【发布时间】：2020-03-26 10:49:28
【问题描述】：
我正在尝试构建一个应用程序，该应用程序可以从 ListView 中的博客加载无穷无尽的提要。在顶部，用户可以通过“类别”菜单选择根据某个类别过滤提要。当用户点击“类别”菜单时，会出现另一个 ListView，其中包含所有可用类别。当用户点击所需的类别时，应用程序应返回到提要 ListView 仅显示该类别下的帖子。
预期结果：
- 应用调用 API 并检索 10 个最新帖子
- 随着用户滚动，接下来的 10 个帖子将通过连续的 API 调用检索
- 用户点击“类别”菜单并打开带有类别的 ListView。
- 用户点击所需的类别，应用程序返回到提要列表视图，创建一个 API 调用以检索该类别的前 10 个帖子。
- 当用户滚动时，该类别的下 10 个帖子将通过连续 API 检索 来电。
观察结果：
- 应用调用 API 并检索 10 个最新帖子
- 随着用户滚动，接下来的 10 个帖子将通过连续的 API 调用检索
- 用户点击“类别”菜单并打开带有类别的 ListView。
- 用户点击所需的类别，应用程序返回到提要列表视图，创建一个 API 调用以检索该类别的前 10 个帖子。
- 所需类别的帖子被附加到 ListView 并且仅出现在帖子之后 之前已加载。
我的问题：
我必须如何修改我的状态或我的 Bloc，才能获得想要的结果？
相关截图
我的结构：
PostBloc - 我的 bloc 组件，其中包含 Articles 和 ArticleCategory StreamBuilders 的流定义。还包含对 API 进行调用的方法 获取文章和文章类别。
class PostBloc extends Bloc<PostEvent, PostState> {
final http.Client httpClient;
int _currentPage = 1;
int _limit = 10;
int _totalResults = 0;
int _numberOfPages = 0;
int _categoryId;
bool hasReachedMax = false;
var cachedData = new Map<int, Article>();
PostBloc({@required this.httpClient}) {
//Listen to when user taps a category in the ArticleCategory ListView
_articleCategoryController.stream.listen((articleCategory) {
if (articleCategory.id != null) {
_categoryId = articleCategory.id;
_articlesSubject.add(UnmodifiableListView(null));
_currentPage = 1;
_fetchPosts(_currentPage, _limit, _categoryId)
.then((articles) {
_articlesSubject.add(UnmodifiableListView(articles));
});
_currentPage++;
dispatch(Fetch());
}
});
_currentPage++;
}
List<Article> _articles = <Article>[];
// Category Sink for listening to the tapped category
final _articleCategoryController = StreamController<ArticleCategory>();
Sink<ArticleCategory> get getArticleCategory =>
_articleCategoryController.sink;
//Article subject for populating articles ListView
Stream<UnmodifiableListView<Article>> get articles => _articlesSubject.stream;
final _articlesSubject = BehaviorSubject<UnmodifiableListView<Article>>();
//Categories subjet for the article categories
Stream<UnmodifiableListView<ArticleCategory>> get categories => _categoriesSubject.stream;
final _categoriesSubject = BehaviorSubject<UnmodifiableListView<ArticleCategory>>();
void dispose() {
_articleCategoryController.close();
}
@override
Stream<PostState> transform(
Stream<PostEvent> events,
Stream<PostState> Function(PostEvent event) next,
) {
return super.transform(
(events as Observable<PostEvent>).debounceTime(
Duration(milliseconds: 500),
),
next,
);
}
@override
get initialState => PostUninitialized();
@override
Stream<PostState> mapEventToState(PostEvent event) async* {
//This event is triggered when user taps on categories menu
if (event is ShowCategory) {
_currentPage = 1;
await _fetchCategories(_currentPage, _limit).then((categories) {
_categoriesSubject.add(UnmodifiableListView(categories));
});
yield PostCategories();
}
// This event is triggered when user taps on a category
if(event is FilterCategory){
yield PostLoaded(hasReachedMax: false);
}
// This event is triggered when app loads and when user scrolls to the bottom of articles
if (event is Fetch && !_hasReachedMax(currentState)) {
try {
//First time the articles feed opens
if (currentState is PostUninitialized) {
_currentPage = 1;
await _fetchPosts(_currentPage, _limit).then((articles) {
_articlesSubject.add(UnmodifiableListView(articles)); //Send to stream
});
this.hasReachedMax = false;
yield PostLoaded(hasReachedMax: false);
_currentPage++;
return;
}
//User scrolls to bottom of ListView
if (currentState is PostLoaded) {
await _fetchPosts(_currentPage, _limit, _categoryId)
.then((articles) {
_articlesSubject.add(UnmodifiableListView(articles));//Append to stream
});
_currentPage++;
// Check if last page has been reached or not
if(_currentPage > _numberOfPages){
this.hasReachedMax = true;
}
else{
this.hasReachedMax = false;
}
yield (_currentPage > _numberOfPages)
? (currentState as PostLoaded).copyWith(hasReachedMax: true)
: PostLoaded(
hasReachedMax: false,
);
}
} catch (e) {
print(e.toString());
yield PostError();
}
}
}
bool _hasReachedMax(PostState state) =>
state is PostLoaded && this.hasReachedMax;
Article _getArticle(int index) {
if (cachedData.containsKey(index)) {
Article data = cachedData[index];
return data;
}
throw Exception("Article could not be fetched");
}
/**
* Fetch all articles
*/
Future<List<Article>> _fetchPosts(int startIndex, int limit,
[int categoryId]) async {
String query =
'https://www.batatolandia.de/api/batatolandia/articles?page=$startIndex&limit=$limit';
if (categoryId != null) {
query += '&category_id=$categoryId';
}
final response = await httpClient.get(query);
if (response.statusCode == 200) {
final data = json.decode(response.body);
ArticlePagination res = ArticlePagination.fromJson(data);
_totalResults = res.totalResults;
_numberOfPages = res.numberOfPages;
for (int i = 0; i < res.data.length; i++) {
_articles.add(res.data[i]);
}
return _articles;
} else {
throw Exception('error fetching posts');
}
}
/**
* Fetch article categories
*/
Future<List<ArticleCategory>> _fetchCategories(int startIndex, int limit,
[int categoryId]) async {
String query =
'https://www.batatolandia.de/api/batatolandia/articles/categories?page=$startIndex&limit=$limit';
final response = await httpClient.get(query);
if (response.statusCode == 200) {
final data = json.decode(response.body);
ArticleCategoryPagination res = ArticleCategoryPagination.fromJson(data);
_totalResults = res.totalResults;
_numberOfPages = res.numberOfPages;
List<ArticleCategory> categories = <ArticleCategory>[];
categories.add(ArticleCategory(id: 0 , title: 'Todos', color: '#000000'));
for (int i = 0; i < res.data.length; i++) {
categories.add(res.data[i]);
}
return categories;
} else {
throw Exception('error fetching categories');
}
}
}
Articles - 包含一个 BlocProvider 来读取 PostBloc 中设置的当前状态并显示 对应的视图。
class Articles extends StatelessWidget{
PostBloc _postBloc;
@override
Widget build(BuildContext context) {
return BlocProvider(
builder: (context) =>
PostBloc(httpClient: http.Client())..dispatch(Fetch()),
child: BlocBuilder<PostBloc, PostState>(
builder: (context, state){
_postBloc = BlocProvider.of<PostBloc>(context);
// Displays circular progress indicator while posts are being retrieved
if (state is PostUninitialized) {
return Center(
child: CircularProgressIndicator(),
);
}
// Shows the feed Listview when API responds with the posts data
if (state is PostLoaded) {
return ArticlesList(postBloc:_postBloc );
}
// Shows the Article categories Listview when user clicks on menu
if(state is PostCategories){
return ArticlesCategoriesList(postBloc: _postBloc);
}
//Shows error if there are any problems while fetching posts
if (state is PostError) {
return Center(
child: Text('Failed to fetch posts'),
);
}
return null;
}
)
);
}
}
ArticlesList - 包含一个 StreamBuilder，它从 PostBloc 读取文章数据并加载到提要 ListView 中。
class ArticlesList extends StatelessWidget {
ScrollController _scrollController = new ScrollController();
int currentPage = 1;
int _limit = 10;
int totalResults = 0;
int numberOfPages = 0;
final _scrollThreshold = 200.0;
Completer<void> _refreshCompleter;
PostBloc postBloc;
ArticlesList({Key key, this.postBloc}) : super(key: key);
@override
Widget build(BuildContext context) {
_scrollController.addListener(_onScroll);
_refreshCompleter = Completer<void>();
return Scaffold(
appBar: AppBar(
title: Text("Posts"),
),
body: StreamBuilder<UnmodifiableListView<Article>>(
stream: postBloc.articles,
initialData: UnmodifiableListView<Article>([]),
builder: (context, snapshot) {
if(snapshot.hasData && snapshot != null) {
if(snapshot.data.length > 0){
return Column(
mainAxisSize: MainAxisSize.max,
children: <Widget>[
ArticlesFilterBar(),
Expanded(
child: RefreshIndicator(
child: ListView.builder(
itemBuilder: (BuildContext context,
int index) {
return index >= snapshot.data.length
? BottomLoader()
: ArticlesListItem(
article: snapshot.data.elementAt(
index));
},
itemCount: postBloc.hasReachedMax
? snapshot.data.length
: snapshot.data.length + 1,
controller: _scrollController,
),
onRefresh: _refreshList,
),
)
],
);
}
else if (snapshot.data.length==0){
return Center(
child: CircularProgressIndicator(),
);
}
}
else{
Text("Error!");
}
return CircularProgressIndicator();
}
)
);
}
@override
void dispose() {
_scrollController.dispose();
}
void _onScroll() {
final maxScroll = _scrollController.position.maxScrollExtent;
final currentScroll = _scrollController.position.pixels;
if (maxScroll - currentScroll <= _scrollThreshold) {
postBloc.dispatch(Fetch());
}
}
Future<void> _refreshList() async {
postBloc.dispatch(Fetch());
return null;
}
}
ArticlesCategoriesList - 一个 StreamBuilder，它从 PostBloc 读取类别并加载到 ListView。
class ArticlesCategoriesList extends StatelessWidget {
PostBloc postBloc;
ArticlesCategoriesList({Key key, this.postBloc}) : super(key: key);
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text("Categorias"),
),
body:
SafeArea(
child: StreamBuilder<UnmodifiableListView<ArticleCategory>>(
stream: postBloc.categories,
initialData: UnmodifiableListView<ArticleCategory>([]),
builder: (context, snapshot) {
return ListView.separated(
itemBuilder: (BuildContext context, int index) {
return new Container(
decoration: new BoxDecoration(
color: Colors.white,
),
child: ListTile(
dense: true,
leading: Icon(Icons.fiber_manual_record,color: HexColor(snapshot.data[index].color)),
trailing: Icon(Icons.keyboard_arrow_right),
title: Text(snapshot.data[index].title),
onTap: () {
postBloc.getArticleCategory.add(snapshot.data[index]);
},
));
},
separatorBuilder: (context, index) => Divider(
color: Color(0xff666666),
height: 1,
),
itemCount: snapshot.data.length);
},
)));
}
}
【问题讨论】：
-
所有这些代码？伙计，你需要转行。