Move setBridge: off main thread

Summary:
Previously, if a module implemented `setBridge:` we assumed that it needs to be initialised on the main thread. This assumption was not really warranted however, and it was a barrier to deferring module initialization.

This diff tweaks the rules so that only modules that override `init` or `constantsToExport**` are assumed to require main thread initialization, and others can be created lazily when they are first used.

WARNING: this will be a breaking change to any 3rd party modules that are assuming `setBridge:` is called on the main thread. Those modules should be rewritten to move any code that requires the main thread into `init` or `constantsToExport` instead.

`**` We will also be examining whether `constantsToExport` can be done lazily, but for now any module that uses it will still be created eagerly when the bridge starts up.

Reviewed By: javache

Differential Revision: D3240682

fb-gh-sync-id: 48f309e3158bbccb52141032baf70def3e609371
fbshipit-source-id: 48f309e3158bbccb52141032baf70def3e609371

Examples/UIExplorer/UIExplorerUnitTests/RCTModuleInitTests.m

@@ -221,10 +221,16 @@ RCT_EXPORT_MODULE()
 
 - (void)testCustomSetBridgeModuleInitializedAtBridgeStartup
 {
-  RUN_RUNLOOP_WHILE(!_customSetBridgeModuleNotificationSent);
+  XCTAssertFalse(_customSetBridgeModuleNotificationSent);
+
+  __block RCTTestCustomSetBridgeModule *module;
+  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
+    module = [_bridge moduleForClass:[RCTTestCustomSetBridgeModule class]];
+  });
+
+  RUN_RUNLOOP_WHILE(!module);
   XCTAssertTrue(_customSetBridgeModuleNotificationSent);
-  RCTTestCustomSetBridgeModule *module = [_bridge moduleForClass:[RCTTestCustomSetBridgeModule class]];
-  XCTAssertTrue(module.setBridgeOnMainThread);
+  XCTAssertFalse(module.setBridgeOnMainThread);
   XCTAssertEqual(module.bridge, _bridge.batchedBridge);
   XCTAssertNotNil(module.methodQueue);
 }

React/Base/RCTModuleData.m

@@ -37,19 +37,15 @@
   _instanceLock = [NSLock new];
 
   static IMP objectInitMethod;
-  static SEL setBridgeSelector;
   static dispatch_once_t onceToken;
   dispatch_once(&onceToken, ^{
     objectInitMethod = [NSObject instanceMethodForSelector:@selector(init)];
-    setBridgeSelector = NSSelectorFromString(@"setBridge:");
   });
 
-  // If a module overrides `init`, `setBridge:` then we must assume that it
-  // expects for both of those methods to be called on the main thread, because
-  // they may need to access UIKit.
-  _requiresMainThreadSetup =
-  [_moduleClass instancesRespondToSelector:setBridgeSelector] ||
-  (!_instance && [_moduleClass instanceMethodForSelector:@selector(init)] != objectInitMethod);
+  // 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 &&
+  [_moduleClass instanceMethodForSelector:@selector(init)] != objectInitMethod;
 
   // If a module overrides `constantsToExport` then we must assume that it
   // must be called on the main thread, because it may need to access UIKit.

React/Base/RCTUtils.m

@@ -258,6 +258,10 @@ CGFloat RCTScreenScale()
 
 CGSize RCTScreenSize()
 {
+  // FIXME: this caches the bounds at app start, whatever those were, and then
+  // doesn't update when the device is rotated. We need to find another thread-
+  // safe way to get the screen size.
+
   static CGSize size;
   static dispatch_once_t onceToken;
   dispatch_once(&onceToken, ^{

React/Modules/RCTAppState.m

@@ -43,13 +43,20 @@ RCT_EXPORT_MODULE()
 
 #pragma mark - Lifecycle
 
+- (instancetype)init
+{
+  if ((self = [super init])) {
+
+    // Needs to be called on the main thread, as it accesses UIApplication
+    _lastKnownState = RCTCurrentAppBackgroundState();
+  }
+  return self;
+}
+
 - (void)setBridge:(RCTBridge *)bridge
 {
   _bridge = bridge;
 
-  // Is this thread-safe?
-  _lastKnownState = RCTCurrentAppBackgroundState();
-
   for (NSString *name in @[UIApplicationDidBecomeActiveNotification,
                            UIApplicationDidEnterBackgroundNotification,
                            UIApplicationDidFinishLaunchingNotification,

React/Modules/RCTDevLoadingView.m

@@ -40,6 +40,13 @@ RCT_EXPORT_MODULE()
   [[NSNotificationCenter defaultCenter] removeObserver:self];
 }
 
+- (instancetype)init
+{
+  // We're only overriding this to ensure the module gets created at startup
+  // TODO (D3175632): Remove once we have more declarative control over module setup.
+  return [super init];
+}
+
 - (void)setBridge:(RCTBridge *)bridge
 {
   _bridge = bridge;

React/Modules/RCTUIManager.m

@@ -730,7 +730,7 @@ RCT_EXPORT_METHOD(removeSubviewsFromContainerWithID:(nonnull NSNumber *)containe
 
 - (void)_removeChildren:(NSArray<id<RCTComponent>> *)children
           fromContainer:(id<RCTComponent>)container
-              permanent: (BOOL)permanent
+              permanent:(BOOL)permanent
 {
   RCTLayoutAnimation *layoutAnimation = _layoutAnimation;
   RCTAnimation *deleteAnimation = layoutAnimation.deleteAnimation;

React/Profiler/RCTPerfMonitor.m

@@ -125,6 +125,25 @@ static vm_size_t RCTGetResidentMemorySize(void)
 
 RCT_EXPORT_MODULE()
 
+- (instancetype)init
+{
+  // We're only overriding this to ensure the module gets created at startup
+  // TODO (D3175632): Remove once we have more declarative control over module setup.
+  return [super init];
+}
+
+- (dispatch_queue_t)methodQueue
+{
+  return dispatch_get_main_queue();
+}
+
+- (void)setBridge:(RCTBridge *)bridge
+{
+  _bridge = bridge;
+
+  [_bridge.devMenu addItem:self.devMenuItem];
+}
+
 - (void)invalidate
 {
   [self hide];
@@ -272,18 +291,6 @@ RCT_EXPORT_MODULE()
   return _metrics;
 }
 
-- (dispatch_queue_t)methodQueue
-{
-  return dispatch_get_main_queue();
-}
-
-- (void)setBridge:(RCTBridge *)bridge
-{
-  _bridge = bridge;
-
-  [_bridge.devMenu addItem:self.devMenuItem];
-}
-
 - (void)show
 {
   if (_container) {

React/Views/RCTViewManager.h

@@ -30,7 +30,7 @@ typedef void (^RCTViewManagerUIBlock)(RCTUIManager *uiManager, NSDictionary<NSNu
  * allowing the manager (or the views that it manages) to manipulate the view
  * hierarchy and send events back to the JS context.
  */
-@property (nonatomic, weak, readonly) RCTBridge *bridge;
+@property (nonatomic, weak) RCTBridge *bridge;
 
 /**
  * This method instantiates a native view to be managed by the module. Override