Position change when parent resizes

[JobRecrd]

How do I properly change the x, y of an object so that it changes its position when the parent is resized? There is, I will introduce that if I drag the rectangle to the middle, then when the window is resized, it should remain in the middle. (middle for example only, rectangle can be moved freely)

import QtQuick 2.9
import QtQuick.Window 2.2
import QtQuick.Controls 2.3

Window {
    visible: true
    width: 640
    height: 480

onWidthChanged: {
            block.x -= block.previousWidth - width
            block.previousWidth = width
        }

onHeightChanged: {
            block.y -= block.previousHeight - height
            block.previousHeight = height
        }

    Rectangle {
        id: block
        color: "red"
        width: 50
        height:50
        x: 100
        y: 50

       property int previousWidth: 0
       property int previousHeight:0

       Component.onCompleted: { 
          previousWidth = parent.width
          previousHeight  = parent.height
}

        MouseArea {
            anchors.fill: parent
            drag.target: block
        }
    }
}