Android Fragment that holds lifecycle for being hidden

HidableFragment.kt

import android.os.Bundle
import androidx.annotation.CallSuper
import androidx.fragment.app.FragmentTransaction
import androidx.lifecycle.Lifecycle
import androidx.lifecycle.LifecycleOwner
import androidx.lifecycle.LifecycleRegistry

/**
 * By default [FragmentTransaction.hide] does not trigger any lifecycle events (Fragment is
 * still started and resumed even if invisible).
 *
 * This class provides a solution with [onFragmentDisplayed], [onFragmentHidden] callbacks and
 * [visibleViewLifecycle] which is Lifecycle that starts or stops depending on invisibility.
 *
 * This class is a workaround for the https://github.com/Zhuinden/simple-stack/issues/133
 */
abstract class HidableFragment : Fragment() {
    /**
     * Lifecycle that based on [onFragmentDisplayed] and [onFragmentHidden] callbacks
     */
    protected val visibleViewLifecycle = SimpleLifecycleOwner()

    private var activityStarted : Boolean = false

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        visibleViewLifecycle.handleLifecycleEvent(Lifecycle.Event.ON_CREATE)
    }

    override fun onStart() {
        super.onStart()

        activityStarted = true

        if (!isHidden) {
            onFragmentDisplayed()
        }
    }

    override fun onStop() {
        activityStarted = false
        if (!isHidden) {
            onFragmentHidden()
        }
        super.onStop()
    }

    override fun onDestroy() {
        visibleViewLifecycle.handleLifecycleEvent(Lifecycle.Event.ON_DESTROY)

        super.onDestroy()
    }

    /**
     * Callback that gets called whenever fragment is no longer visible to the user,
     * either due to whole app not being visible or due to just fragment being invisible
     */
    @CallSuper
    open fun onFragmentHidden() {
        visibleViewLifecycle.handleLifecycleEvent(Lifecycle.Event.ON_STOP)
    }

    /**
     * Callback that gets called whenever fragment becomes longer visible to the user,
     * either due to whole app becoming visible or due to just fragment being un-hidden
     */
    @CallSuper
    open fun onFragmentDisplayed() {
        visibleViewLifecycle.handleLifecycleEvent(Lifecycle.Event.ON_START)
    }

    override fun onHiddenChanged(hidden: Boolean) {
        if (hidden) {
            if (activityStarted) {
                onFragmentHidden()
            }
        } else if (activityStarted) {
            onFragmentDisplayed()
        }
    }

    protected class SimpleLifecycleOwner : LifecycleOwner {
        private val lifecycle = LifecycleRegistry(this)

        override fun getLifecycle(): Lifecycle {
            return lifecycle
        }

        internal fun handleLifecycleEvent(event: Lifecycle.Event) {
            lifecycle.handleLifecycleEvent(event)
        }
    }
}