A solution to allow use of `targetFragment` in conjunction with the Android Jetpack Navigation Component

AutoTargetFragment.kt

import android.util.Log
import androidx.fragment.app.Fragment
import androidx.fragment.app.FragmentFactory
import androidx.fragment.app.FragmentManager

/** Performs [work] on the [Fragment.getTargetFragment] of type [TargetFragmentType] */
inline fun <reified TargetFragmentType> Fragment.withCallback(work: TargetFragmentType.() -> Unit) {
    val typedTarget = targetFragment as? TargetFragmentType
    if (typedTarget == null) {
        Log.e(
            "AutoTargetFragment",
            "Target Fragment is not of expected type ${TargetFragmentType::class.qualifiedName}"
        )
        return
    }
    typedTarget.work()
}

/**
 * Automatically sets the target fragment of newly created fragments to the previous fragment.
 * This therefore allows you to use 'targetFragment' in conjunction with the
 * Jetpack Navigation Component, for example.
 *
 * You usually want to call this function on your Activity top-level FragmentManager.
 */
fun FragmentManager.autoTarget() {
    fragmentFactory = ChildManagerFragmentFactory(this)
}

/** An [AutoTargetFragmentFactory] which determines current fragment via the child [FragmentManager] of [fragmentManager] */
class ChildManagerFragmentFactory(
    private val fragmentManager: FragmentManager
) : AutoTargetFragmentFactory() {
    override fun getCurrentFragment() =
        fragmentManager.primaryNavigationFragment?.childFragmentManager?.fragments?.firstOrNull()
}

abstract class AutoTargetFragmentFactory : FragmentFactory() {
    abstract fun getCurrentFragment(): Fragment?

    override fun instantiate(classLoader: ClassLoader, className: String): Fragment {
        val fragment = super.instantiate(classLoader, className)
        val currentFragment = getCurrentFragment()
        fragment.setTargetFragment(currentFragment, REQUEST_CODE)
        return fragment
    }

    companion object {
        const val REQUEST_CODE = 0
    }
}

Simply setup like so:

class MainActivity : AppCompatActivity(R.layout.activity_main) {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        supportFragmentManager.autoTarget()
    }
}

And then to use a callback, use:

withCallback<MyCallbackType> { myCallback() }