Flutter - 初嘗BloC

BLoC Example - Counter

使用Library - flutter_bloc
此筆記不同於官方的Counter例子，使用較完整的結構
如果有誤解，請指正，我也是新手!
Source Code

2019-12-26_20-30-48

我理解的流程

Flutter_bloc

安裝插件

Bloc Code Generator，這個插件可以幫助我們生成flutter_bloc所需的檔案

生成步驟:

-w784

-w360

插件會生成會生成以下檔案

bloc
 ├── counter_bloc.dart  // 所有business logic, 例如加數，減數
 ├── counter_state.dart // 所有state(狀態), 例如Added(已加), Decreased(已減)
 ├── counter_event.dart // 所有event(動作), 例如Add(我要加), Remove(我要減)
 └── bloc.dart //不用改，只是export以上3個檔，使用的時候import這個檔案就好

---

定義BLoC

counter_event.dart

簡單定義了三個所需的event Add, Remove 和 Refresh

abstract class CounterEvent {}

class Add extends CounterEvent {}

class Remove extends CounterEvent {}

class Refresh extends CounterEvent {}

counter_state.dart

定義了4個State(狀態), Initial, Added, Decreased 和 Reset
一開始定義了count 是為了在wight可以很方便獲得count

abstract class CounterState {
  int count;

  CounterState(this.count);
}

class Initial extends CounterState {
  Initial(int count) : super(count);
}

class Added extends CounterState {
  Added(int count) : super(count);
}

class Decreased extends CounterState {
  Decreased(int count) : super(count);
}

class Reset extends CounterState {
  Reset(int count) : super(count);
}

counter_bloc.dart

這裡處理所有business logic

import 'package:bloc/bloc.dart';
import './bloc.dart';

class CounterBloc extends Bloc {
  int _initCount = 0;

  //一開始必須定義初始化的State(狀態)
  @override
  CounterState get initialState => Initial(_initCount);

  @override
  Stream mapEventToState(
    CounterEvent event,
  ) async* {
    //如果是event是`Add`, 將count+1 再用相應的State 傳出去
    if (event is Add) {
      _initCount++;
      yield Added(_initCount);
    } else if (event is Remove) {
      _initCount--;
      yield Decreased(_initCount);
    } else if (event is Refresh) {
      _initCount = 0;
      yield Reset(_initCount);
    }
  }
}

---

介面

BlocProvider --包住-> BlocBuilder

Flutter_bloc -2-

main.dart

import 'dart:developer';
import 'package:flutter/material.dart';
import 'package:flutter_bloc/flutter_bloc.dart';
import 'package:try_count/bloc/bloc.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,
      ),
      home: BlocProvider(
          create: (context) => CounterBloc(),
          child: MyBLoCHomePage(title: 'Flutter Demo Home Page')),
    );
  }
}

class MyBLoCHomePage extends StatelessWidget {
  final String title;

  const MyBLoCHomePage({Key key, this.title}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(title),
      ),
      body: Center(
        child:
            BlocBuilder(
                builder: (context, state) {
          return Column(
            mainAxisSize: MainAxisSize.min,
            children: [
              Text(
                state.toString(),
              ),
              Text(
                state.count.toString(),
                style: Theme.of(context).textTheme.display1,
              ),
            ],
          );
        }),
      ),
      floatingActionButton: Stack(
        children: [
          Align(
            alignment: Alignment.bottomRight,
            child: BlocBuilder(
              builder: (context, state) {
                CounterBloc counterBloc = BlocProvider.of(context);
                return Column(
                  mainAxisSize: MainAxisSize.min,
                  children: [
                    Padding(
                      padding: const EdgeInsets.all(5.0),
                      child: FloatingActionButton(
                        onPressed: () {
                          counterBloc.add(Add());
                        },
                        tooltip: 'Increment',
                        child: Icon(Icons.add),
                      ),
                    ),
                    Padding(
                      padding: const EdgeInsets.all(5.0),
                      child: FloatingActionButton(
                        onPressed: () {
                          counterBloc.add(Remove());
                        },
                        tooltip: 'Increment',
                        child: Icon(Icons.remove),
                      ),
                    ),
                    Padding(
                      padding: const EdgeInsets.all(5.0),
                      child: FloatingActionButton(
                        onPressed: () {
                          counterBloc.add(Refresh());
                        },
                        tooltip: 'Increment',
                        child: Icon(Icons.refresh),
                      ),
                    ),
                  ],
                );
              },
            ),
          ),
        ],
      ),
    );
  }
}