Key 是什么

key是用来作为Widget、Element和SemanticsNode的标示，仅仅用来更新widget->key相同的小部件的状态

Key子类包含LocalKey和GlobalKey

LocalKey

用作diff算法的核心所在！用作Element和Widget进行比较！

abstract class LocalKey extends Key {
  const LocalKey() : super.empty();
}

LocalKey子类包含ValueKey/ObjectKey/UniqueKey

• ValueKey 以一个数据作为Key。如：数字、字符

 StfulItem(
      'aaaaa',
      key: ValueKey(111),
    ),

• ObjectKey以Object对象作为Key

    StfulItem(
      'bbbbb',
      key: ObjectKey(Text('222')),
    ),

• UniqueKey可以保证Key的唯一性！（一旦使用Uniquekey那么就不存在Element复用 了！）

  StfulItem(
      'ccccc',
      key: UniqueKey(),
    ),

具体代码如下

import 'dart:math';

import 'package:flutter/material.dart';

class KeyDemo extends StatefulWidget {
  @override
  _KeyDemoState createState() => _KeyDemoState();
}

class _KeyDemoState extends State<KeyDemo> {
  List<Widget> items = [
    StfulItem(
      'aaaaa',
      key: ValueKey(111),
    ),
    StfulItem(
      'bbbbb',
      key: ObjectKey(Text('222')),
    ),
    StfulItem(
      'ccccc',
      key: UniqueKey(),
    ),
  ];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('keyDemo'),
      ),
      body: Row(
        mainAxisAlignment: MainAxisAlignment.center,
        children: items,
      ),
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.add),
        onPressed: () {
          setState(() {
            items.removeAt(0);
          });
        },
      ),
    );
  }
}

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

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

class _StfulItemState extends State<StfulItem> {
  final _color = Color.fromRGBO(
      Random().nextInt(256), Random().nextInt(256), Random().nextInt(256), 1.0);
  @override
  Widget build(BuildContext context) {
    return Container(
      width: 100,
      height: 100,
      color: _color,
      child: Text(widget.title),
    );
  }
}

//做一个正方形!
class StlItem extends StatelessWidget {
  final title;
  StlItem(this.title);
  final _color = Color.fromRGBO(
      Random().nextInt(256), Random().nextInt(256), Random().nextInt(256), 1.0);
  @override
  Widget build(BuildContext context) {
    return Container(
      width: 100,
      height: 100,
      color: _color,
      child: Text(title),
    );
  }
}

GlobalKey

GlobalKey可以获取到对应的Widget的State对象！关于stateFul尽量在末端!在树的"叶子"处
具体代码如下

import 'package:flutter/material.dart';

class GlobalKeyDemo extends StatelessWidget {
  final GlobalKey<_ChildPageState> _globalKey = GlobalKey();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('GlobalKeyDemo'),
      ),
      body: ChildPage(
        key: _globalKey,
      ),
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.add),
        onPressed: () {
          _globalKey.currentState.data =
              'old:' + _globalKey.currentState.count.toString();
          _globalKey.currentState.count++;

          _globalKey.currentState.setState(() {});
        },
      ),
    );
  }
}

class ChildPage extends StatefulWidget {
  ChildPage({Key key}) : super(key: key);
  @override
  _ChildPageState createState() => _ChildPageState();
}

class _ChildPageState extends State<ChildPage> {
  int count = 0;
  String data = 'hello';
  @override
  Widget build(BuildContext context) {
    return Center(
      child: Column(
        children: <Widget>[
          Text(count.toString()),
          Text(data),
        ],
      ),
    );
  }
}