@Singleton
class ApplicationInjector @Inject constructor(
        private val injectorFactories: Map<Class<out Activity>, @JvmSuppressWildcards Provider<ProvidesActivityInjectorFactories.Builder>>
) : Application.ActivityLifecycleCallbacks {

    //region < Unused callback methods >

    override fun onActivityPaused(activity: Activity) = Unit
    override fun onActivityResumed(activity: Activity) = Unit
    override fun onActivityStarted(activity: Activity) = Unit
    override fun onActivityDestroyed(activity: Activity) = Unit
    override fun onActivitySaveInstanceState(activity: Activity, outState: Bundle?) = Unit

    //endregion

    /**
     * Map of components that survive a configuration change.
     */
    private val screenComponents: MutableMap<Class<out Activity>, ProvidesActivityInjectorFactories> = HashMap()

    override fun onActivityStopped(activity: Activity) {
        if (activity.isFinishing) {
            // remove finished activity component
            screenComponents.remove(activity.javaClass)
        }
    }

    override fun onActivityCreated(activity: Activity, savedInstanceState: Bundle?) {
        val screenComponent: ProvidesActivityInjectorFactories = screenComponents[activity.javaClass]
                ?: injectorFactories[activity.javaClass]!!.get().build().also {
                    screenComponents[activity.javaClass] = it
                }

        @Suppress("UNCHECKED_CAST")
        val activityInjectorFactory: AndroidInjector.Factory<Activity> = (screenComponent.provideInjectors()[activity.javaClass] as AndroidInjector.Factory<Activity>)

        activityInjectorFactory.create(activity).inject(activity)
    }

}