reactBridgeDidFinishTransaction was removed from RCTTabBar

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: D6549586

fbshipit-source-id: 589b814f83b91ed8fabf7e638e7554ab3c9d286e

React/Views/RCTTabBar.h

@@ -19,4 +19,6 @@
 @property (nonatomic, assign) UIBarStyle barStyle;
 #endif
 
+- (void)uiManagerDidPerformMounting;
+
 @end

React/Views/RCTTabBar.m

@@ -73,7 +73,7 @@ RCT_NOT_IMPLEMENTED(- (instancetype)initWithCoder:(NSCoder *)aDecoder)
 
 - (void)didUpdateReactSubviews
 {
-  // Do nothing, as subviews are managed by `reactBridgeDidFinishTransaction`
+  // Do nothing, as subviews are managed by `uiManagerDidPerformMounting`
 }
 
 - (void)layoutSubviews
@@ -83,7 +83,7 @@ RCT_NOT_IMPLEMENTED(- (instancetype)initWithCoder:(NSCoder *)aDecoder)
   _tabController.view.frame = self.bounds;
 }
 
-- (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 whenever a transaction has finished

React/Views/RCTTabBarManager.m

@@ -11,6 +11,8 @@
 
 #import "RCTBridge.h"
 #import "RCTTabBar.h"
+#import "RCTUIManager.h"
+#import "RCTUIManagerObserverCoordinator.h"
 
 @implementation RCTConvert (UITabBar)
 
@@ -22,13 +24,39 @@ RCT_ENUM_CONVERTER(UITabBarItemPositioning, (@{
 
 @end
 
+@interface RCTTabBarManager () <RCTUIManagerObserver>
+
+@end
+
 @implementation RCTTabBarManager
+{
+  // The main thread only.
+  NSHashTable<RCTTabBar *> *_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 [RCTTabBar new];
+  if (!_viewRegistry) {
+    _viewRegistry = [NSHashTable hashTableWithOptions:NSPointerFunctionsWeakMemory];
+  }
+
+  RCTTabBar *view = [RCTTabBar new];
+  [_viewRegistry addObject:view];
+  return view;
 }
 
 RCT_EXPORT_VIEW_PROPERTY(unselectedTintColor, UIColor)
@@ -41,4 +69,15 @@ RCT_EXPORT_VIEW_PROPERTY(barStyle, UIBarStyle)
 RCT_EXPORT_VIEW_PROPERTY(itemPositioning, UITabBarItemPositioning)
 RCT_EXPORT_VIEW_PROPERTY(unselectedItemTintColor, UIColor)
 
+#pragma mark - RCTUIManagerObserver
+
+- (void)uiManagerDidPerformMounting:(__unused RCTUIManager *)manager
+{
+  RCTExecuteOnMainQueue(^{
+    for (RCTTabBar *view in self->_viewRegistry) {
+      [view uiManagerDidPerformMounting];
+    }
+  });
+}
+
 @end