Getting binding loop when checking mousearea.containsMouse on a drag operation

[fcarney]

I cannot tell why I am getting a binding loop here.

main.qml:

import QtQuick 2.12
import QtQuick.Window 2.12

Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Dragon Drop: drag to window")

    ColorAnimation on color {
        id: rejectAnimation

        running: false
        from: "#FCC"
        to: "#EEE"
        duration: 1000
    }

    DropArea {
        id: dropArea
        anchors.fill: parent
        keys: ["amet/displaykey"]
        onEntered: (drag) => {
            if(drag.proposedAction != Qt.CopyAction){
                drag.accepted = false
                rejectAnimation.start()
            }
        }
        onDropped: (drop) => {
            var textdata = drop.getDataAsString(keys[0])
            if (textdata.length) {
                if (drop.proposedAction == Qt.CopyAction) {
                    console.log(textdata)
                    drop.acceptProposedAction()
                }
            }
        }
    }

    Window {
        id: side_window

        title: "drag from window"
        visible: true

        width: 400
        height: 400

        MouseArea {
            id: mousearea

            hoverEnabled: true

            width: parent.width
            height: parent.height
        }

        DragIcon {
            visible: mousearea.containsMouse

            dropdata: ({
                "display": "some data"
            })
            dragtext: "✋"
            dragkeys: ["displaykey"]
            dragmime: {"amet/displaykey":dropdata}
        }
    }
}

DragIcon.qml:

import QtQuick 2.15
import QtQuick.Controls 2.15

Rectangle {
    id: grab_handle

    width: 20
    height: 20

    property int dragwidth: 20
    property int dragheight: 20

    property string dragtext
    property string draggedtext: dragtext
    property var dragkeys: []
    property var dragmime: ({})
    property var dropdata: ({})
    property var dragparent

    color: "transparent"
    border.color: "steelblue"
    border.width: 1

    Text {
        id: drag_text

        anchors.centerIn: parent

        text: grab_handle.dragtext
    }

    MouseArea {
        id: drag_mousearea

        anchors.fill: parent

        drag.target: draggable

        onReleased: {
            draggable.Drag.drop()
        }

        onPressed: draggable.grabToImage(function(result) {
            draggable.Drag.imageSource = result.url
        })

        Rectangle {
            id: draggable

            objectName: "draggable"

            visible: drag_mousearea.drag.active

            width: grab_handle.dragwidth
            height: grab_handle.dragheight
            color: "transparent"

            border.color: grab_handle.border.color
            border.width: 1

            property var dropdata: grab_handle.dropdata

            Drag.active: drag_mousearea.drag.active
            Drag.hotSpot.x: 0
            Drag.hotSpot.y: 0
            Drag.keys: grab_handle.dragkeys
            Drag.dragType: Drag.Automatic
            Drag.supportedActions: Qt.CopyAction
            Drag.mimeData: dragmime

            Drag.onDragFinished: {
                Qt.callLater(fixpos, x,y) // fixes drag glitches
            }
            function fixpos(x, y){
                draggable.x = x
                draggable.y =y
            }

            //states: State {
                //when: drag_mousearea.drag.active
                //ParentChange { target: draggable; parent: grab_handle.dragparent }
                //AnchorChanges { target: draggable; anchors.verticalCenter: undefined; anchors.horizontalCenter: undefined }
            //}

            Text {
                id: dragged_text

                anchors.centerIn: parent

                text: grab_handle.draggedtext
            }
        }
    }
}

I think it has something to do with the mousearea active, but I just don't see why there is a loop.

The error occurs once when starting the drag:

qrc:/DragIcon.qml:47:9: QML Rectangle (parent or ancestor of QQuickDragAttached): Binding loop detected for property "active"

If I don't use mousearea.containsMouse in main.qml to determine visibility it doesn't show this binding loop. Which makes no sense to me.
Everything works as expected so I have been ignoring this. I don't want this to bite me later.

Qt 5.15.2 64 bit
Linux

[fcarney]

I found where I learned how to bind mousearea.drag.active to Drag.active. So that part "should" be good. I upgraded from 5.15.1. to 5.15.2 to see if this is a library issue. Not that I can tell.

I just checked and I get the binding loop detection in 5.12.10. So it was not introduced with 5.15.

Are binding loop detections really sensitive? I put an on changed routine on the Drag.active and it only changes once:

Drag.onActiveChanged: console.log("Drag.active:", Drag.active)

[fcarney]

Okay, I found how to make it go away. I am not sure if this is a good solution, but it seems to get rid of the binding loop message and does not seem to affect functionality.

Change this:

Drag.active: drag_mousearea.drag.active

To this:

// prevents binding loop with unknown cause
Binding on Drag.active {
  value: drag_mousearea.drag.active
  delayed: true
}