Measure touch events from nearest "root view"

Summary:
This makes RCTTouchHandler follow the same logic when sending touch event
coordinates as `UIManager.measure`.
This is important as UIManager.measure is used in `Touchable.js` aka the mother
of all touch events in RN.

In particular, this will make touch events correctly handled for places where RN is embedded into other screens.

Reviewed By: shergin

Differential Revision: D6838102

fbshipit-source-id: 70cad52606ea931cb637d8aa2d4845818eea0647

Author: Mehdi Mulani

React/Base/RCTTouchHandler.m

@@ -39,6 +39,8 @@ @implementation RCTTouchHandler
   NSMutableArray<NSMutableDictionary *> *_reactTouches;
   NSMutableArray<UIView *> *_touchViews;
 
+  __weak UIView *_cachedRootView;
+
   uint16_t _coalescingKey;
 }
 
@@ -150,7 +152,8 @@ - (void)_updateReactTouchAtIndex:(NSInteger)touchIndex
 {
   UITouch *nativeTouch = _nativeTouches[touchIndex];
   CGPoint windowLocation = [nativeTouch locationInView:nativeTouch.window];
-  CGPoint rootViewLocation = [nativeTouch.window convertPoint:windowLocation toView:self.view];
+  RCTAssert(_cachedRootView, @"We were unable to find a root view for the touch");
+  CGPoint rootViewLocation = [nativeTouch.window convertPoint:windowLocation toView:_cachedRootView];
 
   UIView *touchView = _touchViews[touchIndex];
   CGPoint touchViewLocation = [nativeTouch.window convertPoint:windowLocation toView:touchView];
@@ -231,6 +234,26 @@ - (void)_updateAndDispatchTouches:(NSSet<UITouch *> *)touches
   [_eventDispatcher sendEvent:event];
 }
 
+/***
+ * To ensure compatibilty when using UIManager.measure and RCTTouchHandler, we have to adopt
+ * UIManager.measure's behavior in finding a "root view".
+ * Usually RCTTouchHandler is already attached to a root view but in some cases (e.g. Modal),
+ * we are instead attached to some RCTView subtree. This is also the case when embedding some RN
+ * views inside a seperate ViewController not controlled by RN.
+ * This logic will either find the nearest rootView, or go all the way to the UIWindow.
+ * While this is not optimal, it is exactly what UIManager.measure does, and what Touchable.js
+ * relies on.
+ * We cache it here so that we don't have to repeat it for every touch in the gesture.
+ */
+- (void)_cacheRootView
+{
+  UIView *rootView = self.view;
+  while (rootView.superview && ![rootView isReactRootView]) {
+    rootView = rootView.superview;
+  }
+  _cachedRootView = rootView;
+}
+
 #pragma mark - Gesture Recognizer Delegate Callbacks
 
 static BOOL RCTAllTouchesAreCancelledOrEnded(NSSet<UITouch *> *touches)
@@ -262,6 +285,8 @@ - (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
 {
   [super touchesBegan:touches withEvent:event];
 
+  [self _cacheRootView];
+
   // "start" has to record new touches *before* extracting the event.
   // "end"/"cancel" needs to remove the touch *after* extracting the event.
   [self _recordNewTouches:touches];
@@ -333,6 +358,8 @@ - (void)reset
     [_nativeTouches removeAllObjects];
     [_reactTouches removeAllObjects];
     [_touchViews removeAllObjects];
+
+    _cachedRootView = nil;
   }
 }