didUpdateWidget

main_2.dart

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: ParentWidget(title: 'Flutter Demo Home Page'),
    );
  }
}

class ParentWidget extends StatefulWidget {
  ParentWidget({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _ParentWidgetState createState() => _ParentWidgetState();
}

class _ParentWidgetState extends State<ParentWidget> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            ChildWidget(_counter)
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}

class ChildWidget extends StatefulWidget {
  final int counter;

  ChildWidget(this.counter);

  @override
  _ChildWidgetState createState() => _ChildWidgetState();
}

class _ChildWidgetState extends State<ChildWidget> {
  int _counter = 0;

  @override
  void initState() {
    debugPrint('Child widget: initState(), counter = $_counter');
    _counter = widget.counter;
    super.initState();
  }

  @override
  void didChangeDependencies() {
    debugPrint('Child widget: didChangeDependencies(), counter = $_counter');
    super.didChangeDependencies();
  }


  @override
  void didUpdateWidget(ChildWidget oldWidget) {
    super.didUpdateWidget(oldWidget);
    debugPrint('Child widget: didUpdateWidget(), counter = ${widget.counter}');
    _counter = widget.counter;
  }

  @override
  Widget build(BuildContext context) {
    print("ChildWidget build");
    return Text(
      '$_counter',
      style: Theme.of(context).textTheme.display1,
    );
  }
}