Calling lifecycle.addObserver from a Kotlin abstract class

Question

I have an abstract class that implements DefaultLifecycleObserver. I'd like to call lifecycle.addObserver(this) from the init block, but it says "Leaking 'this' in constructor of non-final class MyAbstractClass".

My code:

abstract class MyAbstractClass(protected val activity: AppCompatActivity) : DefaultLifecycleObserver {
    init {
        activity.lifecycle.addObserver(this)
    }
  .
  .
  .
}

I can move this line of code to the init block of each final class that extends this abstract class, but I don't like the idea, especially because I want to guarantee that each new class that will extend MyAbstractClass in the future will call it as well. Is there a better place to call this without creating a leak?

score 1 · Accepted Answer · answered Aug 07 '20 at 02:48

I suppose you could post your call so it only happens after the object is fully instantiated:

abstract class MyAbstractClass(protected val activity: AppCompatActivity) : DefaultLifecycleObserver {
    init {
        Handler(Looper.getMainLooper()).post {
            activity.lifecycle.addObserver(this)
        }
    }
}

Or it might be less surprising to create an extension function you can tack onto your constructor calls. Then you can explicitly start the observation immediately. You'd have to make activity public, though. By defining it in an extension like this, your subclasses can call this and return themselves so you can chain it to constructor calls.

fun <T: MyAbstractClass> T.alsoBegin(): T {
    activity.lifecycle.addObserver(this)
    return this
}

val foo = SomeImplementation(myActivity).alsoBegin()

Calling lifecycle.addObserver from a Kotlin abstract class

1 Answers1