limit fake scroll event emitting

Summary:A need for sending a scroll events outside of scrollview made D3092854 a bit clunky. This diff kinda fixes it by tightening up emitting of fake scroll events just to the only usecase we have right now.

Why not just simply construct the event in `RCTNavigator`, so we can drop the code from `RCTScrollView` altogether?
`RCTScrollEvent` is private to `RCTScrollView`, and that's good. We don't want anyone have an ability to make up scroll events. Even this existing functionality should be sunset one day when we better integrate with native gesture recognizers.

Depends on D3092867.

Reviewed By: javache

Differential Revision: D3120751

fb-gh-sync-id: 6519c055b983cfd48c4b4a9d619c4452e12efda1
fbshipit-source-id: 6519c055b983cfd48c4b4a9d619c4452e12efda1

React/Views/RCTNavigator.m

@@ -449,11 +449,7 @@ RCT_NOT_IMPLEMENTED(- (instancetype)initWithCoder:(NSCoder *)aDecoder)
 
 - (void)dispatchFakeScrollEvent
 {
-  [_bridge.eventDispatcher sendScrollEventWithType:RCTScrollEventTypeMove
-                                          reactTag:self.reactTag
-                                        scrollView:nil
-                                          userData:nil
-                                     coalescingKey:0];
+  [_bridge.eventDispatcher sendFakeScrollEvent:self.reactTag];
 }
 
 /**

React/Views/RCTScrollView.h

@@ -56,13 +56,8 @@
 @interface RCTEventDispatcher (RCTScrollView)
 
 /**
- * Send a scroll event.
- * (You can send a fake scroll event by passing nil for scrollView).
+ * Send a fake scroll event.
  */
-- (void)sendScrollEventWithType:(RCTScrollEventType)type
-                       reactTag:(NSNumber *)reactTag
-                     scrollView:(UIScrollView *)scrollView
-                       userData:(NSDictionary *)userData
-                  coalescingKey:(uint16_t)coalescingKey;
+- (void)sendFakeScrollEvent:(NSNumber *)reactTag;
 
 @end

React/Views/RCTScrollView.m

@@ -931,29 +931,26 @@ RCT_SET_AND_PRESERVE_OFFSET(setScrollIndicatorInsets, scrollIndicatorInsets, UIE
     _coalescingKey++;
     _lastEmittedEventType = type;
   }
-  [_eventDispatcher sendScrollEventWithType:type
-                                   reactTag:reactTag
-                                 scrollView:scrollView
-                                   userData:userData
-                              coalescingKey:_coalescingKey];
+  RCTScrollEvent *scrollEvent = [[RCTScrollEvent alloc] initWithType:type
+                                                            reactTag:reactTag
+                                                          scrollView:scrollView
+                                                            userData:userData
+                                                       coalescingKey:_coalescingKey];
+  [_eventDispatcher sendEvent:scrollEvent];
 }
 
 @end
 
 @implementation RCTEventDispatcher (RCTScrollView)
 
-- (void)sendScrollEventWithType:(RCTScrollEventType)type
-                       reactTag:(NSNumber *)reactTag
-                     scrollView:(UIScrollView *)scrollView
-                       userData:(NSDictionary *)userData
-                  coalescingKey:(uint16_t)coalescingKey
+- (void)sendFakeScrollEvent:(NSNumber *)reactTag
 {
-  RCTScrollEvent *scrollEvent = [[RCTScrollEvent alloc] initWithType:type
+  RCTScrollEvent *fakeScrollEvent = [[RCTScrollEvent alloc] initWithType:RCTScrollEventTypeMove
                                                             reactTag:reactTag
-                                                          scrollView:scrollView
-                                                            userData:userData
-                                                       coalescingKey:coalescingKey];
-  [self sendEvent:scrollEvent];
+                                                          scrollView:nil
+                                                            userData:nil
+                                                       coalescingKey:0];
+  [self sendEvent:fakeScrollEvent];
 }
 
 @end