RCTScrollEvent: get all required values injected rather than accessing the scroll view

Summary:
This PR fixes #15006 by removing all UI API calls from RCTScrollEvent.

`-[RCTScrollEvent arguments]` can now be called from a background thread.
The Main Thread Checker of Xcode 9 will not any longer produce runtime issues when calling this method.

1. create a React Native (version: this PR) project with a scroll view
2. open it in Xcode 9
3. launch it
4. scroll the scroll view
5. observe the runtime issues in Xcode. There should not contain "UI API called from background thread"-issues.

I verified my changes on this branch: https://github.com/HeEAaD/Demo-ReactNative-UI-not-on-main-thread/tree/fix

<!--
Thank you for sending the PR!

If you changed any code, please provide us with clear instructions on how you verified your changes work. In other words, a test plan is *required*. Bonus points for screenshots and videos!

Please read the Contribution Guidelines at https://github.com/facebook/react-native/blob/master/CONTRIBUTING.md to learn more about contributing to React Native.

Happy contributing!
-->
Closes #15008

Differential Revision: D5424734

Pulled By: shergin

fbshipit-source-id: 56beec2d7603ea6782d55622567509f3758a4517

Author: Steffen Matthischke

React/Views/RCTScrollView.m

@@ -27,15 +27,23 @@ @interface RCTScrollEvent : NSObject <RCTEvent>
 
 - (instancetype)initWithEventName:(NSString *)eventName
                          reactTag:(NSNumber *)reactTag
-                       scrollView:(UIScrollView *)scrollView
+          scrollViewContentOffset:(CGPoint)scrollViewContentOffset
+           scrollViewContentInset:(UIEdgeInsets)scrollViewContentInset
+            scrollViewContentSize:(CGSize)scrollViewContentSize
+                  scrollViewFrame:(CGRect)scrollViewFrame
+              scrollViewZoomScale:(CGFloat)scrollViewZoomScale
                          userData:(NSDictionary *)userData
                     coalescingKey:(uint16_t)coalescingKey NS_DESIGNATED_INITIALIZER;
 
 @end
 
 @implementation RCTScrollEvent
 {
-  UIScrollView *_scrollView;
+  CGPoint _scrollViewContentOffset;
+  UIEdgeInsets _scrollViewContentInset;
+  CGSize _scrollViewContentSize;
+  CGRect _scrollViewFrame;
+  CGFloat _scrollViewZoomScale;
   NSDictionary *_userData;
   uint16_t _coalescingKey;
 }
@@ -45,7 +53,11 @@ @implementation RCTScrollEvent
 
 - (instancetype)initWithEventName:(NSString *)eventName
                          reactTag:(NSNumber *)reactTag
-                       scrollView:(UIScrollView *)scrollView
+          scrollViewContentOffset:(CGPoint)scrollViewContentOffset
+           scrollViewContentInset:(UIEdgeInsets)scrollViewContentInset
+            scrollViewContentSize:(CGSize)scrollViewContentSize
+                  scrollViewFrame:(CGRect)scrollViewFrame
+              scrollViewZoomScale:(CGFloat)scrollViewZoomScale
                          userData:(NSDictionary *)userData
                     coalescingKey:(uint16_t)coalescingKey
 {
@@ -54,7 +66,11 @@ - (instancetype)initWithEventName:(NSString *)eventName
   if ((self = [super init])) {
     _eventName = [eventName copy];
     _viewTag = reactTag;
-    _scrollView = scrollView;
+    _scrollViewContentOffset = scrollViewContentOffset;
+    _scrollViewContentInset = scrollViewContentInset;
+    _scrollViewContentSize = scrollViewContentSize;
+    _scrollViewFrame = scrollViewFrame;
+    _scrollViewZoomScale = scrollViewZoomScale;
     _userData = userData;
     _coalescingKey = coalescingKey;
   }
@@ -72,24 +88,24 @@ - (NSDictionary *)body
 {
   NSDictionary *body = @{
     @"contentOffset": @{
-      @"x": @(_scrollView.contentOffset.x),
-      @"y": @(_scrollView.contentOffset.y)
+      @"x": @(_scrollViewContentOffset.x),
+      @"y": @(_scrollViewContentOffset.y)
     },
     @"contentInset": @{
-      @"top": @(_scrollView.contentInset.top),
-      @"left": @(_scrollView.contentInset.left),
-      @"bottom": @(_scrollView.contentInset.bottom),
-      @"right": @(_scrollView.contentInset.right)
+      @"top": @(_scrollViewContentInset.top),
+      @"left": @(_scrollViewContentInset.left),
+      @"bottom": @(_scrollViewContentInset.bottom),
+      @"right": @(_scrollViewContentInset.right)
     },
     @"contentSize": @{
-      @"width": @(_scrollView.contentSize.width),
-      @"height": @(_scrollView.contentSize.height)
+      @"width": @(_scrollViewContentSize.width),
+      @"height": @(_scrollViewContentSize.height)
     },
     @"layoutMeasurement": @{
-      @"width": @(_scrollView.frame.size.width),
-      @"height": @(_scrollView.frame.size.height)
+      @"width": @(_scrollViewFrame.size.width),
+      @"height": @(_scrollViewFrame.size.height)
     },
-    @"zoomScale": @(_scrollView.zoomScale ?: 1),
+    @"zoomScale": @(_scrollViewZoomScale ?: 1),
   };
 
   if (_userData) {
@@ -893,7 +909,11 @@ - (void)sendScrollEventWithName:(NSString *)eventName
   }
   RCTScrollEvent *scrollEvent = [[RCTScrollEvent alloc] initWithEventName:eventName
                                                                  reactTag:self.reactTag
-                                                               scrollView:scrollView
+                                                  scrollViewContentOffset:scrollView.contentOffset
+                                                   scrollViewContentInset:scrollView.contentInset
+                                                    scrollViewContentSize:scrollView.contentSize
+                                                          scrollViewFrame:scrollView.frame
+                                                      scrollViewZoomScale:scrollView.zoomScale
                                                                  userData:userData
                                                             coalescingKey:_coalescingKey];
   [_eventDispatcher sendEvent:scrollEvent];
@@ -909,7 +929,11 @@ - (void)sendFakeScrollEvent:(NSNumber *)reactTag
   NSString *eventName = NSStringFromSelector(@selector(onScroll));
   RCTScrollEvent *fakeScrollEvent = [[RCTScrollEvent alloc] initWithEventName:eventName
                                                                      reactTag:reactTag
-                                                                   scrollView:nil
+                                                      scrollViewContentOffset:CGPointZero
+                                                       scrollViewContentInset:UIEdgeInsetsZero
+                                                        scrollViewContentSize:CGSizeZero
+                                                              scrollViewFrame:CGRectZero
+                                                          scrollViewZoomScale:0
                                                                      userData:nil
                                                                 coalescingKey:0];
   [self sendEvent:fakeScrollEvent];