How to make an item drag inside a circle in QML?

Question

Below code allows the small red coloured rectangle to be dragged in an area which is a rectangle defined by minimum and maximum drag values.

I want it to go on only up t

Answer 1

So I found some time to provide the aforementioned smoother solution.

import QtQuick 2.5
import QtQuick.Window 2.2

Window {
    id: root
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    property int radius: 100

    Rectangle {
        id: circle
        width: 2 * radius
        height: 2 * radius
        radius: root.radius

        color: 'blue'
    }

    Rectangle {
        id: mark
        width: 20
        height: 20
        x: (dragObj.dragRadius <= root.radius ? dragObj.x : root.radius + ((dragObj.x - root.radius) * (root.radius / dragObj.dragRadius))) - 10
        y: (dragObj.dragRadius <= root.radius ? dragObj.y : root.radius + ((dragObj.y - root.radius) * (root.radius / dragObj.dragRadius))) - 10
        color: 'red'

        MouseArea {
            id: markArea
            anchors.fill: parent
            drag.target: dragObj
            onPressed: {
                dragObj.x = mark.x + 10
                dragObj.y = mark.y + 10
            }
        }
    }

    Item {
        id: dragObj
        readonly property real dragRadius: Math.sqrt(Math.pow(x - root.radius, 2) + Math.pow(y - root.radius, 2))
        x: root.radius
        y: root.radius

        onDragRadiusChanged: console.log(dragRadius)
    }
}

I use the dragObj to avoid the limitations of my dragging position. This spans a vector from the center of the circle. As long as the dragObj itself is contained in the circle, I will use it's position as the position of the marker.
But once it leaves the circle, I will project the vector on the circle, so it will stay within the limits.

To ensure, that every drag starts again on the mark, I will reset the position of the dragObj to the position of the mark, when ever the mouse is pressed again (precondition for a new drag-event)

Have fun with it.