How to implement onVideoCompletion Callback ?? Help Required

After watch video I have to mark it watched so I need a onCompletion Callback. I tried with listener as `class _ChewieDemoState extends State {
TargetPlatform _platform;
VideoPlayerController _controller;
VoidCallback listener;
bool isCompeleted = false;

@override
void initState() {
super.initState();
_controller = new VideoPlayerController.network(
'http://commondatastorage.googleapis.com/gtv-videos-bucket/sample/ForBiggerEscapes.mp4',
);
listener = () {
// if (!mounted) {
// return;
// }
//debugPrint("listening .....");

if(_controller.value.duration!=null){
debugPrint("duration"+_controller.value.duration.inMilliseconds.toString()+" _ position _"+"duration"+_controller.value.position.inMilliseconds.toString());
//debugPrint(_controller.value.position.inMilliseconds.toString());
if(_controller.value.duration.inMilliseconds==_controller.value.position.inMilliseconds){
debugPrint("competed .....");
_controller.removeListener(listener);
}

}
// if (isCompeleted == _controller.value.completed) {
// initialized = _controller.value.initialized;
// setState(() {});
// }
};
_controller.addListener(listener);

}`

But it don't work and _controller.value.duration.inMilliseconds and _controller.value.position.inMilliseconds never become same ,quite surprising why ? Thanks in advance