How to pass delegates from Unity C# to a native Kotlin Android SDK?

Question

I have a native Android library I'm integrating into Unity which require me to pass C# delegates to the native SDK. As a design decision, I'm passing functions to the native SDK to be called when various events happen. I can't figure out how to pass a delegate to the native code which defines the handlers as public variables on the class like var onMyEvent: () -> Unit = {}. How can I pass a Unity C# function to the generated setOnMyEvent method?

Answer 1

I had a hard time with this problem since I was still thinking in terms of function pointers from the iOS integration. I tried using the low level AndroidJNI functions to force the method signature and creating function pointers from delegates to no avail. However, the solution was a lot simpler than what I was thinking. After looking at the decompiled AAR, I realized that the setOnMyEvent function expected a Function0 parameter, which is an object that conforms to the kotlin.jvm.functions.Function0 interface, defined as such:

/** A function that takes 0 arguments. */
public interface Function0<out R> : Function<R> {
    /** Invokes the function. */
    public operator fun invoke(): R
}

Using this, it is trivial to implement using the builtin Unity AndroidJavaProxy class:

using UnityEngine;
// ...
class Function0 : AndroidJavaProxy {
  private MyDelegate _callback;
  public Function0(MyDelegate callback) : base("kotlin.jvm.functions.Function0") {
    this._callback = callback;
  }

  public void invoke() { _callback(); }
}

In the Unity C# wrapper class all I needed to do was the following:

public void SetOnMyEvent(MyDelegate callback) {
  _myAndroidJavaObject.Call("setOnMyEvent", new Function0(callback));
}

I hope this helps anyone running into similar issues since I couldn't find any previous references on this topic. I spent more time than I'd like to admit on this.