Flutter Redux Navigator GlobalKey.currentState 返回 null答案

【问题标题】：Flutter Redux Navigator GlobalKey.currentState returns nullFlutter Redux Navigator GlobalKey.currentState 返回 null
【发布时间】：2018-05-12 06:18:08
【问题描述】：

我正在使用 Redux 开发 Flutter。

当用户启动应用程序时，我希望 Redux 自动 dispatch 和 action。此操作将使Navigator 依赖推送不同的路由。

这个sn-p provided by a Flutter dev member 使用GlobalKey 在middleware 中使用Navigator。

在此之后，我将我的代码组织如下：

main.dart

void main() {
  final store = new Store(appStateReducer,
      middleware:createRouteMiddleware()
  );
  runApp(new MyApp(store));
}

class MyApp extends StatelessWidget {
  final Store<AppState> store;

  MyApp(this.store);

  @override
  Widget build(BuildContext context) {
    return new StoreProvider<AppState>(
        store: store,
        child: MaterialApp(
            routes: {
              Routes.REGISTER: (context) {
                return RegisterScreenContainer();
              },
              Routes.SET_PROFILE: (context) {
                return SetProfileScreenContainer();
              },
              //Routes.HOME = "/" so this route will be run first
              Routes.HOME: (context) {
                return StoreBuilder<AppState>(
                  //onInit is called to dispatch an action automatically when the application starts. The middleware will catch this and navigate to the appropriate route.
                  onInit: (store) => store.dispatch(
                      ChangeRoute(routeStateType: RouteStateType.Register)),
                  builder: (BuildContext context, Store vm) {
                    return RegisterScreenContainer();
                  },
                );
              },
            }));
  }
}

middleware.dart

Middleware<AppState> createRouteMiddleware(
    {@required GlobalKey<NavigatorState> navigatorKey}) {
  final changeRoute = _createChangeRouteMiddleware(navigatorKey: navigatorKey);
  return TypedMiddleware<AppState, ChangeRoute>(changeRoute);
}

Middleware<AppState> _createChangeRouteMiddleware(
    {@required GlobalKey<NavigatorState> navigatorKey}) {
  print(navigatorKey.currentState);
  return (Store store, action, NextDispatcher next) async {
    switch ((action.routeStateType as RouteStateType)) {
      case RouteStateType.Home:
        navigatorKey.currentState.pushNamed(Routes.HOME);
        break;
      case RouteStateType.Register:
        //The middleware intercepts and push the appropriate route depending on the action
        navigatorKey.currentState.pushNamed(Routes.REGISTER);
        break;
      default:
        break;
    }
    next(action);
  };
}

这是我得到的错误

[ERROR:topaz/lib/tonic/logging/dart_error.cc(16)] Unhandled exception: E/flutter ( 2544): NoSuchMethodError: The method 'pushNamed' was called on null. E/flutter ( 2544): Receiver: null E/flutter ( 2544): Tried calling: pushNamed("/register")

这意味着action已成功调度，但是navigatorKey的currentState是null。

我在这里错过了什么？

请注意，我知道 this seemingly similar question 并不真正适用于我的问题。即使我将 main.dart 和 middleware.dart 合并到一个文件中，它仍然不起作用。

【问题讨论】：

标签： redux flutter redux-middleware

【解决方案1】：

我通过将全局导航键放在单独的文件中解决了这个问题。然后我在我的 materialApp 和中间件中使用了它。

我将导航键放在我的 keys.dart 文件中：

import 'package:flutter/widgets.dart';
class NoomiKeys {
  static final navKey = new GlobalKey<NavigatorState>();
}

在我的 main.dart 文件中添加了 MaterialApp 小部件“navigatorKey: NoomiKeys.navKey”的键（删除了很多代码以加快阅读速度）：

import 'package:noomi_nursing_home_app/keys.dart';


@override
Widget build(BuildContext context) {
return StoreProvider<AppState>(
  store: store,
  child: MaterialApp(

    //Add the key to your materialApp widget
    navigatorKey: NoomiKeys.navKey,

    localizationsDelegates: [NoomiLocalizationsDelegate()],
    onGenerateRoute: (RouteSettings settings) {
      switch (settings.name) {

并在 navigate_middleware.dart 中使用；

import 'package:noomi_nursing_home_app/actions/actions.dart';
import 'package:noomi_nursing_home_app/keys.dart';
import 'package:redux/redux.dart';
import 'package:noomi_nursing_home_app/models/models.dart';

List<Middleware<AppState>> navigateMiddleware() {

  final navigatorKey = NoomiKeys.navKey;
  
  final navigatePushNamed = _navigatePushNamed(navigatorKey);
  return ([
    TypedMiddleware<AppState, NavigatePushNamedAction>(navigatePushNamed),
  ]);
}

Middleware<AppState> _navigatePushNamed(navigatorKey) {
  return (Store<AppState> store, action, NextDispatcher next) {
    next(action);
    navigatorKey.currentState.pushNamed(action.to);
  };
}

【讨论】：

【解决方案2】：

您好像忘记声明并将 navigatorKey 传递给中间件

final navigatorKey = GlobalKey<NavigatorState>();
void main() {
  final store = Store(appStateReducer,
      middleware:createRouteMiddleware(navigatorKey: navigatorKey)
  );
  runApp(MyApp(store));
}

而且您的 MaterialApp 也缺少 navigatorKey

MaterialApp(
  navigatorKey: navigatorKey
  routes: /* your routes */
)

【讨论】：