add support to provide fallback-sourceURL: in case primary-sourceURL fails to load

Reviewed By: javache

Differential Revision: D3339692

fbshipit-source-id: 93fa1821bf4abca878832d4f75c6b9968d8d0460

React/Base/RCTBatchedBridge.m

@@ -170,7 +170,18 @@ RCT_EXTERN NSArray<Class> *RCTGetModuleClasses(void);
   if ([self.delegate respondsToSelector:@selector(loadSourceForBridge:withBlock:)]) {
     [self.delegate loadSourceForBridge:_parentBridge withBlock:onSourceLoad];
   } else if (self.bundleURL) {
+    [RCTJavaScriptLoader loadBundleAtURL:self.bundleURL onComplete:^(NSError *error, NSData *source) {
+      if (error && [self.delegate respondsToSelector:@selector(fallbackSourceURLForBridge:)]) {
+        NSURL *fallbackURL = [self.delegate fallbackSourceURLForBridge:_parentBridge];
+        if (fallbackURL && ![fallbackURL isEqual:self.bundleURL]) {
+          RCTLogError(@"Failed to load bundle(%@) with error:(%@)", self.bundleURL, error.localizedDescription);
+          self.bundleURL = fallbackURL;
           [RCTJavaScriptLoader loadBundleAtURL:self.bundleURL onComplete:onSourceLoad];
+          return;
+        }
+      }
+      onSourceLoad(error, source);
+    }];
   } else {
     // Allow testing without a script
     dispatch_async(dispatch_get_main_queue(), ^{

React/Base/RCTBridgeDelegate.h

@@ -23,6 +23,15 @@ typedef void (^RCTSourceLoadBlock)(NSError *error, NSData *source);
 
 @optional
 
+/**
+ * The bridge will attempt to load the JS source code from the location specified
+ * by the `sourceURLForBridge:` method, if loading fails, you can implement this
+ * method to specify fallbackSourceURL.
+ * NOTE: We don't plan to support this API permanently (this method will be
+ * removed after we track down why a valid sourceURL fails to load sometimes).
+ */
+- (NSURL *)fallbackSourceURLForBridge:(RCTBridge *)bridge;
+
 /**
  * The bridge initializes any registered RCTBridgeModules automatically, however
  * if you wish to instantiate your own module instances, you can return them