上篇也说过,Redux 的好处之一就是状态管理的形式是统一的,三个元素 Action、Store 和 Reducer 缺一不可,因此,我们先来梳理联系人列表业务中对应的这三个元素的内容。
首先来定义 Action,列表页面交互上会涉及 2 个Action,刷新和加载更多。但逻辑上还有另外两个动作:获取数据成功和获取数据失败,因此一共有 4 个Action。
刷新:获取第一页的数据,定义为 RefreshAction,在交互时使用下来刷新时调度该 Action。class RefreshAction {} class LoadAction {} class SuccessAction { final List<dynamic> jsonItems; final int currentPage; SuccessAction(this.jsonItems, this.currentPage); } class FailedAction { final String errorMessage; FailedAction(this.errorMessage); }接下来是 Store的状态对象,我们要明确需要哪些数据。首先肯定的是,需要有网络请求成功后的联系人列表数据;其次是当前请求的页码,我们在加载更多的时候需要根据该页面请求下一页数据;之后是 Loading 状态标记和错误信息,Loading 状态标记在某些场合可以用于提示,而错误信息则用于错误提醒。因此,Store 对应的状态数据有:
class ContactorState { final List<dynamic> contactors; final isLoading; final String? errorMessage; final int currentPage; ContactorState(this.contactors, {this.isLoading = false, this.errorMessage, this.currentPage = 1}); factory ContactorState.initial() => ContactorState(List.unmodifiable([])); }最后是 Reducer 了,Reducer 定义是一个函数,根据旧的状态对象和当前的 Action 来返回新的状态对象。这里的业务逻辑如下:
// 堆代码 duidaima.com ContactorState contactorReducer(ContactorState state, dynamic action) { if (action is RefreshAction) { ContactorState contactorState = ContactorState(state.contactors, isLoading: true, errorMessage: null, currentPage: 1); return contactorState; } if (action is LoadAction) { ContactorState contactorState = ContactorState(state.contactors, isLoading: true, errorMessage: null, currentPage: state.currentPage + 1); return contactorState; } if (action is SuccessAction) { int currentPage = action.currentPage; List<dynamic> contactors = state.contactors; if (currentPage > 1) { contactors += action.jsonItems; } else { contactors = action.jsonItems; } ContactorState contactorState = ContactorState(contactors, isLoading: false, errorMessage: null, currentPage: currentPage); return contactorState; } if (action is FailedAction) { ContactorState contactorState = ContactorState( state.contactors, isLoading: false, errorMessage: action.errorMessage, ); return contactorState; } return state; }
void (Store<T> store, action, NextDispatcher next)在这里,我们定义的中间件方法名为:fetchContactorMiddleware,需要在构建 Store 对象时加入到 middleware 参数中。middleware本身是一个数组,因此我们可以添加多种中间件,以便进行不同的处理。
final Store<ContactorState> store = Store( contactorReducer, initialState: ContactorState.initial(), middleware: [ fetchContactorMiddleware, ], );在中间件中我们可以获取到当前的 Action和状态,因此可以根据 Action 做不同的业务。在这里我们只需要处理刷新和加载更多:
void fetchContactorMiddleware( Store<ContactorState> store, action, NextDispatcher next) { const int pageSize = 10; if (action is RefreshAction) { // 刷新取第一页数据 ContactorService.list(1, pageSize).then((response) { if (response != null && response.statusCode == 200) { store.dispatch(SuccessAction(response.data, 1)); } else { store.dispatch(FailedAction('请求失败')); } }).catchError((error, trace) { store.dispatch(FailedAction(error.toString())); }); } if (action is LoadAction) { // 加载更多时页码+1 int currentPage = store.state.currentPage + 1; ContactorService.list(currentPage, pageSize).then((response) { if (response != null && response.statusCode == 200) { store.dispatch(SuccessAction(response.data, currentPage)); } else { store.dispatch(FailedAction('请求失败')); } }).catchError((error, trace) { store.dispatch(FailedAction(error.toString())); }); } next(action); }
class _ViewModel { final List<_ContactorViewModel> contactors; _ViewModel(this.contactors); factory _ViewModel.create(Store<ContactorState> store) { List<_ContactorViewModel> items = store.state.contactors .map((dynamic item) => _ContactorViewModel.fromJson(item)) .toList(); return _ViewModel(items); } } class _ContactorViewModel { final String followedUserId; final String nickname; final String avatar; final String description; _ContactorViewModel({ required this.followedUserId, required this.nickname, required this.avatar, required this.description, }); static _ContactorViewModel fromJson(Map<String, dynamic> json) { return _ContactorViewModel( followedUserId: json['followedUserId'], nickname: json['nickname'], avatar: UploadService.uploadBaseUrl + 'image/' + json['avatar'], description: json['description']); } }页面的build方法如下,可以看到页面中没有体现中间件部分的代码,而是在 dispatch 过程中自动完成了。
@override Widget build(BuildContext context) { return StoreProvider<ContactorState>( store: store, child: Scaffold( //省略 appBar body: StoreConnector<ContactorState, _ViewModel>( converter: (Store<ContactorState> store) => _ViewModel.create(store), builder: (BuildContext context, _ViewModel viewModel) { return EasyRefresh( child: ListView.builder( itemBuilder: (context, index) { return ListTile( leading: _getRoundImage(viewModel.contactors[index].avatar, 50), title: Text(viewModel.contactors[index].nickname), subtitle: Text( viewModel.contactors[index].description, style: TextStyle(fontSize: 14.0, color: Colors.grey), ), ); }, itemCount: viewModel.contactors.length, ), onRefresh: () async { store.dispatch(RefreshAction()); }, onLoad: () async { store.dispatch(LoadAction()); }, firstRefresh: true, ); }, ), // 省略其他代码 ), );这里需要注意,EasyRefresh 组件要放置在 StoreConnector 的下一级,否则会因为在刷新的时候找不到下级ScrollView,报null错误。