Expose UIManager queue via a static function to prevent race conditions

Summary: Having UI modules access the shadowQueue via UIManager.methodQueue is fragile and leads to race conditions in startup, sometimes resulting in an error where the methodQueue is set twice, or not at all.

Reviewed By: javache

Differential Revision: D3304890

fbshipit-source-id: 7198d28314dbec798877fcaaf17ae017d50157e9

Author: nicklockwood

React/Modules/RCTUIManager.h

@@ -15,6 +15,11 @@
 #import "RCTViewManager.h"
 #import "RCTRootView.h"
 
+/**
+ * UIManager queue
+ */
+RCT_EXTERN dispatch_queue_t RCTGetUIManagerQueue(void);
+
 /**
  * Default name for the UIManager queue
  */

React/Modules/RCTUIManager.m

@@ -91,7 +91,7 @@ static UIViewAnimationOptions UIViewAnimationOptionsFromRCTAnimationType(RCTAnim
 
 // Use a custom initialization function rather than implementing `+initialize` so that we can control
 // when the initialization code runs. `+initialize` runs immediately before the first message is sent
-// to the class which may be too late for us. By this time, we may have missed some 
+// to the class which may be too late for us. By this time, we may have missed some
 // `UIKeyboardWillChangeFrameNotification`s.
 + (void)initializeStatics
 {
@@ -208,8 +208,6 @@ - (instancetype)initWithDictionary:(NSDictionary *)config callback:(RCTResponseS
 
 @implementation RCTUIManager
 {
-  dispatch_queue_t _shadowQueue;
-
   // Root views are only mutated on the shadow queue
   NSMutableSet<NSNumber *> *_rootViewTags;
   NSMutableArray<dispatch_block_t> *_pendingUIBlocks;
@@ -235,7 +233,7 @@ @implementation RCTUIManager
 - (void)didReceiveNewContentSizeMultiplier
 {
   __weak RCTUIManager *weakSelf = self;
-  dispatch_async(self.methodQueue, ^{
+  dispatch_async(RCTGetUIManagerQueue(), ^{
     RCTUIManager *strongSelf = weakSelf;
     if (strongSelf) {
       [[NSNotificationCenter defaultCenter] postNotificationName:RCTUIManagerWillUpdateViewsDueToContentSizeMultiplierChangeNotification
@@ -342,22 +340,29 @@ - (void)setBridge:(RCTBridge *)bridge
                                            selector:@selector(interfaceOrientationWillChange:)
                                                name:UIApplicationWillChangeStatusBarOrientationNotification
                                              object:nil];
-  
+
   [RCTAnimation initializeStatics];
 }
 
-- (dispatch_queue_t)methodQueue
+dispatch_queue_t RCTGetUIManagerQueue(void)
 {
-  if (!_shadowQueue) {
+  static dispatch_queue_t shadowQueue;
+  static dispatch_once_t onceToken;
+  dispatch_once(&onceToken, ^{
     if ([NSOperation instancesRespondToSelector:@selector(qualityOfService)]) {
       dispatch_queue_attr_t attr = dispatch_queue_attr_make_with_qos_class(DISPATCH_QUEUE_SERIAL, QOS_CLASS_USER_INTERACTIVE, 0);
-      _shadowQueue = dispatch_queue_create(RCTUIManagerQueueName, attr);
+      shadowQueue = dispatch_queue_create(RCTUIManagerQueueName, attr);
     } else {
-      _shadowQueue = dispatch_queue_create(RCTUIManagerQueueName, DISPATCH_QUEUE_SERIAL);
-      dispatch_set_target_queue(_shadowQueue, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0));
+      shadowQueue = dispatch_queue_create(RCTUIManagerQueueName, DISPATCH_QUEUE_SERIAL);
+      dispatch_set_target_queue(shadowQueue, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0));
     }
-  }
-  return _shadowQueue;
+  });
+  return shadowQueue;
+}
+
+- (dispatch_queue_t)methodQueue
+{
+  return RCTGetUIManagerQueue();
 }
 
 - (void)registerRootView:(UIView *)rootView withSizeFlexibility:(RCTRootViewSizeFlexibility)sizeFlexibility
@@ -378,7 +383,7 @@ - (void)registerRootView:(UIView *)rootView withSizeFlexibility:(RCTRootViewSize
 
   // Register shadow view
   __weak RCTUIManager *weakSelf = self;
-  dispatch_async(_shadowQueue, ^{
+  dispatch_async(RCTGetUIManagerQueue(), ^{
     RCTUIManager *strongSelf = weakSelf;
     if (!_viewRegistry) {
       return;
@@ -419,7 +424,7 @@ - (void)setFrame:(CGRect)frame forView:(UIView *)view
   }
 
   NSNumber *reactTag = view.reactTag;
-  dispatch_async(_shadowQueue, ^{
+  dispatch_async(RCTGetUIManagerQueue(), ^{
     RCTShadowView *shadowView = _shadowViewRegistry[reactTag];
     RCTAssert(shadowView != nil, @"Could not locate shadow view with tag #%@", reactTag);
 
@@ -452,7 +457,7 @@ - (void)setIntrinsicContentSize:(CGSize)size forView:(UIView *)view
   RCTAssertMainThread();
 
   NSNumber *reactTag = view.reactTag;
-  dispatch_async(_shadowQueue, ^{
+  dispatch_async(RCTGetUIManagerQueue(), ^{
     RCTShadowView *shadowView = _shadowViewRegistry[reactTag];
     RCTAssert(shadowView != nil, @"Could not locate root view with tag #%@", reactTag);
 
@@ -469,7 +474,7 @@ - (void)setBackgroundColor:(UIColor *)color forView:(UIView *)view
   NSNumber *reactTag = view.reactTag;
 
   __weak RCTUIManager *weakSelf = self;
-  dispatch_async(_shadowQueue, ^{
+  dispatch_async(RCTGetUIManagerQueue(), ^{
     RCTUIManager *strongSelf = weakSelf;
     if (!_viewRegistry) {
       return;
@@ -505,9 +510,9 @@ - (void)_purgeChildren:(NSArray<id<RCTComponent>> *)children
 
 - (void)addUIBlock:(RCTViewManagerUIBlock)block
 {
-  RCTAssertThread(_shadowQueue,
+  RCTAssertThread(RCTGetUIManagerQueue(),
                   @"-[RCTUIManager addUIBlock:] should only be called from the "
-                  "UIManager's _shadowQueue (it may be accessed via `bridge.uiManager.methodQueue`)");
+                  "UIManager's queue (get this using `RCTGetUIManagerQueue()`)");
 
   if (!block) {
     return;
@@ -1103,7 +1108,8 @@ - (void)_layoutAndMount
 
 - (void)flushUIBlocks
 {
-  RCTAssertThread(_shadowQueue, @"flushUIBlocks can only be called from the shadow queue");
+  RCTAssertThread(RCTGetUIManagerQueue(),
+                  @"flushUIBlocks can only be called from the shadow queue");
 
   // First copy the previous blocks into a temporary variable, then reset the
   // pending blocks to a new array. This guards against mutation while

React/Views/RCTViewManager.m

@@ -52,10 +52,7 @@ @implementation RCTViewManager
 
 - (dispatch_queue_t)methodQueue
 {
-  RCTAssert(_bridge, @"Bridge not set");
-  RCTAssert(_bridge.uiManager || !_bridge.valid, @"UIManager not initialized");
-  RCTAssert(_bridge.uiManager.methodQueue || !_bridge.valid, @"UIManager.methodQueue not initialized");
-  return _bridge.uiManager.methodQueue;
+  return RCTGetUIManagerQueue();
 }
 
 - (UIView *)view