使用BLOC模式构建Flutter项目(一)

BLoC可视化架构图

image.png

上图展示了数据流如何从UI层到数据层，反之亦然。BLoC不会调用任何UI层的widget。UI层侦听(observe)BLOC类的变化。
1、首先建立一个新项目并删掉main.dart文件中的所有代码，在终端中输入以下代码：

flutter create my_movies

在main.dart文件中输入以下代码：

import 'package:flutter/material.dart';
import 'src/app.dart';

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

在lib包下创建src包，并在src包下创建一个名为app.dart的文件。文件内容如下：

import 'package:flutter/material.dart';
import 'ui/movie_list.dart';

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return MaterialApp(
        theme: ThemeData.dark(),
        home: Scaffold(
          body: MovieList(),
        ),
      );
  }
}

在src包下创建名为blocs,models,resources和ui的包，下图展示项目目录结构：

image.png
blocs存放BLOC相关文件。models存放POJO类或服务端响应的JSON返回值。resources存放repository类和网络调用实现类。ui存放可视化用户界面组件。
在pubspec.yaml里增加rxdart和http两个三方库：
image.png
创建item_model.dart。通过https://javiercbk.github.io/json_to_dart/
把服务响应的JSON数据转为POJO响应model。 内容如下：

class ItemModel {
  int page;
  int totalResults;
  int totalPages;
  List<Results> results;

  ItemModel({this.page, this.totalResults, this.totalPages, this.results});

  ItemModel.fromJson(Map<String, dynamic> json) {
    page = json['page'];
    totalResults = json['total_results'];
    totalPages = json['total_pages'];
    if (json['results'] != null) {
      results = new List<Results>();
      json['results'].forEach((v) {
        results.add(new Results.fromJson(v));
      });
    }
  }

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = new Map<String, dynamic>();
    data['page'] = this.page;
    data['total_results'] = this.totalResults;
    data['total_pages'] = this.totalPages;
    if (this.results != null) {
      data['results'] = this.results.map((v) => v.toJson()).toList();
    }
    return data;
  }
}

class Results {
  int voteCount;
  int id;
  bool video;
  var voteAverage;
  String title;
  double popularity;
  String posterPath;
  String originalLanguage;
  String originalTitle;
  List<int> genreIds;
  String backdropPath;
  bool adult;
  String overview;
  String releaseDate;

  Results(
      {this.voteCount,
      this.id,
      this.video,
      this.voteAverage,
      this.title,
      this.popularity,
      this.posterPath,
      this.originalLanguage,
      this.originalTitle,
      this.genreIds,
      this.backdropPath,
      this.adult,
      this.overview,
      this.releaseDate});

  Results.fromJson(Map<String, dynamic> json) {
    voteCount = json['vote_count'];
    id = json['id'];
    video = json['video'];
    voteAverage = json['vote_average'];
    title = json['title'];
    popularity = json['popularity'];
    posterPath = json['poster_path'];
    originalLanguage = json['original_language'];
    originalTitle = json['original_title'];
    genreIds = json['genre_ids'].cast<int>();
    backdropPath = json['backdrop_path'];
    adult = json['adult'];
    overview = json['overview'];
    releaseDate = json['release_date'];
  }

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = new Map<String, dynamic>();
    data['vote_count'] = this.voteCount;
    data['id'] = this.id;
    data['video'] = this.video;
    data['vote_average'] = this.voteAverage;
    data['title'] = this.title;
    data['popularity'] = this.popularity;
    data['poster_path'] = this.posterPath;
    data['original_language'] = this.originalLanguage;
    data['original_title'] = this.originalTitle;
    data['genre_ids'] = this.genreIds;
    data['backdrop_path'] = this.backdropPath;
    data['adult'] = this.adult;
    data['overview'] = this.overview;
    data['release_date'] = this.releaseDate;
    return data;
  }
}

在resources包中创建名为movie_api_provider.dart文件，内容如下：

import 'dart:async';
import 'package:http/http.dart' show Client;
import 'dart:convert';
import '../models/item_model.dart';

class MovieApiProvider {
  Client client = Client();
  final _apiKey = '802b2c4b88ea1183e50e6b285a27696e';

  Future<ItemModel> fetchMovieList() async {
    print("entered");
    final response = await client
        .get("http://api.themoviedb.org/3/movie/popular?api_key=$_apiKey");
    print(response.body.toString());
    if (response.statusCode == 200) {
      // If the call to the server was successful, parse the JSON
      return ItemModel.fromJson(json.decode(response.body));
    } else {
      // If that call was not successful, throw an error.
      throw Exception('Failed to load post');
    }
  }
}

在resources包中创建名为respository.dart的文件，内容如下：

import 'dart:async';
import 'movie_api_provider.dart';
import '../models/item_model.dart';

class Repository {
  final moviesApiProvider = MovieApiProvider();

  Future<ItemModel> fetchAllMovies() => moviesApiProvider.fetchMovieList();
}

导入movie_api_provider.dart文件并调用fetchMovieList()方法。Respository类将为BLoC提供数据流出点。
在blocs包中创建movies_bloc.dart文件，内容如下:

import '../resources/repository.dart';
import 'package:rxdart/rxdart.dart';
import '../models/item_model.dart';

class MoviesBloc {
  final _repository = Repository();
  final _moviesFetcher = PublishSubject<ItemModel>();

  Observable<ItemModel> get allMovies => _moviesFetcher.stream;

  fetchAllMovies() async {
    ItemModel itemModel = await _repository.fetchAllMovies();
    _moviesFetcher.sink.add(itemModel);
  }

  dispose() {
    _moviesFetcher.close();
  }
}

final bloc = MoviesBloc();

在ui包中创建movie_list.dart文件。内容如下：

import 'package:flutter/material.dart';
import '../models/item_model.dart';
import '../blocs/movies_bloc.dart';

class MovieList extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    bloc.fetchAllMovies();
    return Scaffold(
      appBar: AppBar(
        title: Text('Popular Movies'),
      ),
      body: StreamBuilder(
        stream: bloc.allMovies,
        builder: (context, AsyncSnapshot<ItemModel> snapshot) {
          if (snapshot.hasData) {
            return buildList(snapshot);
          } else if (snapshot.hasError) {
            return Text(snapshot.error.toString());
          }
          return Center(child: CircularProgressIndicator());
        },
      ),
    );
  }

  Widget buildList(AsyncSnapshot<ItemModel> snapshot) {
    return GridView.builder(
        itemCount: snapshot.data.results.length,
        gridDelegate:
            new SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 2),
        itemBuilder: (BuildContext context, int index) {
          return Image.network(
            'https://image.tmdb.org/t/p/w185${snapshot.data
                .results[index].posterPath}',
            fit: BoxFit.cover,
          );
        });
  }
}

运行结果如下图:

image.png