RxFragmentDelegates.kt

import com.trello.rxlifecycle2.android.FragmentEvent
import io.reactivex.subjects.BehaviorSubject

/**
* Owner of a Fragment Lifecycle. Your Fragment should implement this interface. 
*/
interface FragmentLifeCycleOwner {
    val lifecycle: BehaviorSubject<FragmentEvent>
}

/**
 * Base delegate that sets and disposes the fragment's listener when the parent is
 * attached/detached.
 */
abstract class RxBaseParentDelegate<T>(private var fragment: Fragment?)
        ReadOnlyProperty<Fragment, T> {

    private var value: T? = null

    init {
        (fragment!! as FragmentLifeCycleOwner).lifecycle.subscribe { event ->
            if (event === FragmentEvent.ATTACH) {
                value = extractValue(fragment)
            } else if (event === FragmentEvent.DETACH) {
                value = null
                fragment = null
            }
        }
    }

    /**
     * Extract [T] from the fragment. This is called when the fragment is attached to the parent.
     */
    abstract fun extractValue(fragment: Fragment): T?

    operator fun setValue(thisRef: Fragment, property: KProperty<*>, value: T) {
        throw IllegalStateException("Cannot set value on a ReadOnlyProperty")
    }
          
    override operator fun getValue(thisRef: Fragment, property: KProperty<*>): T {
        return value!!
    }
}