Send layout events on shadow queue

Summary:
We currently wait until after views have been updated on the main thread before sending layout events. This means that any code that relies on those events to update the UI will lag the atual layout by at least one frame.

This changes the RCTUIManager to send the event immediately after layout has occured on the shadow thread. This noticably improves the respinsiveness of the layout example in UIExplorer, which now updates the dimension labels immediately instead of waiting until after the layout animation has completed.

Examples/UIExplorer/LayoutEventsExample.js

@@ -85,8 +85,7 @@ var LayoutEventExample = React.createClass({
     return (
       <View style={this.state.containerStyle}>
         <Text>
-          onLayout events are called on mount and whenever layout is updated,
-          including after layout animations complete.{'  '}
+          layout events are called on mount and whenever layout is recalculated. Note that the layout event will typically be received <Text style={styles.italicText}>before</Text> the layout has updated on screen, especially when using layout animations.{'  '}
           <Text style={styles.pressText} onPress={this.animateViewLayout}>
             Press here to change layout.
           </Text>
@@ -136,6 +135,9 @@ var styles = StyleSheet.create({
   pressText: {
     fontWeight: 'bold',
   },
+  italicText: {
+    fontStyle: 'italic',
+  },
 });
 
 exports.title = 'Layout Events';

Libraries/Components/View/View.js

@@ -185,6 +185,10 @@ var View = React.createClass({
      * Invoked on mount and layout changes with
      *
      *   {nativeEvent: { layout: {x, y, width, height}}}.
+     *
+     * This event is fired immediately once the layout has been calculated, but
+     * the new layout may not yet be reflected on the screen at the time the
+     * event is received, especially if a layout animation is in progress.
      */
     onLayout: PropTypes.func,
 

React/Modules/RCTUIManager.m

@@ -448,29 +448,12 @@ - (RCTViewManagerUIBlock)uiBlockWithLayoutUpdateForRootView:(RCTShadowView *)roo
   NSMutableArray *frames = [NSMutableArray arrayWithCapacity:viewsWithNewFrames.count];
   NSMutableArray *areNew = [NSMutableArray arrayWithCapacity:viewsWithNewFrames.count];
   NSMutableArray *parentsAreNew = [NSMutableArray arrayWithCapacity:viewsWithNewFrames.count];
-  NSMutableArray *onLayoutEvents = [NSMutableArray arrayWithCapacity:viewsWithNewFrames.count];
 
   for (RCTShadowView *shadowView in viewsWithNewFrames) {
     [frameReactTags addObject:shadowView.reactTag];
     [frames addObject:[NSValue valueWithCGRect:shadowView.frame]];
     [areNew addObject:@(shadowView.isNewView)];
     [parentsAreNew addObject:@(shadowView.superview.isNewView)];
-
-    // TODO (#8214142): this can be greatly simplified by sending the layout
-    // event directly from the shadow thread, which may be better anyway.
-    id event = (id)kCFNull;
-    if (shadowView.onLayout) {
-      event = @{
-        @"target": shadowView.reactTag,
-        @"layout": @{
-          @"x": @(shadowView.frame.origin.x),
-          @"y": @(shadowView.frame.origin.y),
-          @"width": @(shadowView.frame.size.width),
-          @"height": @(shadowView.frame.size.height),
-        },
-      };
-    }
-    [onLayoutEvents addObject:event];
   }
 
   for (RCTShadowView *shadowView in viewsWithNewFrames) {
@@ -486,7 +469,20 @@ - (RCTViewManagerUIBlock)uiBlockWithLayoutUpdateForRootView:(RCTShadowView *)roo
   for (RCTShadowView *shadowView in viewsWithNewFrames) {
     RCTViewManager *manager = [_componentDataByName[shadowView.viewName] manager];
     RCTViewManagerUIBlock block = [manager uiBlockToAmendWithShadowView:shadowView];
-    if (block) [updateBlocks addObject:block];
+    if (shadowView.onLayout) {
+      CGRect frame = shadowView.frame;
+      shadowView.onLayout(@{
+        @"layout": @{
+          @"x": @(frame.origin.x),
+          @"y": @(frame.origin.y),
+          @"width": @(frame.size.width),
+          @"height": @(frame.size.height),
+        },
+      });
+    }
+    if (block) {
+      [updateBlocks addObject:block];
+    }
   }
 
   // Perform layout (possibly animated)
@@ -497,7 +493,6 @@ - (RCTViewManagerUIBlock)uiBlockWithLayoutUpdateForRootView:(RCTShadowView *)roo
       NSNumber *reactTag = frameReactTags[ii];
       UIView *view = viewRegistry[reactTag];
       CGRect frame = [frames[ii] CGRectValue];
-      id event = onLayoutEvents[ii];
 
       BOOL isNew = [areNew[ii] boolValue];
       RCTAnimation *updateAnimation = isNew ? nil : _layoutAnimation.updateAnimation;
@@ -506,9 +501,6 @@ - (RCTViewManagerUIBlock)uiBlockWithLayoutUpdateForRootView:(RCTShadowView *)roo
 
       void (^completion)(BOOL) = ^(BOOL finished) {
         completionsCalled++;
-        if (event != (id)kCFNull) {
-          [self.bridge.eventDispatcher sendInputEventWithName:@"layout" body:event];
-        }
         if (callback && completionsCalled == frames.count - 1) {
           callback(@[@(finished)]);
         }