MediaPlayer QML duration

[apic]

Hello, i have following qml:

import QtQuick 2.1
import QtMultimedia 5.7
  
 Rectangle {
	 x: 0
	 y: 0
	 width: 1152
	 height: 675
	 color:"black"
	 radius:20
 
	 MediaPlayer {
		 id: mediaPlayer
		 objectName: "mediaPlayer"
		 autoLoad: true
		 volume: 1
		 playlist: Playlist {
			 id: playlist
			 PlaylistItem { source: "/testVideo.mp4"; }
		   }
		 onPlaying: {                                  
			console.log("[qmlvideo] duration " + duration) 
		 }
	 }
 
	 VideoOutput {
		 id:videoOutput
		 source:mediaPlayer
		 anchors.fill: parent
	 }
	 function stop() {
		 mediaPlayer.stop();
	 }
	 function setVolume(volume) {
		 mediaPlayer.setVolume(volume);
	 }
 }

this is used in a widgets application through QQuickWidget.
When i call MediaPlayer::play() video plays correctly
and onPlay is fired the problem is that duration property always return -1.
Tryed also to connect playbackStateChanged() signal and also here duration is always -1
both on start and stop.

How can i get video duration?

Thank you