Qt Quick Demo - Photo Viewer

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 import QtQuick 2.0

 Item {
     id: container

     property real progress: 0

     Behavior on opacity { NumberAnimation { duration: 600 } }

     Rectangle { anchors.fill: parent; color: "black"; opacity: 0.5 }

     Rectangle {
         id: fill; color: "white"; height: container.height
         width: container.width * container.progress
     }
 }