ConnectivityResult在Flutter应用的启动过程中返回null？

Question

我正在使用连接性：^ 0.4.6插件，用于检查flutter应用程序中的连接性。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());
}

在应用启动后连接发生更改时，它可以完美地工作。但是，如果在初始状态下没有任何连接，它将无法正常工作。 connectivityResult这次为空。

我该如何解决？

Answer 1

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

我为您添加了一个空的情况：

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());
}