在 Flutter 应用程序的启动中，connectivityResult 返回 null？

Question

我正在使用连接：^0.4.6 插件来检查颤振应用程序中的连接。我的代码如下：

Future<void> main() async {

  StreamSubscription connectivitySubscription;

  ConnectivityResult _previousResult;

  connectivitySubscription = Connectivity()
    .onConnectivityChanged
    .listen((ConnectivityResult connectivityResult) {
      if (connectivityResult == ConnectivityResult.none) {
        nav.currentState.push(CupertinoPageRoute(
          builder: (BuildContext _) => NoConnectivityView()));
      } else if (_previousResult == ConnectivityResult.none) {
        nav.currentState.push(CupertinoPageRoute(
          builder: (BuildContext _) =>  LoginScreen()));
      }

      _previousResult = connectivityResult;
    });

  Future.delayed(Duration(seconds: 3));
  runApp(MyApp());
}

当应用启动后连接发生变化时，它工作得很好。但是，如果在初始状态下没有连接，它就不起作用。 connectivityResult 这次为空。

我该如何解决这个问题？

Answer 1

由于您正在监听设备的连接，此回调将触发多次，因此在某一时刻它可能为空，但在另一时刻它不会为空。

我已经为你添加了一个空案例：

Future<void> main() async {

  StreamSubscription connectivitySubscription;

  ConnectivityResult _previousResult;

  connectivitySubscription = Connectivity()
    .onConnectivityChanged
    .listen((ConnectivityResult connectivityResult) {
      if (connectivityResult == null || connectivityResult == ConnectivityResult.none) {
        nav.currentState.push(CupertinoPageRoute(
          builder: (BuildContext _) => NoConnectivityView()));
      } else if (_previousResult == ConnectivityResult.none) {
        nav.currentState.push(CupertinoPageRoute(
          builder: (BuildContext _) =>  LoginScreen()));
      }

      _previousResult = connectivityResult;
    });

  Future.delayed(Duration(seconds: 3));
  runApp(MyApp());
}