Flutter —— 状态管理 | Provide

///为了更好的理解，我创建了两个model
import 'package:flutter/material.dart';
//model1
class Counter with ChangeNotifier {
  int _value;
  int get value => _value;
  Counter(this._value);
  void increment() {
    _value++;
    notifyListeners();
  }
}
//model2
import 'package:flutter/material.dart';

class PersionModel with ChangeNotifier {
  List<String> persionNames = ['小明', '小红'];
  PersionModel();

  List<String> get getPersionNameList => persionNames;

  void addName(String name) {
    persionNames.insert(0, name);
    notifyListeners();
  }
}

void main() {
  //1.创建 model
  var counter =Counter(0);
  var persionNameModel=PersionModel();
  //2.创建 Providers
  var providers=Providers();
  providers
  //3.将创建的 modle 加入到 providers 中
    ..provide(Provider.value(counter))
    ..provide(Provider.value(persionNameModel));
  runApp(
  //4.通过 ProviderNode 将树包裹
    ProviderNode(child:  MyApp(), providers: providers)
     );
}

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,
      ),
      home: FirstPage(),
    );
  }
}

///方法一: 通过 provide 构建小部件
Provide<Modle>(
  builder: (context, child, modle) {
    return Text('小部件' + modle.value);
  },
),

///方法二: StreamBuilder<Model>构建小部件
StreamBuilder<Counter>(
            initialData:Provide.value<Counter>(context) ,
            stream: Provide.stream<Counter>(context).where((Counter counter)=> counter.value % 2 ==0),
            builder: (context,snapshot){
              return Text(
                '通过StreamBuilder构建Widget->${snapshot.data.value}',
              );
            },
          ),

class FirstPage extends StatefulWidget {
  @override
  FirstPageState createState() => new FirstPageState();
}

class FirstPageState extends State<FirstPage> {
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text('第一个界面'),
      ),
      body: Column(
        children: <Widget>[
        //通过Provide构建一个 Text 小部件
          Provide<PersionModel>(
            builder: (context, child, persionNameModel) {
              return Text('取第一个人的名字' + persionNameModel.getPersionNameList[0]);
            },
          ),
          Provide<Counter>(
            builder: (context, child, counter) {
              return Text('当前数值' + counter.value.toString());
            },
          ),
        //通过StreamBuilder构建一个 Text 小部件
          StreamBuilder<Counter>(
           initialData: Provide.value<Counter>(context),
           //可以操作的 stream 流
           stream: Provide.stream<Counter>(context)
               .where((Counter counter) => counter.value % 2 == 0),
           builder: (context, snapshot) {
             return Text(
               '通过StreamBuilder构建Widget->${snapshot.data.value}',
             );
           },
         ),
        ],
      ),
      floatingActionButton: FloatingActionButton(
          child: Icon(Icons.add),
          onPressed: () {
            Navigator.of(context)
                .push(MaterialPageRoute(builder: (context) => SecondPage()));
          }),
    );
  }
}

1.获取 model 的方法  Provide.value<Model>(context)
2.调用 model 中的数据 Provide.value<Model>(context).方法名
///例如在 SecondPage 界面中
floatingActionButton: FloatingActionButton(
          child: Icon(Icons.add),
          onPressed: () {
            ///value值加一
            Provide.value<Counter>(context).increment();

            ///往人名集合中添加人名
            Provide.value<PersionModel>(context).addName(
                '小紫' + Provide.value<Counter>(context).value.toString());
          }),

内容点1：
providers
   ..provide(Provider.value(counter));
(1) Provider.value(counter)=>返回 Provider 对象

(2) providers.provide()=>
  //将provide添加进map集合
  Map<Type, Provider<dynamic>> _providersForScope(scope) =>
      _providers[scope ?? defaultScope] ??= {};

(3) ProviderNode(child:  MyApp(), providers: providers)
ProviderNode创建一个小部件，返回
_InheritedProviders(
       child: child,
       providers: providers,
       parent: _InheritedProviders.of(context));

同样_InheritedProviders继承了InheritedWidget,通过InheritedWidget向下传递数据

内容点2：如何构建小部件，并且刷新数据
Provide<PersionModel>(
            builder: (context, child, persionNameModel) {
              return Text('取第一个人的名字' + persionNameModel.getPersionNameList[0]);
            },
          ),

class Provide<T> extends StatelessWidget {
  /// Called whenever there is a change.
  final ValueBuilder<T> builder;

  /// The part of the widget tree not rebuilt on change.
  final Widget child;

  /// The scope from which the type is requested
  final ProviderScope scope;

  /// Constructor.
  const Provide({@required this.builder, this.child, this.scope});

  /// 返回model
  static T value<T>(BuildContext context, {ProviderScope scope}) {
    final provider = _InheritedProviders.of(context).getValue<T>(scope: scope);
    assert(provider != null);

    return provider.get(context);
  }

  ///返回一个stream
  static Stream<T> stream<T>(BuildContext context, {ProviderScope scope}) {
    final provider = _InheritedProviders.of(context).getValue<T>(scope: scope);
    assert(provider != null);

    return provider.stream(context);
  }

  @override
  Widget build(BuildContext context) {
    /// 从map集合中获取provider
    final provider = _InheritedProviders.of(context).getValue<T>(scope: scope);
    /// 返回model
    final value = provider.get(context);
    final listenable = _getListenable(provider, value);
    ///关键字is的意思 类型匹配 类似 java 的 instance of
    if (provider is Listenable) {
      return ListeningBuilder(
        listenable: listenable,
        child: child,
        builder: (buildContext, child) =>
            builder(buildContext, child, provider.get(context)),
      );
    } else if (value is Listenable) {
    /// value 就是我们定义的model ,model with ChangeNotifier
    /// ChangeNotifier implements Listenable
      return ListeningBuilder(
        listenable: listenable,
        child: child,
        builder: (buildContext, child) => builder(buildContext, child, value),
      );
    }

    throw ArgumentError('Either the type or the provider of it must'
        ' implement listenable. To get a non-listenable value, use the static'
        ' Provide.value<T>.');
  }
}
对于ListeningBuilder，有这么一段话
/// Widget that rebuilds part of the widget tree whenever
/// the [listenable] changes.
当listenable 发生改变时，widget 会rebuild 重新构建widget