Summary:

In the old architecture, we have the ability to easily change the JavaScript runtime on Android using the overridden getJavaScriptExecutorFactory function in the ReactNativeHost abstract class. This creates a new instance of the factory and an executor with assigned runtime. However, in the new architecture, engine selection occurs in the DefaultReactHost object and depends on the isHermesEnabled property of the ReactNativeHost class. This logic is limited to engine selection between Hermes and JSC and lacks the ability to easily override it (as is the case with iOS code).

This PR includes a modification of this mechanism so that the function responsible for selecting the Runtime is in the DefaultReactNativeHost object – this will allow it to be written from within the project's MainApplication class, e.g:

class MainApplication : Application(), ReactApplication {
  override val reactNativeHost: ReactNativeHost =
    object : DefaultReactNativeHost(this) {
       //...
       override fun getJSRuntimeFactory(): JSRuntimeFactory {
           return CustomJSInstance() // e.g. V8Instance
       }
    }

Exactly as was the case with the old architecture and function:

override fun getJavaScriptExecutorFactory(): JavaScriptExecutorFactory {
  return CustomJSExecutorFactory()
}

This is particularly important when using custom JS engines other than Hermes or JSC in a project, such as the V8 (https://github.com/Kudo/react-native-v8), or smaller engines tailored to selected devices or solutions.

Changelog:

[ANDROID] [CHANGED] - Moving the Javascript Runtime selection to the DefaultReactNativeHost object to allow selection of custom JS runtimes

Test Plan:

All tests were successfully completed.