Replaced isMainThread checks with a proper test for main queue

Summary:
As per https://twitter.com/olebegemann/status/738656134731599872, our use of "main thread" to mean "main queue" seems to be unsafe.

This diff replaces the `NSThread.isMainQueue` checks with dispatch_get_specific(), which is the recommended approach.

I've also replaced all use of "MainThread" terminology with "MainQueue", and taken the opportunity to deprecate the "sync" param of `RCTExecuteOnMainThread()`, which, while we do still use it in a few places, is incredibly unsafe and shouldn't be encouraged.

Reviewed By: javache

Differential Revision: D3384910

fbshipit-source-id: ea7c216013372267b82eb25a38db5eb4cd46a089

Examples/UIExplorer/UIExplorerUnitTests/RCTModuleInitTests.m

@@ -48,7 +48,7 @@ RCT_EXPORT_MODULE()
 
 @interface RCTTestCustomInitModule : NSObject <RCTBridgeModule>
 
-@property (nonatomic, assign) BOOL initializedOnMainThread;
+@property (nonatomic, assign) BOOL initializedOnMainQueue;
 
 @end
 
@@ -62,7 +62,7 @@ RCT_EXPORT_MODULE()
 - (id)init
 {
   if ((self = [super init])) {
-    _initializedOnMainThread = [NSThread isMainThread];
+    _initializedOnMainQueue = RCTIsMainQueue();
   }
   return self;
 }
@@ -72,7 +72,7 @@ RCT_EXPORT_MODULE()
 
 @interface RCTTestCustomSetBridgeModule : NSObject <RCTBridgeModule>
 
-@property (nonatomic, assign) BOOL setBridgeOnMainThread;
+@property (nonatomic, assign) BOOL setBridgeOnMainQueue;
 
 @end
 
@@ -86,7 +86,7 @@ RCT_EXPORT_MODULE()
 - (void)setBridge:(RCTBridge *)bridge
 {
   _bridge = bridge;
-  _setBridgeOnMainThread = [NSThread isMainThread];
+  _setBridgeOnMainQueue = RCTIsMainQueue();
 }
 
 @end
@@ -95,7 +95,7 @@ RCT_EXPORT_MODULE()
 @interface RCTTestExportConstantsModule : NSObject <RCTBridgeModule>
 
 @property (nonatomic, assign) BOOL exportedConstants;
-@property (nonatomic, assign) BOOL exportedConstantsOnMainThread;
+@property (nonatomic, assign) BOOL exportedConstantsOnMainQueue;
 
 @end
 
@@ -109,7 +109,7 @@ RCT_EXPORT_MODULE()
 - (NSDictionary<NSString *, id> *)constantsToExport
 {
   _exportedConstants = YES;
-  _exportedConstantsOnMainThread = [NSThread isMainThread];
+  _exportedConstantsOnMainQueue = RCTIsMainQueue();
   return @{ @"foo": @"bar" };
 }
 
@@ -137,7 +137,7 @@ RCT_EXPORT_MODULE()
   BOOL _customSetBridgeModuleNotificationSent;
   BOOL _exportConstantsModuleNotificationSent;
   BOOL _lazyInitModuleNotificationSent;
-  BOOL _lazyInitModuleNotificationSentOnMainThread;
+  BOOL _lazyInitModuleNotificationSentOnMainQueue;
   BOOL _viewManagerModuleNotificationSent;
   RCTTestInjectedModule *_injectedModule;
 }
@@ -196,7 +196,7 @@ RCT_EXPORT_MODULE()
     _exportConstantsModuleNotificationSent = YES;
   } else if ([module isKindOfClass:[RCTLazyInitModule class]]) {
     _lazyInitModuleNotificationSent = YES;
-    _lazyInitModuleNotificationSentOnMainThread = [NSThread isMainThread];
+    _lazyInitModuleNotificationSentOnMainQueue = RCTIsMainQueue();
   }
 }
 
@@ -214,7 +214,7 @@ RCT_EXPORT_MODULE()
   RUN_RUNLOOP_WHILE(!_customInitModuleNotificationSent);
   XCTAssertTrue(_customInitModuleNotificationSent);
   RCTTestCustomInitModule *module = [_bridge moduleForClass:[RCTTestCustomInitModule class]];
-  XCTAssertTrue(module.initializedOnMainThread);
+  XCTAssertTrue(module.initializedOnMainQueue);
   XCTAssertEqual(module.bridge, _bridge.batchedBridge);
   XCTAssertNotNil(module.methodQueue);
 }
@@ -230,7 +230,7 @@ RCT_EXPORT_MODULE()
 
   RUN_RUNLOOP_WHILE(!module);
   XCTAssertTrue(_customSetBridgeModuleNotificationSent);
-  XCTAssertFalse(module.setBridgeOnMainThread);
+  XCTAssertFalse(module.setBridgeOnMainQueue);
   XCTAssertEqual(module.bridge, _bridge.batchedBridge);
   XCTAssertNotNil(module.methodQueue);
 }
@@ -242,7 +242,7 @@ RCT_EXPORT_MODULE()
   RCTTestExportConstantsModule *module = [_bridge moduleForClass:[RCTTestExportConstantsModule class]];
   RUN_RUNLOOP_WHILE(!module.exportedConstants);
   XCTAssertTrue(module.exportedConstants);
-  XCTAssertTrue(module.exportedConstantsOnMainThread);
+  XCTAssertTrue(module.exportedConstantsOnMainQueue);
   XCTAssertEqual(module.bridge, _bridge.batchedBridge);
   XCTAssertNotNil(module.methodQueue);
 }
@@ -258,7 +258,7 @@ RCT_EXPORT_MODULE()
 
   RUN_RUNLOOP_WHILE(!module);
   XCTAssertTrue(_lazyInitModuleNotificationSent);
-  XCTAssertFalse(_lazyInitModuleNotificationSentOnMainThread);
+  XCTAssertFalse(_lazyInitModuleNotificationSentOnMainQueue);
   XCTAssertNotNil(module);
   XCTAssertEqual(module.bridge, _bridge.batchedBridge);
   XCTAssertNotNil(module.methodQueue);

Libraries/Image/RCTImageLoader.m

@@ -293,7 +293,7 @@ static UIImage *RCTResizeImageIfNeeded(UIImage *image,
   __weak RCTImageLoader *weakSelf = self;
 
   void (^completionHandler)(NSError *error, id imageOrData) = ^(NSError *error, id imageOrData) {
-    if ([NSThread isMainThread]) {
+    if (RCTIsMainQueue()) {
 
       // Most loaders do not return on the main thread, so caller is probably not
       // expecting it, and may do expensive post-processing in the callback
@@ -545,7 +545,7 @@ static UIImage *RCTResizeImageIfNeeded(UIImage *image,
 
   __block volatile uint32_t cancelled = 0;
   void (^completionHandler)(NSError *, UIImage *) = ^(NSError *error, UIImage *image) {
-    if ([NSThread isMainThread]) {
+    if (RCTIsMainQueue()) {
       // Most loaders do not return on the main thread, so caller is probably not
       // expecting it, and may do expensive post-processing in the callback
       dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

Libraries/Image/RCTImageStoreManager.m

@@ -191,7 +191,7 @@ RCT_EXPORT_METHOD(addImageFromBase64:(NSString *)base64String
 
 - (NSString *)storeImage:(UIImage *)image
 {
-  RCTAssertMainThread();
+  RCTAssertMainQueue();
   RCTLogWarn(@"RCTImageStoreManager.storeImage() is deprecated and has poor performance. Use an alternative method instead.");
   __block NSString *imageTag;
   dispatch_sync(_methodQueue, ^{
@@ -202,7 +202,7 @@ RCT_EXPORT_METHOD(addImageFromBase64:(NSString *)base64String
 
 - (UIImage *)imageForTag:(NSString *)imageTag
 {
-  RCTAssertMainThread();
+  RCTAssertMainQueue();
   RCTLogWarn(@"RCTImageStoreManager.imageForTag() is deprecated and has poor performance. Use an alternative method instead.");
   __block NSData *imageData;
   dispatch_sync(_methodQueue, ^{

Libraries/Image/RCTImageView.m

@@ -263,22 +263,18 @@ RCT_NOT_IMPLEMENTED(- (instancetype)init)
         // Blur on a background thread to avoid blocking interaction
         dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
           UIImage *image = RCTBlurredImageWithRadius(loadedImage, blurRadius);
-          RCTExecuteOnMainThread(^{
+          RCTExecuteOnMainQueue(^{
             setImageBlock(image);
-          }, NO);
+          });
         });
       } else {
         // No blur, so try to set the image on the main thread synchronously to minimize image
         // flashing. (For instance, if this view gets attached to a window, then -didMoveToWindow
         // calls -reloadImage, and we want to set the image synchronously if possible so that the
         // image property is set in the same CATransaction that attaches this view to the window.)
-        if ([NSThread isMainThread]) {
+        RCTExecuteOnMainQueue(^{
           setImageBlock(loadedImage);
-        } else {
-          RCTExecuteOnMainThread(^{
-            setImageBlock(loadedImage);
-          }, NO);
-        }
+        });
       }
     }];
   } else {

Libraries/WebSocket/RCTWebSocketExecutor.m

@@ -215,7 +215,7 @@ RCT_EXPORT_MODULE()
 
 - (void)executeBlockOnJavaScriptQueue:(dispatch_block_t)block
 {
-  RCTExecuteOnMainThread(block, NO);
+  RCTExecuteOnMainQueue(block);
 }
 
 - (void)executeAsyncBlockOnJavaScriptQueue:(dispatch_block_t)block

React/Base/RCTAssert.h

@@ -11,6 +11,11 @@
 
 #import "RCTDefines.h"
 
+/*
+ * Defined in RCTUtils.m
+ */
+RCT_EXTERN BOOL RCTIsMainQueue(void);
+
 /**
  * This is the main assert macro that you should use. Asserts should be compiled out
  * in production builds. You can customize the assert behaviour by setting a custom
@@ -58,13 +63,11 @@ RCT_EXTERN NSString *const RCTFatalExceptionName;
 /**
  * A block signature to be used for custom assertion handling.
  */
-typedef void (^RCTAssertFunction)(
-                                  NSString *condition,
+typedef void (^RCTAssertFunction)(NSString *condition,
                                   NSString *fileName,
                                   NSNumber *lineNumber,
                                   NSString *function,
-                                  NSString *message
-                                  );
+                                  NSString *message);
 
 typedef void (^RCTFatalHandler)(NSError *error);
 
@@ -74,17 +77,23 @@ typedef void (^RCTFatalHandler)(NSError *error);
 #define RCTAssertParam(name) RCTAssert(name, @"'%s' is a required parameter", #name)
 
 /**
- * Convenience macro for asserting that we're running on main thread.
+ * Convenience macro for asserting that we're running on main queue.
  */
-#define RCTAssertMainThread() RCTAssert([NSThread isMainThread], \
+#define RCTAssertMainQueue() RCTAssert(RCTIsMainQueue(), \
   @"This function must be called on the main thread")
 
 /**
- * Convenience macro for asserting that we're running off the main thread.
+ * Convenience macro for asserting that we're running off the main queue.
  */
-#define RCTAssertNotMainThread() RCTAssert(![NSThread isMainThread], \
+#define RCTAssertNotMainQueue() RCTAssert(!RCTIsMainQueue(), \
 @"This function must not be called on the main thread")
 
+/**
+ * Deprecated, do not use
+ */
+#define RCTAssertMainThread() RCTAssertMainQueue()
+#define RCTAssertNotMainThread() RCTAssertNotMainQueue()
+
 /**
  * These methods get and set the current assert function called by the RCTAssert
  * macros. You can use these to replace the standard behavior with custom assert

React/Base/RCTAssert.m

@@ -87,7 +87,7 @@ void RCTPerformBlockWithAssertFunction(void (^block)(void), RCTAssertFunction as
 NSString *RCTCurrentThreadName(void)
 {
   NSThread *thread = [NSThread currentThread];
-  NSString *threadName = thread.isMainThread ? @"main" : thread.name;
+  NSString *threadName = RCTIsMainQueue() || thread.isMainThread ? @"main" : thread.name;
   if (threadName.length == 0) {
     const char *label = dispatch_queue_get_label(DISPATCH_CURRENT_QUEUE_LABEL);
     if (label && strlen(label) > 0) {

React/Base/RCTBatchedBridge.m

@@ -366,7 +366,7 @@ RCT_EXTERN NSArray<Class> *RCTGetModuleClasses(void);
   // Synchronously set up the pre-initialized modules
   for (RCTModuleData *moduleData in _moduleDataByID) {
     if (moduleData.hasInstance &&
-        (!moduleData.requiresMainThreadSetup || [NSThread isMainThread])) {
+        (!moduleData.requiresMainQueueSetup || RCTIsMainQueue())) {
       // Modules that were pre-initialized should ideally be set up before
       // bridge init has finished, otherwise the caller may try to access the
       // module directly rather than via `[bridge moduleForClass:]`, which won't
@@ -383,10 +383,10 @@ RCT_EXTERN NSArray<Class> *RCTGetModuleClasses(void);
 
   // Set up modules that require main thread init or constants export
   RCTPerformanceLoggerSet(RCTPLNativeModuleMainThread, 0);
-  NSUInteger modulesOnMainThreadCount = 0;
+  NSUInteger modulesOnMainQueueCount = 0;
   for (RCTModuleData *moduleData in _moduleDataByID) {
     __weak RCTBatchedBridge *weakSelf = self;
-    if (moduleData.requiresMainThreadSetup || moduleData.hasConstantsToExport) {
+    if (moduleData.requiresMainQueueSetup || moduleData.hasConstantsToExport) {
       // Modules that need to be set up on the main thread cannot be initialized
       // lazily when required without doing a dispatch_sync to the main thread,
       // which can result in deadlock. To avoid this, we initialize all of these
@@ -400,12 +400,12 @@ RCT_EXTERN NSArray<Class> *RCTGetModuleClasses(void);
           RCTPerformanceLoggerAppendEnd(RCTPLNativeModuleMainThread);
         }
       });
-      modulesOnMainThreadCount++;
+      modulesOnMainQueueCount++;
     }
   }
 
   RCTPerformanceLoggerEnd(RCTPLNativeModuleInit);
-  RCTPerformanceLoggerSet(RCTPLNativeModuleMainThreadUsesCount, modulesOnMainThreadCount);
+  RCTPerformanceLoggerSet(RCTPLNativeModuleMainThreadUsesCount, modulesOnMainQueueCount);
 }
 
 - (void)setUpExecutor
@@ -509,7 +509,7 @@ RCT_EXTERN NSArray<Class> *RCTGetModuleClasses(void);
 
 - (void)stopLoadingWithError:(NSError *)error
 {
-  RCTAssertMainThread();
+  RCTAssertMainQueue();
 
   if (!_valid || !_loading) {
     return;
@@ -551,7 +551,7 @@ RCT_NOT_IMPLEMENTED(- (instancetype)initWithBundleURL:(__unused NSURL *)bundleUR
 
 - (void)setExecutorClass:(Class)executorClass
 {
-  RCTAssertMainThread();
+  RCTAssertMainQueue();
 
   _parentBridge.executorClass = executorClass;
 }
@@ -599,7 +599,7 @@ RCT_NOT_IMPLEMENTED(- (instancetype)initWithBundleURL:(__unused NSURL *)bundleUR
     return;
   }
 
-  RCTAssertMainThread();
+  RCTAssertMainQueue();
   RCTAssert(_javaScriptExecutor != nil, @"Can't complete invalidation without a JS executor");
 
   _loading = NO;
@@ -1004,7 +1004,7 @@ RCT_NOT_IMPLEMENTED(- (instancetype)initWithBundleURL:(__unused NSURL *)bundleUR
 
 - (void)startProfiling
 {
-  RCTAssertMainThread();
+  RCTAssertMainQueue();
 
   [_javaScriptExecutor executeBlockOnJavaScriptQueue:^{
     RCTProfileInit(self);
@@ -1013,7 +1013,7 @@ RCT_NOT_IMPLEMENTED(- (instancetype)initWithBundleURL:(__unused NSURL *)bundleUR
 
 - (void)stopProfiling:(void (^)(NSData *))callback
 {
-  RCTAssertMainThread();
+  RCTAssertMainQueue();
 
   [_javaScriptExecutor executeBlockOnJavaScriptQueue:^{
     RCTProfileEnd(self, ^(NSString *log) {

React/Base/RCTBridge.m

@@ -117,7 +117,7 @@ static RCTBridge *RCTCurrentBridgeInstance = nil;
     _delegate = delegate;
     _launchOptions = [launchOptions copy];
     [self setUp];
-    RCTExecuteOnMainThread(^{ [self bindKeys]; }, NO);
+    RCTExecuteOnMainQueue(^{ [self bindKeys]; });
   }
   return self;
 }
@@ -134,7 +134,7 @@ static RCTBridge *RCTCurrentBridgeInstance = nil;
     _moduleProvider = block;
     _launchOptions = [launchOptions copy];
     [self setUp];
-    RCTExecuteOnMainThread(^{ [self bindKeys]; }, NO);
+    RCTExecuteOnMainQueue(^{ [self bindKeys]; });
   }
   return self;
 }
@@ -145,7 +145,7 @@ RCT_NOT_IMPLEMENTED(- (instancetype)init)
 {
   /**
    * This runs only on the main thread, but crashes the subclass
-   * RCTAssertMainThread();
+   * RCTAssertMainQueue();
    */
   [[NSNotificationCenter defaultCenter] removeObserver:self];
   [self invalidate];
@@ -153,7 +153,7 @@ RCT_NOT_IMPLEMENTED(- (instancetype)init)
 
 - (void)bindKeys
 {
-  RCTAssertMainThread();
+  RCTAssertMainQueue();
 
   [[NSNotificationCenter defaultCenter] addObserver:self
                                            selector:@selector(reload)
@@ -270,9 +270,9 @@ RCT_NOT_IMPLEMENTED(- (instancetype)init)
   self.batchedBridge = nil;
 
   if (batchedBridge) {
-    RCTExecuteOnMainThread(^{
+    RCTExecuteOnMainQueue(^{
       [batchedBridge invalidate];
-    }, NO);
+    });
   }
 }
 

React/Base/RCTConvert.m

@@ -884,7 +884,7 @@ RCT_ENUM_CONVERTER(RCTAnimationType, (@{
   }
 
   __block UIImage *image;
-  if (![NSThread isMainThread]) {
+  if (!RCTIsMainQueue()) {
     // It seems that none of the UIImage loading methods can be guaranteed
     // thread safe, so we'll pick the lesser of two evils here and block rather
     // than run the risk of crashing

React/Base/RCTKeyCommands.m

@@ -271,7 +271,7 @@ RCT_NOT_IMPLEMENTED(- (instancetype)init)
                       modifierFlags:(UIKeyModifierFlags)flags
                              action:(void (^)(UIKeyCommand *))block
 {
-  RCTAssertMainThread();
+  RCTAssertMainQueue();
 
   if (input.length && flags && RCTIsIOS8OrEarlier()) {
 
@@ -296,7 +296,7 @@ RCT_NOT_IMPLEMENTED(- (instancetype)init)
 - (void)unregisterKeyCommandWithInput:(NSString *)input
                         modifierFlags:(UIKeyModifierFlags)flags
 {
-  RCTAssertMainThread();
+  RCTAssertMainQueue();
 
   for (RCTKeyCommand *command in _commands.allObjects) {
     if ([command matchesInput:input flags:flags]) {
@@ -309,7 +309,7 @@ RCT_NOT_IMPLEMENTED(- (instancetype)init)
 - (BOOL)isKeyCommandRegisteredForInput:(NSString *)input
                          modifierFlags:(UIKeyModifierFlags)flags
 {
-  RCTAssertMainThread();
+  RCTAssertMainQueue();
 
   for (RCTKeyCommand *command in _commands) {
     if ([command matchesInput:input flags:flags]) {
@@ -323,7 +323,7 @@ RCT_NOT_IMPLEMENTED(- (instancetype)init)
                       modifierFlags:(UIKeyModifierFlags)flags
                              action:(void (^)(UIKeyCommand *))block
 {
-  RCTAssertMainThread();
+  RCTAssertMainQueue();
 
   if (input.length && flags && RCTIsIOS8OrEarlier()) {
 
@@ -348,7 +348,7 @@ RCT_NOT_IMPLEMENTED(- (instancetype)init)
 - (void)unregisterDoublePressKeyCommandWithInput:(NSString *)input
                         modifierFlags:(UIKeyModifierFlags)flags
 {
-  RCTAssertMainThread();
+  RCTAssertMainQueue();
 
   for (RCTKeyCommand *command in _commands.allObjects) {
     if ([command matchesInput:input flags:flags]) {
@@ -361,7 +361,7 @@ RCT_NOT_IMPLEMENTED(- (instancetype)init)
 - (BOOL)isDoublePressKeyCommandRegisteredForInput:(NSString *)input
                          modifierFlags:(UIKeyModifierFlags)flags
 {
-  RCTAssertMainThread();
+  RCTAssertMainQueue();
 
   for (RCTKeyCommand *command in _commands) {
     if ([command matchesInput:input flags:flags]) {

React/Base/RCTModuleData.h

@@ -48,7 +48,7 @@
 /**
  * Returns YES if module instance must be created on the main thread.
  */
-@property (nonatomic, assign, readonly) BOOL requiresMainThreadSetup;
+@property (nonatomic, assign, readonly) BOOL requiresMainQueueSetup;
 
 /**
  * Returns YES if module has constants to export.

React/Base/RCTModuleData.m

@@ -44,7 +44,7 @@
 
   // If a module overrides `init` then we must assume that it expects to be
   // initialized on the main thread, because it may need to access UIKit.
-  _requiresMainThreadSetup = !_instance &&
+  _requiresMainQueueSetup = !_instance &&
   [_moduleClass instanceMethodForSelector:@selector(init)] != objectInitMethod;
 
   // If a module overrides `constantsToExport` then we must assume that it
@@ -85,8 +85,8 @@ RCT_NOT_IMPLEMENTED(- (instancetype)init);
   [_instanceLock lock];
   if (!_setupComplete && _bridge.valid) {
     if (!_instance) {
-      if (RCT_DEBUG && _requiresMainThreadSetup) {
-        RCTAssertMainThread();
+      if (RCT_DEBUG && _requiresMainQueueSetup) {
+        RCTAssertMainQueue();
       }
       RCT_PROFILE_BEGIN_EVENT(RCTProfileTagAlways, @"[RCTModuleData setUpInstanceAndBridge] [_moduleClass new]", nil);
       _instance = [_moduleClass new];
@@ -128,13 +128,13 @@ RCT_NOT_IMPLEMENTED(- (instancetype)init);
     // If we're here, then the module is completely initialized,
     // except for what finishSetupForInstance does.  When the instance
     // method is called after moduleSetupComplete,
-    // finishSetupForInstance will run.  If _requiresMainThreadSetup
+    // finishSetupForInstance will run.  If _requiresMainQueueSetup
     // is true, getting the instance will block waiting for the main
     // thread, which could take a while if the main thread is busy
     // (I've seen 50ms in testing).  So we clear that flag, since
     // nothing in finishSetupForInstance needs to be run on the main
     // thread.
-    _requiresMainThreadSetup = NO;
+    _requiresMainQueueSetup = NO;
   }
 }
 
@@ -208,7 +208,7 @@ RCT_NOT_IMPLEMENTED(- (instancetype)init);
 {
   if (!_setupComplete) {
     RCT_PROFILE_BEGIN_EVENT(RCTProfileTagAlways, [NSString stringWithFormat:@"[RCTModuleData instanceForClass:%@]", _moduleClass], nil);
-    if (_requiresMainThreadSetup) {
+    if (_requiresMainQueueSetup) {
       // The chances of deadlock here are low, because module init very rarely
       // calls out to other threads, however we can't control when a module might
       // get accessed by client code during bridge setup, and a very low risk of

React/Base/RCTRootView.m

@@ -58,7 +58,7 @@ NSString *const RCTContentDidAppearNotification = @"RCTContentDidAppearNotificat
                     moduleName:(NSString *)moduleName
              initialProperties:(NSDictionary *)initialProperties
 {
-  RCTAssertMainThread();
+  RCTAssertMainQueue();
   RCTAssert(bridge, @"A bridge instance is required to create an RCTRootView");
   RCTAssert(moduleName, @"A moduleName is required to create an RCTRootView");
 
@@ -175,7 +175,7 @@ RCT_NOT_IMPLEMENTED(- (instancetype)initWithCoder:(NSCoder *)aDecoder)
 
 - (NSNumber *)reactTag
 {
-  RCTAssertMainThread();
+  RCTAssertMainQueue();
   if (!super.reactTag) {
     /**
      * Every root view that is created must have a unique react tag.
@@ -191,14 +191,14 @@ RCT_NOT_IMPLEMENTED(- (instancetype)initWithCoder:(NSCoder *)aDecoder)
 
 - (void)bridgeDidReload
 {
-  RCTAssertMainThread();
+  RCTAssertMainQueue();
   // Clear the reactTag so it can be re-assigned
   self.reactTag = nil;
 }
 
 - (void)javaScriptDidLoad:(NSNotification *)notification
 {
-  RCTAssertMainThread();
+  RCTAssertMainQueue();
   RCTBridge *bridge = notification.userInfo[@"bridge"];
   [self bundleFinishedLoading:bridge];
 }
@@ -250,7 +250,7 @@ RCT_NOT_IMPLEMENTED(- (instancetype)initWithCoder:(NSCoder *)aDecoder)
 
 - (void)setAppProperties:(NSDictionary *)appProperties
 {
-  RCTAssertMainThread();
+  RCTAssertMainQueue();
 
   if ([_appProperties isEqualToDictionary:appProperties]) {
     return;

React/Base/RCTUtils.h

@@ -29,9 +29,19 @@ RCT_EXTERN id RCTJSONClean(id object);
 // Get MD5 hash of a string
 RCT_EXTERN NSString *RCTMD5Hash(NSString *string);
 
-// Execute the specified block on the main thread. Unlike dispatch_sync/async
-// this will not context-switch if we're already running on the main thread.
-RCT_EXTERN void RCTExecuteOnMainThread(dispatch_block_t block, BOOL sync);
+// Check is we are currently on the main queue (not to be confused with
+// the main thread, which is not neccesarily the same thing)
+// https://twitter.com/olebegemann/status/738656134731599872
+RCT_EXTERN BOOL RCTIsMainQueue(void);
+
+// Execute the specified block on the main queue. Unlike dispatch_async()
+// this will execute immediately if we're already on the main queue.
+RCT_EXTERN void RCTExecuteOnMainQueue(dispatch_block_t block);
+
+// Deprecated - do not use.
+RCT_EXTERN void RCTExecuteOnMainThread(dispatch_block_t block, BOOL sync)
+__deprecated_msg("Use RCTExecuteOnMainQueue instead. RCTExecuteOnMainQueue is "
+                 "async. If you need to use the `sync` option... please don't.");
 
 // Get screen metrics in a thread-safe way
 RCT_EXTERN CGFloat RCTScreenScale(void);

React/Base/RCTUtils.m

@@ -19,6 +19,7 @@
 #import <zlib.h>
 #import <dlfcn.h>
 
+#import "RCTAssert.h"
 #import "RCTLog.h"
 
 NSString *const RCTErrorUnspecified = @"EUNSPECIFIED";
@@ -228,9 +229,31 @@ NSString *RCTMD5Hash(NSString *string)
   ];
 }
 
+BOOL RCTIsMainQueue()
+{
+  static void *mainQueueKey = &mainQueueKey;
+  static dispatch_once_t onceToken;
+  dispatch_once(&onceToken, ^{
+    dispatch_queue_set_specific(dispatch_get_main_queue(),
+                                mainQueueKey, mainQueueKey, NULL);
+  });
+  return dispatch_get_specific(mainQueueKey) == mainQueueKey;
+}
+
+void RCTExecuteOnMainQueue(dispatch_block_t block)
+{
+  if (RCTIsMainQueue()) {
+    block();
+  } else {
+    dispatch_async(dispatch_get_main_queue(), ^{
+      block();
+    });
+  }
+}
+
 void RCTExecuteOnMainThread(dispatch_block_t block, BOOL sync)
 {
-  if ([NSThread isMainThread]) {
+  if (RCTIsMainQueue()) {
     block();
   } else if (sync) {
     dispatch_sync(dispatch_get_main_queue(), ^{

React/Modules/RCTAppState.m

@@ -16,7 +16,7 @@
 
 static NSString *RCTCurrentAppBackgroundState()
 {
-  RCTAssertMainThread();
+  RCTAssertMainQueue();
 
   static NSDictionary *states;
   static dispatch_once_t onceToken;

React/Modules/RCTUIManager.m

@@ -366,7 +366,7 @@ dispatch_queue_t RCTGetUIManagerQueue(void)
 
 - (void)registerRootView:(UIView *)rootView withSizeFlexibility:(RCTRootViewSizeFlexibility)sizeFlexibility
 {
-  RCTAssertMainThread();
+  RCTAssertMainQueue();
 
   NSNumber *reactTag = rootView.reactTag;
   RCTAssert(RCTIsReactRootView(reactTag),
@@ -404,13 +404,13 @@ dispatch_queue_t RCTGetUIManagerQueue(void)
 
 - (UIView *)viewForReactTag:(NSNumber *)reactTag
 {
-  RCTAssertMainThread();
+  RCTAssertMainQueue();
   return _viewRegistry[reactTag];
 }
 
 - (void)setFrame:(CGRect)frame forView:(UIView *)view
 {
-  RCTAssertMainThread();
+  RCTAssertMainQueue();
 
   // The following variable has no meaning if the view is not a react root view
   RCTRootViewSizeFlexibility sizeFlexibility = RCTRootViewSizeFlexibilityNone;
@@ -453,7 +453,7 @@ dispatch_queue_t RCTGetUIManagerQueue(void)
 
 - (void)setIntrinsicContentSize:(CGSize)size forView:(UIView *)view
 {
-  RCTAssertMainThread();
+  RCTAssertMainQueue();
 
   NSNumber *reactTag = view.reactTag;
   dispatch_async(RCTGetUIManagerQueue(), ^{
@@ -468,7 +468,7 @@ dispatch_queue_t RCTGetUIManagerQueue(void)
 
 - (void)setBackgroundColor:(UIColor *)color forView:(UIView *)view
 {
-  RCTAssertMainThread();
+  RCTAssertMainQueue();
 
   NSNumber *reactTag = view.reactTag;
 
@@ -534,7 +534,7 @@ dispatch_queue_t RCTGetUIManagerQueue(void)
 
 - (RCTViewManagerUIBlock)uiBlockWithLayoutUpdateForRootView:(RCTRootShadowView *)rootShadowView
 {
-  RCTAssert(![NSThread isMainThread], @"Should be called on shadow thread");
+  RCTAssert(!RCTIsMainQueue(), @"Should be called on shadow queue");
 
   // This is nuanced. In the JS thread, we create a new update buffer
   // `frameTags`/`frames` that is created/mutated in the JS thread. We access
@@ -835,7 +835,7 @@ RCT_EXPORT_METHOD(removeRootView:(nonnull NSNumber *)rootReactTag)
   [_rootViewTags removeObject:rootReactTag];
 
   [self addUIBlock:^(RCTUIManager *uiManager, NSDictionary<NSNumber *, UIView *> *viewRegistry){
-    RCTAssertMainThread();
+    RCTAssertMainQueue();
     UIView *rootView = viewRegistry[rootReactTag];
     [uiManager _purgeChildren:(NSArray<id<RCTComponent>> *)rootView.reactSubviews
                  fromRegistry:(NSMutableDictionary<NSNumber *, id<RCTComponent>> *)viewRegistry];
@@ -1488,7 +1488,7 @@ RCT_EXPORT_METHOD(clearJSResponder)
 
 static NSDictionary *RCTExportedDimensions(BOOL rotateBounds)
 {
-  RCTAssertMainThread();
+  RCTAssertMainQueue();
 
   // Don't use RCTScreenSize since it the interface orientation doesn't apply to it
   CGRect screenSize = [[UIScreen mainScreen] bounds];

React/Views/RCTComponentData.m

@@ -87,7 +87,7 @@ RCT_NOT_IMPLEMENTED(- (instancetype)init)
 
 - (UIView *)createViewWithTag:(NSNumber *)tag
 {
-  RCTAssertMainThread();
+  RCTAssertMainQueue();
 
   UIView *view = [self.manager view];
   view.reactTag = tag;