使用BLOC模式构建Flutter项目(二)

1、首先解决上一章的两个缺陷，第一个是在MoviesBloc类里建立的dispose()方法来关闭streams以避免内存泄露，但未在任何地方引用所以会导致内存泄露。另一个缺陷是把网络调用放在build方法里相当危险。
当前MovieList类是一个StatelessWidget，所有属性生成后不可变更，因此不适合网络请求，也不能调用bloc的dispose方法，因此转换访类为StatfulWidget，把网络调用放在initState()里，bloc的dispose()放入StatefulWidget的dispose()中。
用以下代码替换当前movie_list.dart代码：

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

class MovieList extends StatefulWidget {
  @override
  State createState() {
    return MovieListState();
  }
}

class MovieListState extends State {
  @override
  void initState() {
    super.initState();
    bloc.fetchAllMovies();
  }

  @override
  void dispose() {
    bloc.dispose();
    super.dispose();
  }

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

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

上面代码把bloc.fetchAllMovies()调用放入了initState()，block.dispose()放入了MovieListState类的dispose()中。运行该程序不会发生改变，但也不会发生多次网络请求和内存泄露。
提示：任何网络和数据库调用都不要放到build方法类中。
完成后进行app的下一步设计流程

image.png

流程使用述语：
1）、Movie List Screen：可视化用户界面看所有图片的grid list。
2）、Movie List Bloc：从repository获取数据后传送到Movie List Screen的桥。
3）、Movie Detail Screen：从主页跳转明细页面。
4）、Repository：数据流控制点。
5）、API provider：网络调用接口。
2、单例(Single Instance)和作用域实例(Scoped Instance)
可以通过两种方式（单例和作用域实例）把BLoC类引入各自的用户界面，单例可以在app里的任何部分进行引用；而作用域实例被限制访问仅限关联用户界面引用，如下图：

image.png

如上图，把BLoC放入InheritedWidget，然后包装用户界面其中的widget即可访问BLoC，无上级节点不可以访问BLoC。
增加detail用户界面
在ui包下创建movie_detail.dart，内容如下：

import 'package:flutter/material.dart';

class MovieDetail extends StatefulWidget {
  final posterUrl;
  final description;
  final releaseDate;
  final String title;
  final String voteAverage;
  final int movieId;

  MovieDetail({
    this.title,
    this.posterUrl,
    this.description,
    this.releaseDate,
    this.voteAverage,
    this.movieId,
  });

  @override
  State createState() {
    return MovieDetailState(
      title: title,
      posterUrl: posterUrl,
      description: description,
      releaseDate: releaseDate,
      voteAverage: voteAverage,
      movieId: movieId,
    );
  }
}

class MovieDetailState extends State {
  final posterUrl;
  final description;
  final releaseDate;
  final String title;
  final String voteAverage;
  final int movieId;

  MovieDetailState({
    this.title,
    this.posterUrl,
    this.description,
    this.releaseDate,
    this.voteAverage,
    this.movieId,
  });

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: SafeArea(
        top: false,
        bottom: false,
        child: NestedScrollView(
          headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
            return [
              SliverAppBar(
                expandedHeight: 200.0,
                floating: false,
                pinned: true,
                elevation: 0.0,
                flexibleSpace: FlexibleSpaceBar(
                    background: Image.network(
                  "https://image.tmdb.org/t/p/w500$posterUrl",
                  fit: BoxFit.cover,
                )),
              ),
            ];
          },
          body: Padding(
            padding: const EdgeInsets.all(10.0),
            child: Column(
              crossAxisAlignment: CrossAxisAlignment.start,
              children: [
                Container(margin: EdgeInsets.only(top: 5.0)),
                Text(
                  title,
                  style: TextStyle(
                    fontSize: 25.0,
                    fontWeight: FontWeight.bold,
                  ),
                ),
                Container(margin: EdgeInsets.only(top: 8.0, bottom: 8.0)),
                Row(
                  children: [
                    Icon(
                      Icons.favorite,
                      color: Colors.red,
                    ),
                    Container(
                      margin: EdgeInsets.only(left: 1.0, right: 1.0),
                    ),
                    Text(
                      voteAverage,
                      style: TextStyle(
                        fontSize: 18.0,
                      ),
                    ),
                    Container(
                      margin: EdgeInsets.only(left: 10.0, right: 10.0),
                    ),
                    Text(
                      releaseDate,
                      style: TextStyle(
                        fontSize: 18.0,
                      ),
                    ),
                  ],
                ),
                Container(margin: EdgeInsets.only(top: 8.0, bottom: 8.0)),
                Text(description),
              ],
            ),
          ),
        ),
      ),
    );
  }
}

3、导航
movie_list.dart内容变更如下：

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

class MovieList extends StatefulWidget {
  @override
  State createState() {
    return MovieListState();
  }
}

class MovieListState extends State {
  @override
  void initState() {
    super.initState();
    bloc.fetchAllMovies();
  }

  @override
  void dispose() {
    bloc.dispose();
    super.dispose();
  }

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

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

  openDetailPage(ItemModel data, int index) {
    Navigator.push(
      context,
      MaterialPageRoute(builder: (context) {
        return MovieDetail(
          title: data.results[index].title,
          posterUrl: data.results[index].backdropPath,
          description: data.results[index].overview,
          releaseDate: data.results[index].releaseDate,
          voteAverage: data.results[index].voteAverage.toString(),
          movieId: data.results[index].id,
        );
      }),
    );
  }
}

下一步为明细页面加入预告片，通过下面的链接获取JSON响应文本：
https://api.themoviedb.org/3/movie/299536/videos?api_key=802b2c4b88ea1183e50e6b285a27696e
首先建立POJO类，在models包中创建trailer_model.dart，代码如下：

class TrailerModel  {
  int id;
  List results;

  TrailerModel({this.id, this.results});

  TrailerModel.fromJson(Map json) {
    id = json['id'];
    if (json['results'] != null) {
      results = new List();
      json['results'].forEach((v) {
        results.add(new Results.fromJson(v));
      });
    }
  }

  Map toJson() {
    final Map data = new Map();
    data['id'] = this.id;
    if (this.results != null) {
      data['results'] = this.results.map((v) => v.toJson()).toList();
    }
    return data;
  }
}

class Results {
  String id;
  String iso6391;
  String iso31661;
  String key;
  String name;
  String site;
  int size;
  String type;

  Results(
      {this.id,
      this.iso6391,
      this.iso31661,
      this.key,
      this.name,
      this.site,
      this.size,
      this.type});

  Results.fromJson(Map json) {
    id = json['id'];
    iso6391 = json['iso_639_1'];
    iso31661 = json['iso_3166_1'];
    key = json['key'];
    name = json['name'];
    site = json['site'];
    size = json['size'];
    type = json['type'];
  }

  Map toJson() {
    final Map data = new Map();
    data['id'] = this.id;
    data['iso_639_1'] = this.iso6391;
    data['iso_3166_1'] = this.iso31661;
    data['key'] = this.key;
    data['name'] = this.name;
    data['site'] = this.site;
    data['size'] = this.size;
    data['type'] = this.type;
    return data;
  }
}

创建movie_api_provider.dart实现(implement)网络调用请求，内容如下：

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

class MovieApiProvider {
  Client client = Client();
  final _apiKey = '802b2c4b88ea1183e50e6b285a27696e';
  final _baseUrl = "http://api.themoviedb.org/3/movie";

  Future fetchMovieList() async {
    final response = await client.get("$_baseUrl/popular?api_key=$_apiKey");
    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');
    }
  }

  Future fetchTrailer(int movieId) async {
    final response =
        await client.get("$_baseUrl/$movieId/videos?api_key=$_apiKey");

    if (response.statusCode == 200) {
      return TrailerModel.fromJson(json.decode(response.body));
    } else {
      throw Exception('Failed to load trailers');
    }
  }
}

fetchTrailer(movie_id)方法作用是转换网络请求响应JSON文本到TrailerModel对象，并返回Future。
更新repository.dart文件，增加新的网络实现，代码如下：

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

class Repository {
  final moviesApiProvider = MovieApiProvider();

  Future fetchAllMovies() => moviesApiProvider.fetchMovieList();

  Future fetchTrailers(int movieId) => moviesApiProvider.fetchTrailer(movieId);
}

接下来实现BLoC作用域实例，在blocs包中创建movie_detail_bloc.dart和movie_detail_bloc_provider.dart文件。
movie_detail_bloc_provider.dart代码如下:

import 'package:flutter/material.dart';
import 'movie_detail_bloc.dart';
export 'movie_detail_bloc.dart';

class MovieDetailBlocProvider extends InheritedWidget {
  final MovieDetailBloc bloc;

  MovieDetailBlocProvider({Key key, Widget child})
      : bloc = MovieDetailBloc(),
        super(key: key, child: child);

  @override
  bool updateShouldNotify(_) {
    return true;
  }

  static MovieDetailBloc of(BuildContext context) {
    return (context.inheritFromWidgetOfExactType(MovieDetailBlocProvider)
            as MovieDetailBlocProvider).bloc;
  }
}

这个类通过继承InheritedWidget，通过of(context)访问bloc。context作为of(context)属于InheritedWidget。
movie_detail_bloc.dart代码如下：

import 'dart:async';

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

class MovieDetailBloc {
  final _repository = Repository();
  final _movieId = PublishSubject();
  final _trailers = BehaviorSubject>();

  Function(int) get fetchTrailersById => _movieId.sink.add;
  Observable> get movieTrailers => _trailers.stream;

  MovieDetailBloc() {
    _movieId.stream.transform(_itemTransformer()).pipe(_trailers);
  }

  dispose() async {
    _movieId.close();
    await _trailers.drain();
    _trailers.close();
  }

  _itemTransformer() {
    return ScanStreamTransformer(
      (Future trailer, int id, int index) {
        print(index);
        trailer = _repository.fetchTrailers(id);
        return trailer;
      },
    );
  }
}

上面代码是通过movieId获取预告片API内容，实现该功能需要使用RxDart的Transformers功能。
Transformers主要是通过两个或多个Subjects获取最终结果。意思就是在对一些数据操作后把从一个Subject数据传递到另一个。
在此应用程序中movieId增加到名为_movieId的PublishSubject，然后把movieId送到ScanStreamTransformer中调用预告片API，并把返回结果通过pipe发送到_trailers的BehaviorSubject。如下图所示：

image.png

最后一步是使movieDetailBloc访问MovieDetail。更新movie_list.dart的openDetailPage()方法，代码如下：

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

class MovieList extends StatefulWidget {
  @override
  State createState() {
    return MovieListState();
  }
}

class MovieListState extends State {
  @override
  void initState() {
    super.initState();
    bloc.fetchAllMovies();
  }

  @override
  void dispose() {
    bloc.dispose();
    super.dispose();
  }

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

  Widget buildList(AsyncSnapshot snapshot) {
    return GridView.builder(
        itemCount: snapshot.data.results.length,
        gridDelegate:
        new SliverGridDelegateWithFixedCrossAxisCount(crossAxisCount: 2),
        itemBuilder: (BuildContext context, int index) {
          return GridTile(
            child: InkResponse(
              enableFeedback: true,
              child: Image.network(
                'https://image.tmdb.org/t/p/w185${snapshot.data
                    .results[index].poster_path}',
                fit: BoxFit.cover,
              ),
              onTap: () => openDetailPage(snapshot.data, index),
            ),
          );
        });
  }

  openDetailPage(ItemModel data, int index) {
    Navigator.push(
      context,
      MaterialPageRoute(builder: (context) {
        return MovieDetailBlocProvider(
          child: MovieDetail(
            title: data.results[index].title,
            posterUrl: data.results[index].backdrop_path,
            description: data.results[index].overview,
            releaseDate: data.results[index].release_date,
            voteAverage: data.results[index].vote_average.toString(),
            movieId: data.results[index].id,
          ),
        );
      }),
    );
  }
}

代码中MaterialPageRoute返回MovieDetailBlocProvider(InheritedWidget)，其中包含子组件MovieDetail。因此MovieDetailBloc类能访问明细页面和其下所有组件。
最后，修改movie_detail.dart文件，代码如下：

import 'dart:async';

import 'package:flutter/material.dart';
import '../blocs/movie_detail_bloc_provider.dart';
import '../models/trailer_model.dart';

class MovieDetail extends StatefulWidget {
  final posterUrl;
  final description;
  final releaseDate;
  final String title;
  final String voteAverage;
  final int movieId;

  MovieDetail({
    this.title,
    this.posterUrl,
    this.description,
    this.releaseDate,
    this.voteAverage,
    this.movieId,
  });

  @override
  State createState() {
    return MovieDetailState(
      title: title,
      posterUrl: posterUrl,
      description: description,
      releaseDate: releaseDate,
      voteAverage: voteAverage,
      movieId: movieId,
    );
  }
}

class MovieDetailState extends State {
  final posterUrl;
  final description;
  final releaseDate;
  final String title;
  final String voteAverage;
  final int movieId;

  MovieDetailBloc bloc;

  MovieDetailState({
    this.title,
    this.posterUrl,
    this.description,
    this.releaseDate,
    this.voteAverage,
    this.movieId,
  });

  @override
  void didChangeDependencies() {
    bloc = MovieDetailBlocProvider.of(context);
    bloc.fetchTrailersById(movieId);
    super.didChangeDependencies();
  }

  @override
  void dispose() {
    bloc.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: SafeArea(
        top: false,
        bottom: false,
        child: NestedScrollView(
          headerSliverBuilder: (BuildContext context,
              bool innerBoxIsScrolled) {
            return [
              SliverAppBar(
                expandedHeight: 200.0,
                floating: false,
                pinned: true,
                elevation: 0.0,
                flexibleSpace: FlexibleSpaceBar(
                    background: Image.network(
                      "https://image.tmdb.org/t/p/w500$posterUrl",
                      fit: BoxFit.cover,
                    )),
              ),
            ];
          },
          body: Padding(
            padding: const EdgeInsets.all(10.0),
            child: Column(
              crossAxisAlignment: CrossAxisAlignment.start,
              children: [
                Container(margin: EdgeInsets.only(top: 5.0)),
                Text(
                  title,
                  style: TextStyle(
                    fontSize: 25.0,
                    fontWeight: FontWeight.bold,
                  ),
                ),
                Container(margin: EdgeInsets.only(top: 8.0,
                    bottom: 8.0)),
                Row(
                  children: [
                    Icon(
                      Icons.favorite,
                      color: Colors.red,
                    ),
                    Container(
                      margin: EdgeInsets.only(left: 1.0,
                          right: 1.0),
                    ),
                    Text(
                      voteAverage,
                      style: TextStyle(
                        fontSize: 18.0,
                      ),
                    ),
                    Container(
                      margin: EdgeInsets.only(left: 10.0,
                          right: 10.0),
                    ),
                    Text(
                      releaseDate,
                      style: TextStyle(
                        fontSize: 18.0,
                      ),
                    ),
                  ],
                ),
                Container(margin: EdgeInsets.only(top: 8.0,
                    bottom: 8.0)),
                Text(description),
                Container(margin: EdgeInsets.only(top: 8.0,
                    bottom: 8.0)),
                Text(
                  "Trailer",
                  style: TextStyle(
                    fontSize: 25.0,
                    fontWeight: FontWeight.bold,
                  ),
                ),
                Container(margin: EdgeInsets.only(top: 8.0,
                    bottom: 8.0)),
                StreamBuilder(
                  stream: bloc.movieTrailers,
                  builder:
                      (context, AsyncSnapshot> snapshot) {
                    if (snapshot.hasData) {
                      return FutureBuilder(
                        future: snapshot.data,
                        builder: (context,
                            AsyncSnapshot itemSnapShot) {
                          if (itemSnapShot.hasData) {
                            if (itemSnapShot.data.results.length > 0)
                              return trailerLayout(itemSnapShot.data);
                            else
                              return noTrailer(itemSnapShot.data);
                          } else {
                            return Center(child: CircularProgressIndicator());
                          }
                        },
                      );
                    } else {
                      return Center(child: CircularProgressIndicator());
                    }
                  },
                ),
              ],
            ),
          ),
        ),
      ),
    );
  }

  Widget noTrailer(TrailerModel data) {
    return Center(
      child: Container(
        child: Text("No trailer available"),
      ),
    );
  }

  Widget trailerLayout(TrailerModel data) {
    if (data.results.length > 1) {
      return Row(
        children: [
          trailerItem(data, 0),
          trailerItem(data, 1),
        ],
      );
    } else {
      return Row(
        children: [
          trailerItem(data, 0),
        ],
      );
    }
  }

  trailerItem(TrailerModel data, int index) {
    return Expanded(
      child: Column(
        children: [
          Container(
            margin: EdgeInsets.all(5.0),
            height: 100.0,
            color: Colors.grey,
            child: Center(child: Icon(Icons.play_circle_filled)),
          ),
          Text(
            data.results[index].name,
            maxLines: 1,
            overflow: TextOverflow.ellipsis,
          ),
        ],
      ),
    );
  }
}