使用 BLoC 模式的 Flutter 默认计数器应用程序

Question

我是 Flutter 的新手，我正在尝试使用 BLoC 进行状态管理。所以我正在尝试使用 BLoC 重写 flutter 的默认计数器应用程序。不幸的是，当按下 添加按钮 时，我的 UI 没有重绘。我不知道我做错了什么我正在使用

  bloc: ^6.1.0
  equatable: ^1.2.5
  flutter_bloc: ^6.0.6

这是我的代码

counter_event.dart

part of 'counter_bloc.dart';

abstract class CounterEvent extends Equatable {
  const CounterEvent();

  @override
  List<Object> get props => [];
}

class IncrementEvent extends CounterEvent {}

class DecrementEvent extends CounterEvent {}

counter_state.dart

part of 'counter_bloc.dart';

class CounterState extends Equatable {
  final int counter;
  const CounterState({@required this.counter});

  @override
  List<Object> get props => [counter];
}

counter_bloc.dart

import 'dart:async';

import 'package:bloc/bloc.dart';
import 'package:equatable/equatable.dart';
import 'package:flutter/foundation.dart';

part 'counter_event.dart';
part 'counter_state.dart';

class CounterBloc extends Bloc<CounterEvent, CounterState> {
  CounterBloc() : super(CounterState(counter: 0));

  @override
  Stream<CounterState> mapEventToState(CounterEvent event) async* {
    if (state is IncrementEvent) {
      yield CounterState(counter: state.counter + 1);
    } else if (state is DecrementEvent) {
      yield CounterState(counter: state.counter - 1);
    }
  }
}

这里是主要功能

main.dart

import 'package:counter_bloc/bloc/counter_bloc.dart';
import 'package:flutter/material.dart';
import 'package:flutter_bloc/flutter_bloc.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: Home(title: 'Flutter Demo Home Page'),
    );
  }
}

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

  final String title;

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

class _HomeState extends State<Home> {
  CounterBloc counterBloc = CounterBloc();
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: BlocBuilder<CounterBloc, CounterState>(
        cubit: counterBloc,
        builder: (context, state) {
          return Center(
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
                Text(
                  'You have pushed the button this many times:',
                ),
                Text(
                  state.counter.toString(),
                  style: Theme.of(context).textTheme.headline4,
                ),
              ],
            ),
          );
        },
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          counterBloc.add(IncrementEvent());
        },
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }

  @override
  void dispose() {
    counterBloc.close();
    super.dispose();
  }
}

Answer 1

您可以复制粘贴运行下面的完整代码
第 1 步：if (event is IncrementEvent) 而不是 state
第 2 步：您需要 BlocProvider

home: BlocProvider<CounterBloc>(
        create: (context) => CounterBloc(),
        child: Home(
          title: "test",
        ),
      ),

第 3 步：您需要调用 BlocProvider.of<CounterBloc>

 Widget build(BuildContext context) {
    final CounterBloc counterBloc = BlocProvider.of<CounterBloc>(context);

工作演示

完整代码

import 'package:equatable/equatable.dart';
import 'package:flutter/material.dart';
import 'dart:async';

import 'package:bloc/bloc.dart';

import 'package:flutter/foundation.dart';
import 'package:flutter_bloc/flutter_bloc.dart';

abstract class CounterEvent extends Equatable {
  const CounterEvent();

  @override
  List<Object> get props => [];
}

class IncrementEvent extends CounterEvent {}

class DecrementEvent extends CounterEvent {}

class CounterState extends Equatable {
  final int counter;
  const CounterState({@required this.counter});

  @override
  List<Object> get props => [counter];
}

class CounterBloc extends Bloc<CounterEvent, CounterState> {
  CounterBloc() : super(CounterState(counter: 0));

  @override
  Stream<CounterState> mapEventToState(CounterEvent event) async* {
    print(event.toString());
    if (event is IncrementEvent) {
      print("inc");
      yield CounterState(counter: state.counter + 1);
    } else if (state is DecrementEvent) {
      yield CounterState(counter: state.counter - 1);
    }
  }
}

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: BlocProvider<CounterBloc>(
        create: (context) => CounterBloc(),
        child: Home(
          title: "test",
        ),
      ),
    );
  }
}

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

  final String title;

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

class _HomeState extends State<Home> {
  @override
  Widget build(BuildContext context) {
    final CounterBloc counterBloc = BlocProvider.of<CounterBloc>(context);
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: BlocBuilder<CounterBloc, CounterState>(
        //cubit: counterBloc,
        builder: (context, state) {
          return Center(
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
                Text(
                  'You have pushed the button this many times:',
                ),
                Text(
                  state.counter.toString(),
                  style: Theme.of(context).textTheme.headline4,
                ),
              ],
            ),
          );
        },
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          print("click");
          counterBloc.add(IncrementEvent());
        },
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}

Answer 2

你错了

if (state is IncrementEvent) {
  yield CounterState(counter: state.counter + 1);
} else if (state is DecrementEvent) {
  yield CounterState(counter: state.counter - 1);
}

改为

if (event is IncrementEvent) {
  yield CounterState(counter: state.counter + 1);
} else if (event is DecrementEvent) {
  yield CounterState(counter: state.counter - 1);
}

使用 BLoC 模式的 Flutter 默认计数器应用程序

Flutter default counter app using BLoC pattern

flutter

bloc