coalescingKeys for RCTTouchEvent

Reviewed By: javache

Differential Revision: D3092864

fb-gh-sync-id: b703dca92578e05195eac9171b5b6eee83976cc9
fbshipit-source-id: b703dca92578e05195eac9171b5b6eee83976cc9

Author: majak

React/Base/RCTTouchHandler.m

@@ -39,6 +39,7 @@ @implementation RCTTouchHandler
   BOOL _dispatchedInitialTouches;
   BOOL _recordingInteractionTiming;
   CFTimeInterval _mostRecentEnqueueJS;
+  uint16_t _coalescingKey;
 }
 
 - (instancetype)initWithBridge:(RCTBridge *)bridge
@@ -200,7 +201,7 @@ - (void)_updateAndDispatchTouches:(NSSet<UITouch *> *)touches
   RCTTouchEvent *event = [[RCTTouchEvent alloc] initWithEventName:eventName
                                                      reactTouches:reactTouches
                                                    changedIndexes:changedIndexes
-                                                    coalescingKey:0];
+                                                    coalescingKey:_coalescingKey];
   [_eventDispatcher sendEvent:event];
 }
 
@@ -233,6 +234,7 @@ - (void)handleGestureUpdate:(__unused UIGestureRecognizer *)gesture
 {
   // If gesture just recognized, send all touches to JS as if they just began.
   if (self.state == UIGestureRecognizerStateBegan) {
+    _coalescingKey++;
     [self _updateAndDispatchTouches:_nativeTouches.set eventName:@"topTouchStart" originatingTime:0];
 
     // We store this flag separately from `state` because after a gesture is