get height of a Widget using its GlobalKey in flutter

Question

I am struggling getting the height of a Widget using its GlobalKey. the function that is getting the height is called after the Layout is rendered to make sure the context is available but key.currentState and also key.currentContext still returns null.

import 'package:flutter/material.dart';

class TestPage extends StatefulWidget{
  @override
  State<StatefulWidget> createState() => new TestPageState();
}

class TestPageState extends State<TestPage>{
  final TestWidget testWidget = new TestWidget();

  @override
  initState() {
    //calling the getHeight Function after the Layout is Rendered
    WidgetsBinding.instance
        .addPostFrameCallback((_) => getHeight());

    super.initState();
  }

  void getHeight(){
    final GlobalKey key = testWidget.key;

    //returns null:
    final State state = key.currentState;
    //returns null:
    final BuildContext context = key.currentContext;

    //Error: The getter 'context' was called on null.
    final RenderBox box = state.context.findRenderObject();

    print(box.size.height);
    print(context.size.height);
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      body: testWidget,
    );
  }
}

class TestWidget extends StatefulWidget{
  @override
  Key get key => new GlobalKey<TestWidgetState>();

  @override
  State<StatefulWidget> createState() => new TestWidgetState();
}

class TestWidgetState extends State<TestWidget>{
  @override
  Widget build(BuildContext context) {
    return new Container(
      child: new Text("Test", style: const TextStyle(fontSize: 32.0, fontWeight: FontWeight.bold),),
    );
  }
}

Answer 1

You need to assign that key to a widget using super in the widget constructor. Not add it as a field. That Key also must be constant.

import 'package:flutter/material.dart';

class TestPage extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => new TestPageState();
}

class TestPageState extends State<TestPage> {
  final key = new GlobalKey<TestWidgetState>();

  @override
  initState() {
    //calling the getHeight Function after the Layout is Rendered
    WidgetsBinding.instance.addPostFrameCallback((_) => getHeight());

    super.initState();
  }

  void getHeight() {
    //returns null:
    final State state = key.currentState;
    //returns null:
    final BuildContext context = key.currentContext;

    //Error: The getter 'context' was called on null.
    final RenderBox box = state.context.findRenderObject();

    print(box.size.height);
    print(context.size.height);
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      body: new TestWidget(key: key),
    );
  }
}

class TestWidget extends StatefulWidget {
  TestWidget({Key key}) : super(key: key);

  @override
  State<StatefulWidget> createState() => new TestWidgetState();
}

class TestWidgetState extends State<TestWidget> {
  @override
  Widget build(BuildContext context) {
    return new Container(
      child: new Text(
        "Test",
        style: const TextStyle(fontSize: 32.0, fontWeight: FontWeight.bold),
      ),
    );
  }
}