Fixed race condition in scan peripherals (#490)

* Fixed race condition in scan peripherals

Under certain circumstances a problem can be reported by the native side very quickly. Since monitoring for callbacks started after the call, those responses could be delivered before anyone would start to listen.

Author: dariuszseweryn

lib/src/bridge/scanning_mixin.dart

@@ -1,42 +1,49 @@
 part of _internal;
 
 mixin ScanningMixin on FlutterBLE {
-  Stream<dynamic> _scanEvents;
+  Stream<ScanResult> _scanEvents;
 
   void _prepareScanEventsStream() {
-    _scanEvents =
-        const EventChannel(ChannelName.scanningEvents).receiveBroadcastStream();
+    _scanEvents = const EventChannel(ChannelName.scanningEvents)
+        .receiveBroadcastStream()
+        .handleError(
+          (errorJson) => throw BleError.fromJson(jsonDecode(errorJson.details)),
+          test: (error) => error is PlatformException,
+        )
+        .map(
+          (scanResultJson) =>
+              ScanResult.fromJson(jsonDecode(scanResultJson), _manager),
+        );
   }
 
   Stream<ScanResult> startDeviceScan(
     int scanMode,
     int callbackType,
     List<String> uuids,
     bool allowDuplicates,
-  ) async* {
-    _methodChannel.invokeMethod(
-      MethodName.startDeviceScan,
-      <String, dynamic>{
-        ArgumentName.scanMode: scanMode,
-        ArgumentName.callbackType: callbackType,
-        ArgumentName.uuids: uuids,
-        ArgumentName.allowDuplicates: allowDuplicates,
-      },
-    );
-
+  ) {
     if (_scanEvents == null) {
       _prepareScanEventsStream();
     }
 
-    yield* _scanEvents.handleError(
-      (errorJson) {
-        throw BleError.fromJson(jsonDecode(errorJson.details));
-      },
-      test: (error) => error is PlatformException,
-    ).map((scanResultJson) => ScanResult.fromJson(
-          jsonDecode(scanResultJson),
-          _manager,
-        ));
+    StreamController<ScanResult> streamController = StreamController.broadcast(
+      onListen: () => _methodChannel.invokeMethod(
+        MethodName.startDeviceScan,
+        <String, dynamic>{
+          ArgumentName.scanMode: scanMode,
+          ArgumentName.callbackType: callbackType,
+          ArgumentName.uuids: uuids,
+          ArgumentName.allowDuplicates: allowDuplicates,
+        },
+      ),
+      onCancel: () => stopDeviceScan(),
+    );
+
+    streamController
+        .addStream(_scanEvents, cancelOnError: true)
+        .then((_) => streamController?.close());
+
+    return streamController.stream;
   }
 
   Future<void> stopDeviceScan() async {