Turbo EventEmitters

[Dunedubby]

Hi, I'm having a little trouble understanding how the new EventEmitters are working behind the scenes on iOS, and if there's a disconnect between the documentation and what actually happens. So my understanding is that every function that is created in the JSI module gets a jsi::Value __hostFunction_... function generated in the ...-generated.cpp file, as well as a virtual method declaration in the SpecJSI.h file, among other things. Should we expecting similar functions to exist for a onThingHappens JS method, after we declare a readonly onThingHappens: EventEmitter<blah>? Or is there another syntax to call these events on the JS side?

[Dunedubby]

Okay so it seems that when they get added to the separate eventEmitterMap_ object, they are then checked separately after the method checks, as shown here: https://github.com/facebook/react-native/blob/35f0e1cca2299b5816cbc1847410651eb20a3f02/packages/react-native/ReactCommon/react/nativemodule/core/ReactCommon/TurboModule.h#L143

What scenarios could cause the methodMap_ objects to be recognized as functions, but the eventEmitterMap_ objects to not?

[Dunedubby]

Actually, after investigating more, it appears that in Obj-C modules, this create() function gets overridden in https://github.com/facebook/react-native/blob/main/packages/react-native/ReactCommon/react/nativemodule/core/platform/ios/ReactCommon/RCTInteropTurboModule.mm#L249

Except this implementation doesn't even look at the eventEmitterMap_ or any reference to the event emitters at all. In other words, it seems like Obj-C Turbo modules can't access the new EventEmitter framework at all, contrary to what the docs say. @cipolleschi am I missing something?

[Dunedubby]

I was missing something: bridged/legacy-compatible modules use RCTInteropTurboModule.mm, but bridgeless modules use RCTTurboModule.mm (if I understand correctly). After switching to fully bridgeless the EventEmitters seemed to connect correctly. Maybe this is worth mentioning in the docs (that legacy-compatible modules will have a hard time using the new EventEmitters), but at least the new EventEmitters are working with Swift, yay.

[cipolleschi]

@Dunedubby thanks for the thorough investigation and I apologize for the delay in answering. Last week we were bus with some internal projects and I could not dedicate a lor of time to Github, sadly.

Please consider that the configuration with TurboModule, Fabric and Bridge mode is not supported anymore. We keep providing the possibility to disable bridgeless for debugging purpose, but users are supposed to use the New Architecture as a whole: TurboModule, Fabric and Bridgeless.

I'm also a bit concerned in updating the docs with so low level details. You did a great job investigating and diving deep in the code, but those details will change in the future (for example when we approach the removal of the Old Architecture) so writing those down in the docs also mean that we have to keep them updated. Plus, I don't see how much they will benefit the average users who don't need to go in such details to learn and use React Native.

What do you think?