Get "y" position of container on Flutter

Question

I want to get a 'y' position of container, widget, stack etc. When it's displayed on center of screen, I want to change from image (video preview image) to video file. How can I do?

I found the screen height. I calculated the screen center. I couldn’t find to container place.

I created such a sample, but the item doesnt stand in the center. Possible I have made an algorithm error here?

class _DedectCenterState extends State<DedectCenter> {  Size _wSize; int listSize = 4; List<GlobalKey> _key = [];  ScrollController _sController;  _scrollListener() {  print("scrolling : " + _sController.offset.toString());   setState(() {    getPosition(); });}  @override void initState() {  for(int i=0; i < listSize; i++) {   _key[i] = new GlobalKey(); }  _sController = ScrollController();  _sController.addListener(_scrollListener); super.initState(); }  void getPosition(){  double _wCenterTop = (_wSize.height / 2) - 150.0; double _wCenterBottom = (_wSize.height / 2) + 150.0;  print("center bottom: " + _wCenterBottom.toString()); print("center top: " + _wCenterTop.toString());   for(int i=0; i < listSize; i++) {    //add key to your widget, which position you need to find   RenderBox box = _key[i].currentContext.findRenderObject();   Offset position = box.localToGlobal(Offset.zero); //this is global position   double y = position.dy; //this is y - I think it's what you want     print("key $i y position: " + y.toString());    if(y < _wCenterTop && y > _wCenterBottom) print("key $i center: " + y.toString());  }}   Widget build(BuildContext context) {  _wSize = MediaQuery.of(context).size;  return new Scaffold(   appBar: null,   body: new ListView(     controller: _sController,   children: <Widget>[     Row(       children: <Widget>[         Column(           children: <Widget>[             Container(               key: _key[0],               margin: EdgeInsets.only(top:100.0),               height: 140.0,                 width: _wSize.width,                 child: new Image.asset('assets/images/physical/a1.png'),             )],)],     ),       Row(       children: <Widget>[         Column(           children: <Widget>[             Container(               key: _key[1],               margin: EdgeInsets.only(top:100.0),               height: 140.0,               width: _wSize.width,               child: new Image.asset('assets/images/physical/a2.png'),   ),],)], ),     Row(       children: <Widget>[         Column(           children: <Widget>[             Container(               margin: EdgeInsets.only(top:100.0),               key: _key[2],               height: 140.0,               width: _wSize.width,                 child: new Image.asset('assets/images/physical/a3.png'),             ),],)],),      Row(       children: <Widget>[         Column(           children: <Widget>[             Container(               margin: EdgeInsets.only(top:100.0),               key: _key[3],               width: _wSize.width,                 height: 140.0,                 child: new Image.asset('assets/images/physical/a4.png'),             ),           ],         )       ],),]));}} 

Answer 1

You can use GlobalKey for getting size or position of widget

GlobalKey key = GlobalKey(); // declare a global key 

add key to your widget, whose position you need to find

Container(    key: key,    ... );  

get the position using a renderbox

RenderBox box = key.currentContext.findRenderObject() as RenderBox; Offset position = box.localToGlobal(Offset.zero); //this is global position double y = position.dy; //this is y - I think it's what you want