Perform clip update on RCTScrollView in increments of 50 pt

Summary:
This diff removes calls to `-updateClippedSubviews` by only re-clipping when the scroll view moves by a certain number of pixels.

leeway = 50pt => 46.9% of calls removed
leeway = 10pt => 13.2% " " "

React/React.xcodeproj/project.pbxproj

@@ -256,6 +256,7 @@
 		83CBBA971A6020BB00E9B192 /* RCTTouchHandler.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = RCTTouchHandler.m; sourceTree = "<group>"; };
 		83CBBACA1A6023D300E9B192 /* RCTConvert.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = RCTConvert.h; sourceTree = "<group>"; };
 		83CBBACB1A6023D300E9B192 /* RCTConvert.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = RCTConvert.m; sourceTree = "<group>"; };
+		83F15A171B7CC46900F10295 /* UIView+Private.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; path = "UIView+Private.h"; sourceTree = "<group>"; };
 		E3BBC8EB1ADE6F47001BBD81 /* RCTTextDecorationLineType.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; path = RCTTextDecorationLineType.h; sourceTree = "<group>"; };
 		E9B20B791B500126007A2DA7 /* RCTAccessibilityManager.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = RCTAccessibilityManager.h; sourceTree = "<group>"; };
 		E9B20B7A1B500126007A2DA7 /* RCTAccessibilityManager.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = RCTAccessibilityManager.m; sourceTree = "<group>"; };
@@ -405,6 +406,7 @@
 				13C156041AB1A2840079392D /* RCTWebViewManager.m */,
 				13B080231A694A8400A75B9A /* RCTWrapperViewController.h */,
 				13B080241A694A8400A75B9A /* RCTWrapperViewController.m */,
+				83F15A171B7CC46900F10295 /* UIView+Private.h */,
 				13E067531A70F44B002CDEE1 /* UIView+React.h */,
 				13E067541A70F44B002CDEE1 /* UIView+React.m */,
 			);

React/Views/RCTScrollView.m

@@ -16,6 +16,7 @@
 #import "RCTLog.h"
 #import "RCTUIManager.h"
 #import "RCTUtils.h"
+#import "UIView+Private.h"
 #import "UIView+React.h"
 
 CGFloat const ZINDEX_DEFAULT = 0;
@@ -365,6 +366,7 @@ RCT_NOT_IMPLEMENTED(-init)
   NSTimeInterval _lastScrollDispatchTime;
   NSMutableArray *_cachedChildFrames;
   BOOL _allowNextScrollNoMatterWhat;
+  CGRect _lastClippedToRect;
 }
 
 @synthesize nativeMainScrollDelegate = _nativeMainScrollDelegate;
@@ -374,7 +376,6 @@ RCT_NOT_IMPLEMENTED(-init)
   RCTAssertParam(eventDispatcher);
 
   if ((self = [super initWithFrame:CGRectZero])) {
-
     _eventDispatcher = eventDispatcher;
     _scrollView = [[RCTCustomScrollView alloc] initWithFrame:CGRectZero];
     _scrollView.delegate = self;
@@ -382,6 +383,7 @@ RCT_NOT_IMPLEMENTED(-init)
     _automaticallyAdjustContentInsets = YES;
     _contentInset = UIEdgeInsetsZero;
     _contentSize = CGSizeZero;
+    _lastClippedToRect = CGRectNull;
 
     _scrollEventThrottle = 0.0;
     _lastScrollDispatchTime = CACurrentMediaTime();
@@ -469,6 +471,33 @@ RCT_NOT_IMPLEMENTED(-initWithCoder:(NSCoder *)aDecoder)
   [self updateClippedSubviews];
 }
 
+- (void)updateClippedSubviews
+{
+  // Find a suitable view to use for clipping
+  UIView *clipView = [self react_findClipView];
+  if (!clipView) {
+    return;
+  }
+
+  static const CGFloat leeway = 50.0;
+
+  const CGSize contentSize = _scrollView.contentSize;
+  const CGRect bounds = _scrollView.bounds;
+  const BOOL scrollsHorizontally = contentSize.width > bounds.size.width;
+  const BOOL scrollsVertically = contentSize.height > bounds.size.height;
+
+  const BOOL shouldClipAgain =
+    CGRectIsNull(_lastClippedToRect) ||
+    (scrollsHorizontally && (bounds.size.width < leeway || fabs(_lastClippedToRect.origin.x - bounds.origin.x) >= leeway)) ||
+    (scrollsVertically && (bounds.size.height < leeway || fabs(_lastClippedToRect.origin.y - bounds.origin.y) >= leeway));
+
+  if (shouldClipAgain) {
+    const CGRect clipRect = CGRectInset(clipView.bounds, -leeway, -leeway);
+    [self react_updateClippedSubviewsWithClipRect:clipRect relativeToView:clipView];
+    _lastClippedToRect = bounds;
+  }
+}
+
 - (void)setContentInset:(UIEdgeInsets)contentInset
 {
   CGPoint contentOffset = _scrollView.contentOffset;

React/Views/UIView+Private.h

@@ -0,0 +1,18 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+#import <UIKit/UIKit.h>
+
+@interface UIView (RCTViewUnmounting)
+
+- (void)react_remountAllSubviews;
+- (void)react_updateClippedSubviewsWithClipRect:(CGRect)clipRect relativeToView:(UIView *)clipView;
+- (UIView *)react_findClipView;
+
+@end