SwapTest.qml

import QtQuick 2.0

Rectangle {
    id: root
    property QtObject window
    implicitWidth: 300
    implicitHeight: 150
    color: Qt.rgba(0.5, 0, 0.5, 1)
    Item {
        NumberAnimation on rotation {
            id: animation
            running: true
            from: 0
            to: 360
            loops: Animation.Infinite
            duration: 1000
        }
    }
    Rectangle {
        id: d
        property bool tick
        property real t: Date.now()
        anchors.fill: parent
        anchors.margins: 20
        color: tick ? Qt.rgba(1, 0, 0, 1) : Qt.rgba(0, 0, 1, 1)
    }
    Connections {
        target: root.window
        onAfterAnimating: {
            d.tick = !d.tick;
            var newT = Date.now();
            var dt = newT - d.t;
            if(dt > 30) print("Drop!", dt);
            d.t = newT;
        }
    }
}