Problems inheriting inner java class

Question

I'm creating an android live wallpaper using Kotlin. This requires a class that extends WallpaperService, which contains an inner class that extends WallpaperService.Engine.

So I've written this:

import android.service.wallpaper.WallpaperService
import android.service.wallpaper.WallpaperService.Engine

public class MyWallpaperService : WallpaperService() {

    override fun onCreateEngine(): Engine = MyEngine()

    private inner class MyEngine : Engine() {

    }
}

The problem is that I'm getting the following 2 errors at compile time:

Error:java.lang.RuntimeException: Error generating constructors of class MyEngine with kind IMPLEMENTATION 

Error:java.lang.UnsupportedOperationException: Don't know how to generate outer expression for lazy class MyWallpaperService

I cant figure out why this is happening so any help would be greatly appreciated.

Answer 1

The best solution I have found is to use an intermediate Java class:

public class Intermediate extends WallpaperService.Engine {
    public Intermediate(WatchfaceService outer) {
        outer.super();
    }
}

Then the inner class in your Kotlin WallpaperService should inherit Intermediate, passing the outer class as parameter.

public class MyWallpaperService : WallpaperService() {
    override fun onCreateEngine(): Engine = MyEngine()
​
    private inner class MyEngine : Intermediate(this) {
    }
}