Fixed removeClippedSubviews

Summary:
The `removeClippedSubviews` feature works by umounting views from the hierarchy if they move outside the bounds of their parent.

This was previously restricted to clipping views which had `overflow: hidden`, since we cannot efficiently check whether the subviews of a view go outside its bounds, and so clipping a view that has potentially overflowing children becomes an expensive recursive operation.

The problem with this is that `overflow: visible` is the default, and it's not well documented nor easy to tell that `removeClippedSubviews` has been set up correctly (i.e. with all children having `overflow: hidden`).

When I checked, I found that `removeClippedSubviews` was not working on any of the examples in UIExplorer, nor in several of our internal apps, because the views inside the ListView has `overflow: visible`. This was probably caused by an infra change at some point, but I'm not sure how long it's been broken.

It's vanishingly unlikely that anyone would ever deliberately want subviews to overflow their bounds in this scenario, so I've updated the logic to simply ignore the `overflow` property and assume that views should be clipped if you are using the `removeClippedSubviews` property on the parent.

Cons / Breaking changes: in some rare circumstances, a view might get clipped prematurely if its parent is outside the scrollview bounds, but it itself is inside. This doesn't occur in practice in any of our products, and could be worked around with additional wrapper views if it did.

Pros: removeClippedSubviews is now much easier to use, and much more likely to work as intended, so most list-based apps should see a performance improvement.

Reviewed By: javache

Differential Revision: D3385316

fbshipit-source-id: 1c0064a4c21340a971ba80d794062a356ae6cfb3

React/Views/RCTView.m

@@ -312,43 +312,24 @@ - (void)remountSubview:(UIView *)view
 
 - (void)mountOrUnmountSubview:(UIView *)view withClipRect:(CGRect)clipRect relativeToView:(UIView *)clipView
 {
-  if (view.clipsToBounds) {
-
-    // View has cliping enabled, so we can easily test if it is partially
-    // or completely within the clipRect, and mount or unmount it accordingly
-
-    if (!CGRectIsEmpty(CGRectIntersection(clipRect, view.frame))) {
-
-      // View is at least partially visible, so remount it if unmounted
-      if (view.superview == nil) {
-        [self remountSubview:view];
-      }
-
-      // Then test its subviews
-      if (CGRectContainsRect(clipRect, view.frame)) {
-        [view react_remountAllSubviews];
-      } else {
-        [view react_updateClippedSubviewsWithClipRect:clipRect relativeToView:clipView];
-      }
-
-    } else if (view.superview) {
-
-      // View is completely outside the clipRect, so unmount it
-      [view removeFromSuperview];
-    }
-
-  } else {
-
-    // View has clipping disabled, so there's no way to tell if it has
-    // any visible subviews without an expensive recursive test, so we'll
-    // just add it.
+  if (!CGRectIsEmpty(CGRectIntersection(clipRect, view.frame))) {
 
+    // View is at least partially visible, so remount it if unmounted
     if (view.superview == nil) {
       [self remountSubview:view];
     }
 
-    // Check if subviews need to be mounted/unmounted
-    [view react_updateClippedSubviewsWithClipRect:clipRect relativeToView:clipView];
+    // Then test its subviews
+    if (CGRectContainsRect(clipRect, view.frame)) {
+      [view react_remountAllSubviews];
+    } else {
+      [view react_updateClippedSubviewsWithClipRect:clipRect relativeToView:clipView];
+    }
+
+  } else if (view.superview) {
+
+    // View is completely outside the clipRect, so unmount it
+    [view removeFromSuperview];
   }
 }
 
@@ -375,10 +356,8 @@ - (void)react_updateClippedSubviewsWithClipRect:(CGRect)clipRect relativeToView:
 
   // Convert clipping rect to local coordinates
   clipRect = [clipView convertRect:clipRect toView:self];
+  clipRect = CGRectIntersection(clipRect, self.bounds);
   clipView = self;
-  if (self.clipsToBounds) {
-    clipRect = CGRectIntersection(clipRect, self.bounds);
-  }
 
   // Mount / unmount views
   for (UIView *view in _reactSubviews) {