Flutter

Android (generate flutter)

• Each Java class or interface becomes a Dart extension type over JObject. Constructors are named by their parameters when overloaded (Intent.new$String$Uri), methods keep the Java name for the primary overload.
• Strings: JString by default (strict-native), or Dart String where lossless (ergonomic-dart).
• Java interfaces can be implemented in Dart (Runnable.implement(...)). Callbacks from any Java thread work; a throwing void callback is reported, not rethrown into Java.
• APIs newer than minApi are guarded and throw NativeApiUnavailableException on older devices before touching JNI.
• Java exceptions surface as NativeJavaException with class name, message and stack trace.
• Kotlin suspend functions in library jars become Futures.
• Objects hold JNI references: call release() for deterministic cleanup (otherwise finalizers clean up).

iOS (generate ios)

• Each Objective-C class or protocol becomes a Dart extension type over ObjCObject; Foundation types already provided by package:objective_c (NSString, NSArray, …) are reused.
• Selectors map to named parameters: addItem(item, atIndex: 2). Properties are getters and setters.
• Structs such as CGRect are passed and returned by value. NSError ** becomes a thrown NativeObjCError.
• Swift-only APIs are reachable through generated @objc adapters for the Objective-C-representable subset (iOS).

Size

Release APK of the example with one Android API: 14,842,216 bytes, both when bindings were generated for that API only and for the entire SDK (14,842,216 bytes); the app without bindings: 14,426,402 bytes (+415,814 bytes, mostly package:jni's native helper).

Cost per call

Measured on an Android emulator in profile mode: about 1.00 µs per generated JNI call versus about 271 µs for a hand-written MethodChannel round trip. On the iOS simulator (debug mode only): about 34 ns per objc_msgSend getter versus 20.4 µs per MethodChannel round trip. Emulator and simulator figures are not device figures.