target and QObject::parent().


persistentTranslation : QVector2D

The translation to be applied to the target if it is not null. Otherwise, bindings can be used to do arbitrary things with this value. While the drag gesture is being performed, activeTranslation is continuously added to it; after the gesture ends, it stays the same.


snapMode : enumeration

This property holds the snap mode.

The snap mode configures snapping of the target item's center to the eventPoint.

Possible values:

ConstantDescription
DragHandler.SnapNeverNever snap
DragHandler.SnapAutoThe target snaps if the eventPoint was pressed outside of the target item and the target is a descendant of parent item (default)
DragHandler.SnapWhenPressedOutsideTargetThe target snaps if the eventPoint was pressed outside of the target
DragHandler.SnapAlwaysAlways snap

target : Item

The Item which this handler will manipulate.

By default, it is the same as the parent, the Item within which the handler is declared. However, it can sometimes be useful to set the target to a different Item, in order to handle events within one item but manipulate another; or to null, to disable the default behavior and do something else instead.


xAxis group

xAxis.activeValue : real [read-only]

xAxis.enabled : bool

xAxis.maximum : real

xAxis.minimum : real

xAxis controls the constraints for horizontal dragging.

minimum is the minimum acceptable value of x to be applied to the target. maximum is the maximum acceptable value of x to be applied to the target. If enabled is true, horizontal dragging is allowed. activeValue is the same as activeTranslation.x.

The activeValueChanged signal is emitted when activeValue changes, to provide the increment by which it changed. This is intended for incrementally adjusting one property via multiple handlers.


yAxis group

yAxis.activeValue : real [read-only]

yAxis.enabled : bool

yAxis.maximum : real

yAxis.minimum : real

yAxis controls the constraints for vertical dragging.

minimum is the minimum acceptable value of y to be applied to the target. maximum is the maximum acceptable value of y to be applied to the target. If enabled is true, vertical dragging is allowed. activeValue is the same as activeTranslation.y.

The activeValueChanged signal is emitted when activeValue changes, to provide the increment by which it changed. This is intended for incrementally adjusting one property via multiple handlers:

import QtQuick

Rectangle {
    width: 50; height: 200

    Rectangle {
        id: knob
        width: parent.width; height: width; radius: width / 2
        anchors.centerIn: parent
        color: "lightsteelblue"

        Rectangle {
            antialiasing: true
            width: 4; height: 20
            x: parent.width / 2 - 2
        }

        WheelHandler {
            property: "rotation"
        }
    }

    DragHandler {
        target: null
        dragThreshold: 0
        yAxis.onActiveValueChanged: (delta)=> { knob.rotation -= delta }
    }
}

Signal Documentation

canceled(eventPoint point)

If this handler has already grabbed the given point, this signal is emitted when the grab is stolen by a different Pointer Handler or Item.

Note: The corresponding handler is onCanceled.


grabChanged(PointerDevice::GrabTransition transition, eventPoint point)

This signal is emitted when the grab has changed in some way which is relevant to this handler.

The transition (verb) tells what happened. The point (object) is the point that was grabbed or ungrabbed.

Valid values for transition are:

ConstantDescription
PointerDevice.GrabExclusiveThis handler has taken primary responsibility for handling the point.
PointerDevice.UngrabExclusiveThis handler has given up its previous exclusive grab.
PointerDevice.CancelGrabExclusiveThis handler's exclusive grab has been taken over or cancelled.
PointerDevice.GrabPassiveThis handler has acquired a passive grab, to monitor the point.
PointerDevice.UngrabPassiveThis handler has given up its previous passive grab.
PointerDevice.CancelGrabPassiveThis handler's previous passive grab has terminated abnormally.

Note: The corresponding handler is onGrabChanged.


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