Flutter, How to get the size of a widget?

Question

I came across this example to get the Size/Position of a widget: https://medium.com/@diegoveloper/flutter-widget-size-and-position-b0a9ffed9407

what is wrong with my code?

I get the error:

The following NoSuchMethodError was thrown while handling a gesture:
I/flutter ( 8566): The method 'findRenderObject' was called on null.
I/flutter ( 8566): Receiver: null
I/flutter ( 8566): Tried calling: findRenderObject()

void main() {
      runApp(new MyApp());
    }

    class MyApp extends StatelessWidget {
      @override
      Widget build(BuildContext context) {
        return MaterialApp(home: MyWidgetSizeTMP());
      }
    }

    class MyWidgetSizeTMP extends StatefulWidget{
      @override
      MyWidgetSizeTMPState createState() {
        return new MyWidgetSizeTMPState();
      }
    }

    class MyWidgetSizeTMPState extends State<MyWidgetSizeTMP> {

      //creating Key for red panel
      GlobalKey _keyRed = GlobalKey();

      _getSizes() {
        final RenderBox renderBoxRed = _keyRed.currentContext.findRenderObject();
        final sizeRed = renderBoxRed.size;
        print("SIZE of Red: $sizeRed");
      }

      _getPositions() {
        final RenderBox renderBoxRed = _keyRed.currentContext.findRenderObject();
        final positionRed = renderBoxRed.localToGlobal(Offset.zero);
        print("POSITION of Red: $positionRed ");
      }

      _afterLayout(_) {
        _getSizes();
        _getPositions();
      }

      @override
      void initState() {
        // TODO: implement initState
        super.initState();
        WidgetsBinding.instance.addPostFrameCallback(_afterLayout);
      }

      @override
      Widget build(BuildContext context) {
        return Scaffold(
          appBar: AppBar(
          ),
          body: Column(
            children: <Widget>[
              Flexible(
                flex: 2,
                child: Container(
                  color: Colors.red,
                ),
              ),
              Flexible(
                flex: 1,
                child: Container(
                  color: Colors.purple,
                ),
              ),
              Flexible(
                flex: 3,
                child: Container(
                  color: Colors.green,
                ),
              ),
              Spacer(),
              Padding(
                padding: const EdgeInsets.only(bottom: 8.0),
                child: Row(
                  mainAxisAlignment: MainAxisAlignment.spaceAround,
                  crossAxisAlignment: CrossAxisAlignment.center,
                  children: <Widget>[
                    MaterialButton(
                      elevation: 5.0,
                      padding: EdgeInsets.all(15.0),
                      color: Colors.grey,
                      child: Text("Get Sizes"),
                      onPressed: _getSizes,
                    ),
                    MaterialButton(
                      elevation: 5.0,
                      color: Colors.grey,
                      padding: EdgeInsets.all(15.0),
                      child: Text("Get Positions"),
                      onPressed: _getPositions,
                    )
                  ],
                ),
              )
            ],
          ),
        );
      }
    }

Answer 1

You can wrap in LayoutBuilder:

return LayoutBuilder(
    builder: (BuildContext context, BoxConstraints constraints) {
//      height: constraints.maxHeight,
//      width: constraints.maxWidth
        return YourWidget();
    }
);

Answer 2

This is because the _keyRed is not attached to Context in your code.

to fix it -

 @override
  Widget build(BuildContext context) {
    return Scaffold(
      key: _keyRed,  //Add this.
      appBar: AppBar(