setNotifyValue now properly waits for descriptor to finish writing.

Before this fix, setNotifyValue was immediately returning and not waiting
for the CCCD descriptor to be properly written to.

If you had an app that needed to set notifications on multiple characteristics,
it would error out as a "error when writing the descriptor", see issue #70.

Author: pauldemarco

android/src/main/java/com/pauldemarco/flutterblue/FlutterBluePlugin.java

@@ -58,7 +58,7 @@ public class FlutterBluePlugin implements MethodCallHandler, RequestPermissionsR
     private static final String TAG = "FlutterBluePlugin";
     private static final String NAMESPACE = "plugins.pauldemarco.com/flutter_blue";
     private static final int REQUEST_COARSE_LOCATION_PERMISSIONS = 1452;
-    static final private UUID CCCD_ID = UUID.fromString("000002902-0000-1000-8000-00805f9b34fb");
+    static final private UUID CCCD_ID = UUID.fromString("00002902-0000-1000-8000-00805f9b34fb");
     private final Registrar registrar;
     private final MethodChannel channel;
     private final EventChannel stateChannel;
@@ -453,6 +453,10 @@ public void onMethodCall(MethodCall call, Result result) {
                     value = BluetoothGattDescriptor.DISABLE_NOTIFICATION_VALUE;
                 }
 
+                if(!gattServer.setCharacteristicNotification(characteristic, request.getEnable())){
+                    result.error("set_notification_error", "could not set characteristic notifications to :" + request.getEnable(), null);
+                    return;
+                }
 
                 if(!cccDescriptor.setValue(value)) {
                     result.error("set_notification_error", "error when setting the descriptor value to: " + value, null);
@@ -464,18 +468,7 @@ public void onMethodCall(MethodCall call, Result result) {
                     return;
                 }
 
-                if(!gattServer.setCharacteristicNotification(characteristic, request.getEnable())){
-                    result.error("set_notification_error", "could not set characteristic notifications to :" + request.getEnable(), null);
-                    return;
-                }
-
                 result.success(null);
-
-                // SetNotificationResponse
-                Protos.SetNotificationResponse.Builder q = Protos.SetNotificationResponse.newBuilder();
-                q.setRemoteId(gattServer.getDevice().getAddress());
-                q.setCharacteristic(ProtoMaker.from(characteristic, gattServer));
-                channel.invokeMethod("SetNotificationResponse", q.build().toByteArray());
                 break;
             }
 
@@ -803,7 +796,7 @@ public void onCharacteristicChanged(BluetoothGatt gatt, BluetoothGattCharacteris
 
         @Override
         public void onDescriptorRead(BluetoothGatt gatt, BluetoothGattDescriptor descriptor, int status) {
-            Log.d(TAG, "onDescriptorRead: ");
+            Log.d(TAG, "onDescriptorRead: " + descriptor.getUuid().toString() + " status: " + status);
             if(descriptorReadSink != null) {
                 // Rebuild the ReadAttributeRequest and send back along with response
                 Protos.ReadDescriptorRequest.Builder q = Protos.ReadDescriptorRequest.newBuilder();
@@ -829,11 +822,12 @@ public void onDescriptorRead(BluetoothGatt gatt, BluetoothGattDescriptor descrip
                 p.setValue(ByteString.copyFrom(descriptor.getValue()));
                 descriptorReadSink.success(p.build().toByteArray());
             }
+
         }
 
         @Override
         public void onDescriptorWrite(BluetoothGatt gatt, BluetoothGattDescriptor descriptor, int status) {
-            Log.d(TAG, "onDescriptorWrite: ");
+            Log.d(TAG, "onDescriptorWrite: " + descriptor.getUuid().toString() + " status: " + status);
             Protos.WriteDescriptorRequest.Builder request = Protos.WriteDescriptorRequest.newBuilder();
             request.setRemoteId(gatt.getDevice().getAddress());
             request.setDescriptorUuid(descriptor.getUuid().toString());
@@ -843,6 +837,14 @@ public void onDescriptorWrite(BluetoothGatt gatt, BluetoothGattDescriptor descri
             p.setRequest(request);
             p.setSuccess(status == BluetoothGatt.GATT_SUCCESS);
             channel.invokeMethod("WriteDescriptorResponse", p.build().toByteArray());
+
+            if(descriptor.getUuid().compareTo(CCCD_ID) == 0) {
+                // SetNotificationResponse
+                Protos.SetNotificationResponse.Builder q = Protos.SetNotificationResponse.newBuilder();
+                q.setRemoteId(gatt.getDevice().getAddress());
+                q.setCharacteristic(ProtoMaker.from(descriptor.getCharacteristic(), gatt));
+                channel.invokeMethod("SetNotificationResponse", q.build().toByteArray());
+            }
         }
 
         @Override