2025-03-18 - react-native-ios-utilities Crash Investigation.md

2025-03-18 - react-native-ios-utilities - Crash Investigation

Gist URL

Things to check:

• AppDelegate
• Package Config - Dependencies
• Package Config - Codegen
• React Native Config
• Metro Config

• In the RN template, the AppDelegate inherits from RCTAppDelegate.
• A newly added property called: RCTAppDelegate.dependencyProvider is initialized with an instance of RCTAppDependencyProvider
  • This property accepts an object that conforms to RCTDependencyProvider
  • It essential acts as a registry for class names and their corresponding class types.
  • The requirements for this protocol defines properties that hold dictionaries (i.e. mapping from string to class types), or arrays that hold class names that conform to some protocol (e.g. (NSArray<NSString *> *)imageURLLoaderClassNames).
  • The registry/map for the "third party components" are in the RCTComponentViewProtocol.thirdPartyFabricComponents property, and is defined as a "string to class type" dictionary (i.e. NSDictionary<NSString *, Class<RCTComponentViewProtocol>>).
  • I.e. a mapping of the "component handle" (class name), to the class types that conform to RCTComponentViewProtocol.
    
    
• Since RCTAppDelegate.dependencyProvider is initialized with an instance of RCTAppDependencyProvider, it conforms to RCTDependencyProvider.
• The RCTAppDependencyProvider.thirdPartyFabricComponents is a computed lazy static property (lazily initialized once).
  • This is because it uses a dispatch_once_t token, and uses a dispatch_once block to cache _thirdPartyFabricComponents ivar.
  • As such every time RCTAppDependencyProvider.thirdPartyFabricComponents is invoked, it just returns the cache value from _thirdPartyFabricComponents ivar.
  • The value returned from this computed static property is from: RCTThirdPartyComponentsProvider.thirdPartyFabricComponents (important).
    
    
• The initialization of RCTAppDependencyProvider.thirdPartyFabricComponents is where the crash originates.
  • Error Message: com.facebook.react.runtime.JavaScript (18): "-[__NSPlaceholderDictionary initWithObjects:forKeys:count:]: attempt to insert nil object from objects[17].
  • Error Desc: "An abort signal terminated the process. Such crashes often happen because of an uncaught exception or unrecoverable error or calling the abort() function"
  • For whatever reason, whenever RCTAppDependencyProvider.thirdPartyFabricComponents is "read" for the first time, it triggers a crash related to a dictionary being empty (or in this case, being malformed).

• We know that RCTThirdPartyComponentsProvider.thirdPartyFabricComponents comes from codegen/turbo modules.
  • To recap, when codegen runs during pod install, It generates stubs and implementations for all the fabric components (defined via the turbo module system).
  • Codegen searches for all the files that ends with: NativeComponent.ts and reads the codegenNativeComponent config/spec defined for the given fabric component.
  • Additionally, in codegenConfig property in your library package.json defines extra settings, most notably: codegenConfig.componentProvider (Record<string, string>).
  • So the crash originates from codegen, specifically the mapping between the handle. i.e. name of the fabric component to its associated class type (component registry).
    
    
• If we check RCTThirdPartyComponentsProvider.mm (in this particular project, it's located at: example/ios/build/generated/ios/RCTThirdPartyComponentsProvider.mm), we can see the mapping the codegen generates.
  • From screenshot, we can see the generated dict. is malformed.