flutter/packages/unit/test/widget/stateful_components_test.dart

import 'package:flutter/widgets.dart';
import 'package:test/test.dart';

import 'widget_tester.dart';

class InnerComponent extends StatefulComponent {
  InnerComponent({ Key key }) : super(key: key);
  InnerComponentState createState() => new InnerComponentState();
}

class InnerComponentState extends State<InnerComponent> {
  bool _didInitState = false;

  void initState() {
    super.initState();
    _didInitState = true;
  }

  Widget build(BuildContext context) {
    return new Container();
  }
}

class OuterContainer extends StatefulComponent {
  OuterContainer({ Key key, this.child }) : super(key: key);

  final InnerComponent child;

  OuterContainerState createState() => new OuterContainerState();
}

class OuterContainerState extends State<OuterContainer> {
  Widget build(BuildContext context) {
    return config.child;
  }
}

void main() {
  test('resync stateful widget', () {
    testWidgets((WidgetTester tester) {
      Key innerKey = new Key('inner');
      Key outerKey = new Key('outer');

      InnerComponent inner1 = new InnerComponent(key: innerKey);
      InnerComponent inner2;
      OuterContainer outer1 = new OuterContainer(key: outerKey, child: inner1);
      OuterContainer outer2;

      tester.pumpWidget(outer1);

      StatefulComponentElement innerElement = tester.findElementByKey(innerKey);
      InnerComponentState innerElementState = innerElement.state;
      expect(innerElementState.config, equals(inner1));
      expect(innerElementState._didInitState, isTrue);
      expect(innerElement.renderObject.attached, isTrue);

      inner2 = new InnerComponent(key: innerKey);
      outer2 = new OuterContainer(key: outerKey, child: inner2);

      tester.pumpWidget(outer2);

      expect(tester.findElementByKey(innerKey), equals(innerElement));
      expect(innerElement.state, equals(innerElementState));

      expect(innerElementState.config, equals(inner2));
      expect(innerElementState._didInitState, isTrue);
      expect(innerElement.renderObject.attached, isTrue);

      StatefulComponentElement outerElement = tester.findElementByKey(outerKey);
      expect(outerElement.state.config, equals(outer2));
      outerElement.state.setState(() {});
      tester.pump();

      expect(tester.findElementByKey(innerKey), equals(innerElement));
      expect(innerElement.state, equals(innerElementState));
      expect(innerElementState.config, equals(inner2));
      expect(innerElement.renderObject.attached, isTrue);
    });
  });
}