TopActivityProviderBasic.kt

class TopActivityProvider {

  var topActivity: Activity? = null
    private set

  private val callbacks: Application.ActivityLifecycleCallbacks = object :  ActivityLifecycleCallbacks {
    override fun onActivityResumed(activity: Activity) {
      topActivity = activity
    }

    override fun onActivityPaused(activity: Activity) {
      topActivity = null
    }

    // Other methods are empty
  }

  private fun setupWith(app: Application) {
    app.registerActivityLifecycleCallbacks(callbacks)
  }
}