Flutter - 异步 function 在真正完成之前返回？

Question

我有一个 function scanAndConnect()应该扫描 BLE 设备并连接到具有指定服务 ID 的设备。 这个 function 应该是async的并且应该返回 Future。

问题是scanAndConnect()打印99999并在不等待flutterReactiveBle.statusStream.listen()完成的情况下返回，尽管我在它之前使用了await 。

Future scanAndConnect(Uuid serviceId, Uuid charctId) async {
    StreamSubscription<BleStatus>? bleStatusStreamSubscription;
    StreamSubscription<DiscoveredDevice>? deviceStreamSubscription;
    Stream<DiscoveredDevice> stream;
    bleStatusStreamSubscription =
        await flutterReactiveBle.statusStream.listen((bleStatus) async {
      print("new listen ${bleStatus.toString()}");
      if (bleStatus == BleStatus.ready) {
        await bleStatusStreamSubscription!.cancel();
        connectionStatus = BLEConnectionStatus.Connecting;
        stream = await flutterReactiveBle.scanForDevices(
          withServices: [serviceId],
          scanMode: ScanMode.lowLatency,
        );
      }
    });

    print("9999999");
  }

....

Future connectToDevice() async {
  await ble.scanAndConnect(BLE_SERVICE_UUID, BLE_CHAR_UUID)
  print("Statement after await in main");
  setState(() {
    loading = false;
    print("Changing state to ${loading.toString()}");
  });
}

这是我在 Xcode 中得到的 output：

flutter: 9999999
flutter: Statement after await in main
flutter: Changing state to false
flutter: new listen BleStatus.unknown
flutter: new listen BleStatus.ready

我怎样才能让scanAndConnect在真正完成之前不返回？

Answer 1

根据文档， FlutterReactiveBle.scanForDevices()返回一个Stream ，而不是Future ，所以await在这里不起作用。 您可以使用

• await for
• listen()
• await stream.first()

等待来自Stream的数据。