import ‘package:scoped_model/scoped_model.dart’;
class CounterModel extends Model {
int _counter = 0;
int get counter => _counter;
void increment() {
_counter++;
notifyListeners();
}
void decrement() {
_counter–;
notifyListeners();
}
}
其中数据变化的部分会通知listeners, 它们收到通知后会rebuild.
在上层初始化并提供数据类, 用ScopeModel.
访问数据有两种方法:
-
用
ScopedModelDescendant包裹widget. -
用
ScopedModel.of静态方法.
使用的时候注意要提供泛型类型, 会帮助我们找到离得最近的上层ScopedModel.
ScopedModelDescendant(
builder: (context, child, model) {
return Text(
model.counter.toString(),
);
}),
数据改变后, 只有ScopedModelDescendant会收到通知, 从而rebuild.
ScopedModelDescendant有一个rebuildOnChange属性, 这个值默认是true.
对于button来说, 它只是控制改变, 自身并不需要重绘, 可以把这个属性置为false.
ScopedModelDescendant(
rebuildOnChange: false,
builder: (context, child, model) {
return FloatingActionButton(
onPressed: model.increment,
tooltip: ‘Increment’,
child: Icon(Icons.add),
);
},
),
scoped model这个库帮我们解决了数据访问和通知的问题, 但是rebuild范围需要自己控制.
-
access state
-
notify other widgets
-
minimal rebuild -> X -> 因为需要开发者自己来决定哪一部分是否需要被重建, 容易被忘记.
代码分支: scoped-model
Provider
Provider是官方文档的例子用的方法.
去年的Google I/O 2019也推荐了这个方法.
和BLoC的流式思想相比, Provider是一个观察者模式, 状态改变时要notifyListeners().
有一个counter版本的sample: https://github.com/flutter/samples/tree/master/provider_counter
Provider的实现在内部还是利用了InheritedWidget.
Provider的好处: dispose指定后会自动被调用, 支持MultiProvider.
Provider实现
- model类继承
ChangeNotifer, 也可以用with.
class CounterModel extends ChangeNotifier {
int value = 0;
void increment() {
value++;
notifyListeners();
}
void decrement() {
value–;
notifyListeners();
}
}
- 数据提供者:
ChangeNotifierProvider.
void main() => runApp(ChangeNotifierProvider(
create: (context) => CounterModel(),
child: MyApp(),
));
- 数据消费者/操纵者, 有两种方式:
Consumer包裹, 用Provider.of.
Consumer(
builder: (context, counter, child) => Text(
‘${counter.value}’,
),
),
FAB:
FloatingActionButton(
onPressed: () =>
Provider.of(context, listen: false).increment(),
),
这里listen置为false表明状态变化时并不需要rebuild FAB widget.
Provider性能相关的实现细节
-
Consumer包裹的范围要尽量小. -
listen变量.
-
child的处理.
Consumer中builder方法的第三个参数.
可以用于缓存一些并不需要重建的widget:
return Consumer(
builder: (context, cart, child) => Stack(
children: [
// Use SomeExpensiveWidget here, without rebuilding every time.
child,
Text(“Total price: ${cart.totalPrice}”),
],
),
// Build the expensive widget here.
child: SomeExpensiveWidget(),
);
代码分支: provider.
BLoC
BLoC模式的全称是: business logic component.
所有的交互都是a stream of asynchronous events.
Widgets + Streams = Reactive.
BLoC的实现的主要思路: Events in -> BloC -> State out.
Google I/O 2018上推荐的还是这个, 2019就推荐Provider了.
当然也不是说这个模式不好, 架构模式本来也没有对错之分, 只是技术选型不同.
BLoC手动实现
不添加任何依赖可以手动实现BLoC, 利用:
-
Dart SDK > dart:async >
Stream. -
Flutter的
StreamBuilder: 输入是一个stream, 有一个builder方法, 每次stream中有新值, 就会rebuild.
可以有多个stream, UI只在自己感兴趣的信息发生变化的时候重建.
BLoC中:
-
输入事件:
Sink<Event> input. -
输出数据:
Stream<Data> output.
CounterBloc类:
class CounterBloc {
int _counter = 0;
final _counterStateController = StreamController();
StreamSink get _inCounter => _counterStateController.sink;
Stream get counter => _counterStateController.stream;
final _counterEventController = StreamController();
Sink get counterEventSink => _counterEventController.sink;
CounterBloc() {
_counterEventController.stream.listen(_mapEventToState);
}
void _mapEventToState(CounterEvent event) {
if (event is IncrementEvent) {
_counter++;
} else if (event is DecrementEvent) {
_counter–;
}
_inCounter.add(_counter);
}
void dispose() {
_counterStateController.close();
_counterEventController.close();
}
}
有两个StreamController, 一个控制state, 一个控制event.
读取状态值要用StreamBuilder:
StreamBuilder(
stream: _bloc.counter,
initialData: 0,
builder: (BuildContext context, AsyncSnapshot snapshot) {
return Text(
‘${snapshot.data}’,
);
},
)
而改变状态是发送事件:
FloatingActionButton(
onPressed: () => _bloc.counterEventSink.add(IncrementEvent()),
),
实现细节:
-
每个屏幕有自己的BLoC.
-
每个BLoC必须有自己的
dispose()方法. -> BLoC必须和StatefulWidget一起使用, 利用其生命周期释放.
代码分支: bloc
BLoC传递: 用InheritedWidget
手动实现的BLoC模式, 可以结合InheritedWidget, 写一个Provider, 用来做BLoC的传递.
代码分支: bloc-with-provider
BLoC rxdart实现
用了rxdart package之后, bloc模块的实现可以这样写:
class CounterBloc {
int _counter = 0;
final _counterSubject = BehaviorSubject();
Stream get counter => _counterSubject.stream;
final _counterEventController = StreamController();
Sink get counterEventSink => _counterEventController.sink;
CounterBloc() {
_counterEventController.stream.listen(_mapEventToState);
}
void _mapEventToState(CounterEvent event) {
if (event is IncrementEvent) {
_counter++;
} else if (event is DecrementEvent) {
_counter–;
}
_counterSubject.add(_counter);
}
void dispose() {
_counterSubject.close();
_counterEventController.close();
}
}
BehaviorSubject也是一种StreamController, 它会记住自己最新的值, 每次注册监听, 会立即给你最新的值.
代码分支: bloc-rxdart.
BLoC Library
可以用这个package来帮我们简化代码: https://pub.dev/packages/flutter_bloc
自己只需要定义Event和State的类型并传入, 再写一个逻辑转化的方法:
class CounterBloc extends Bloc<CounterEvent, CounterState> {
@override
CounterState get initialState => CounterState.initial();
@override
Stream mapEventToState(CounterEvent event) async* {
if (event is IncrementEvent) {
yield CounterState(counter: state.counter + 1);
} else if (event is DecrementEvent) {
yield CounterState(counter: state.counter - 1);
}
}
}
用BlocProvider来做bloc的传递, 从而不用在构造函数中一传到底.
访问的时候用BlocBuilder或BlocProvider.of<CounterBloc>(context).
BlocBuilder(
bloc: BlocProvider.of(context),
builder: (BuildContext context, CounterState state) {
return Text(
‘${state.counter}’,
);
},
),
这里bloc参数如果没有指定, 会自动向上寻找.
BlocBuilder有一个参数condition, 是一个返回bool的函数, 用来精细控制是否需要rebuild.
FloatingActionButton(
onPressed: () =>
BlocProvider.of(context).add(IncrementEvent()),
),
代码分支: bloc-library.
rxdart
这是个原始版本的流式处理.
和BLoC相比, 没有专门的逻辑模块, 只是改变了数据的形式.
利用rxdart, 把数据做成流:
class CounterModel {
BehaviorSubject _counter = BehaviorSubject.seeded(0);
get stream$ => _counter.stream;
int get current => _counter.value;
increment() {
_counter.add(current + 1);
}
decrement() {
_counter.add(current - 1);
}
}
获取数据用StreamBuilder, 包围的范围尽量小.
StreamBuilder(
stream: counterModel.stream$,
builder: (BuildContext context, AsyncSnapshot snapshot) {
return Text(
‘${snapshot.data}’,
);
},
),
Widget dispose的时候会自动解绑.
数据传递的部分还需要进一步处理.
代码分支: rxdart.
Redux
Redux是前端流行的, 一种单向数据流架构.
概念:
-
Store: 用于存储State对象, 代表整个应用的状态. -
Action: 事件操作. -
Reducer: 用于处理和分发事件的方法, 根据收到的Action, 用一个新的State来更新Store. -
View: 每次Store接到新的State,View就会重建.
Reducer是唯一的逻辑处理部分, 它的输入是当前State和Action, 输出是一个新的State.
Flutter Redux状态管理实现
首先定义好action, state:
enum Actions {
Increment,
Decrement,
}
class CounterState {
int _counter;
int get counter => _counter;
CounterState(this._counter);
}
reducer方法根据action和当前state产生新的state:
CounterState reducer(CounterState prev, dynamic action) {
if (action == Actions.Increment) {
return new CounterState(prev.counter + 1);
} else if (action == Actions.Decrement) {
return new CounterState(prev.counter - 1);
} else {
return prev;
}
}
- 数据提供者:
StoreProvider.
放在上层:
StoreProvider(
store: store,
child: MaterialApp(
title: ‘Flutter Demo’,
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: MyHomePage(title: ‘Flutter Demo Home Page’),
),
);
- 数据消费者:
StoreConnector, 可读可写.
读状态:
StoreConnector<CounterState, String>(
converter: (store) => store.state.counter.toString(),
builder: (context, count) {
return 
Text(
‘$count’,
);
},
)
改变状态: 发送action:
StoreConnector<CounterState, VoidCallback>(
converter: (store) {
return () => store.dispatch(action.Actions.Increment);
},
builder: (context, callback) {
return FloatingActionButton(
onPressed: callback,
);
},
),
代码分支: redux.
MobX
MobX本来是一个JavaScript的状态管理库, 它迁移到dart的版本: mobxjs/mobx.dart.
核心概念:
-
Observables
-
Actions
-
Reactions
MobX状态管理实现
官网提供了一个counter的指导: https://mobx.netlify.com/getting-started
这个库的实现需要先生成一些代码.
先写类:
import ‘package:mobx/mobx.dart’;
part ‘counter.g.dart’;
class Counter = _Counter with _$Counter;
abstract class _Counter with Store {
@observable
int value = 0;
@action
void increment() {
value++;
}
@action
void decrement() {
=> store.state.counter.toString(),
builder: (context, count) {
return [外链图片转存中…(img-8RIN0tAq-1642929754290)]
Text(
‘$count’,
);
},
)
改变状态: 发送action:
StoreConnector<CounterState, VoidCallback>(
converter: (store) {
return () => store.dispatch(action.Actions.Increment);
},
builder: (context, callback) {
return FloatingActionButton(
onPressed: callback,
);
},
),
代码分支: redux.
MobX
MobX本来是一个JavaScript的状态管理库, 它迁移到dart的版本: mobxjs/mobx.dart.
核心概念:
-
Observables
-
Actions
-
Reactions
MobX状态管理实现
官网提供了一个counter的指导: https://mobx.netlify.com/getting-started
这个库的实现需要先生成一些代码.
先写类:
import ‘package:mobx/mobx.dart’;
part ‘counter.g.dart’;
class Counter = _Counter with _$Counter;
abstract class _Counter with Store {
@observable
int value = 0;
@action
void increment() {
value++;
}
@action
void decrement() {