reactBridgeDidFinishTransaction was removed from RCTNavigator

Summary:
We are removing `reactBridgeDidFinishTransaction`.
Why?
 * It is a performance drain. Supporting this requires dispatching main-thread block on every single transaction complete;
 * It has "too broad" non-conceptual semantic which encouraged using this as a "band-aid solution" for poorly designed components;
 * It is conceptually incompatible with new approaches that we are trying to implement to optimize the render layer;
 * It was deprecated for very long time.

This diff replaces usage of `reactBridgeDidFinishTransaction` with `uiManagerDidPerformMounting` which has very similar semantic except that fact that `uiManagerDidPerformMounting` is called asynchronously on the next run loop tick. And this should be okay because new React partial rendering does not guarantee synchronous execution anyways.

Reviewed By: mmmulani

Differential Revision: D6549217

fbshipit-source-id: 2649e943e82e6fbe02c7678583a97db3f5800201

Author: shergin

Diff for: React/Views/RCTNavigator.h

@@ -31,4 +31,6 @@
  */
 - (BOOL)requestSchedulingJavaScriptNavigation;
 
+- (void)uiManagerDidPerformMounting;
+
 @end

Diff for: React/Views/RCTNavigator.m

@@ -466,7 +466,7 @@ - (void)insertReactSubview:(RCTNavItem *)view atIndex:(NSInteger)atIndex
 
 - (void)didUpdateReactSubviews
 {
-  // Do nothing, as subviews are managed by `reactBridgeDidFinishTransaction`
+  // Do nothing, as subviews are managed by `uiManagerDidPerformMounting`
 }
 
 - (void)layoutSubviews
@@ -510,7 +510,7 @@ - (UIView *)reactSuperview
   return superview ?: self.reactNavSuperviewLink;
 }
 
-- (void)reactBridgeDidFinishTransaction
+- (void)uiManagerDidPerformMounting
 {
   // we can't hook up the VC hierarchy in 'init' because the subviews aren't
   // hooked up yet, so we do it on demand here

Diff for: React/Views/RCTNavigatorManager.m

@@ -13,15 +13,42 @@
 #import "RCTConvert.h"
 #import "RCTNavigator.h"
 #import "RCTUIManager.h"
+#import "RCTUIManagerObserverCoordinator.h"
 #import "UIView+React.h"
 
+@interface RCTNavigatorManager () <RCTUIManagerObserver>
+
+@end
+
 @implementation RCTNavigatorManager
+{
+  // The main thread only.
+  NSHashTable<RCTNavigator *> *_viewRegistry;
+}
+
+- (void)setBridge:(RCTBridge *)bridge
+{
+  [super setBridge:bridge];
+
+  [self.bridge.uiManager.observerCoordinator addObserver:self];
+}
+
+- (void)invalidate
+{
+  [self.bridge.uiManager.observerCoordinator removeObserver:self];
+}
 
 RCT_EXPORT_MODULE()
 
 - (UIView *)view
 {
-  return [[RCTNavigator alloc] initWithBridge:self.bridge];
+  if (!_viewRegistry) {
+    _viewRegistry = [NSHashTable hashTableWithOptions:NSPointerFunctionsWeakMemory];
+  }
+
+  RCTNavigator *view = [[RCTNavigator alloc] initWithBridge:self.bridge];
+  [_viewRegistry addObject:view];
+  return view;
 }
 
 RCT_EXPORT_VIEW_PROPERTY(requestedTopOfStack, NSInteger)
@@ -44,4 +71,15 @@ - (UIView *)view
   }];
 }
 
+#pragma mark - RCTUIManagerObserver
+
+- (void)uiManagerDidPerformMounting:(__unused RCTUIManager *)manager
+{
+  RCTExecuteOnMainQueue(^{
+    for (RCTNavigator *view in self->_viewRegistry) {
+      [view uiManagerDidPerformMounting];
+    }
+  });
+}
+
 @end