画面遷移用DSL

HogeFragment.kt

class HogeFragment: Fragment, Screen {

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        hogeButton.setOnClickListener { navigate(Event.HogeOkClicked()) }
    }
}

SampleActivity.kt

class SampleActivity : AppCompatActivity(), Screen {
    private val route: Route by Extra()

    private val navigator by lazy { createNavigator(route, supportFragmentManager, R.id.fragment_nav_container, this) }

    override fun onCreate(savedInstanceState: Bundle?) {
            /**  Activity初期生成の場合のみ[navigate]によって表示するFragmentを指定する。Activity再生成の場合は、Activity生成前に表示していたFragmentがAndroidOSによって再表示されれば良いので、何もしない。 */
        if (savedInstanceState == null) navigate(this, Event.OnRootCreated)
    }
}

SampleNavigator.kt

interface Screen

sealed class Event {
    object HogeOkClicked : Event()
    object FugaOkClicked : Event()
    object FugaCancelClicked : Event()
}

fun createNavigator(fm: FragmentManager, @IdRes container: Int, activity: Activity): ScreenNavigator<Screen, Event> =
    navigator {
        screen<SampleActivity> {
          on<OnRootCreated> { /** 遷移処理 */ }
        }
        screen<HogeFragment> {
            on<HogeOkClicked> { /** 遷移処理 */ }
        }
        screen<FugaFragment> {
            on<FugaOkClicked> { /** 遷移処理 */ }
            on<FugaCancelClicked> { /** 遷移処理 */ }
        }
    }
}

ScreenNavigator.kt

data class ScreenNavigator<SCREEN, EVENT>(
    private val screenDefinitions: Map<Class<out SCREEN>, ScreenDefinition<SCREEN, EVENT>>) {

    /**
     * [navigator]で登録した中から、[fromScreen]と[event]に対応する遷移関数を探し実行する
     * @throws NavigationNotFoundException
     */
    fun navigate(fromScreen: SCREEN, event: EVENT): SCREEN =
        findScreenDefinition(fromScreen)?.findEventDefinition(event)?.navigate(fromScreen, event)
            ?: throw NavigationNotFoundException("fromScreen:$fromScreen, event:$event")

    private fun findScreenDefinition(screen: SCREEN): ScreenDefinition<SCREEN, EVENT>? =
        screenDefinitions
            .filterKeys { screenClass -> screenClass.isInstance(screen) }
            .values
            .firstOrNull()

    data class ScreenDefinition<SCREEN, EVENT>(private val eventDefinitions: Map<Class<out EVENT>, EventDefinition<SCREEN, EVENT>>) {
        fun findEventDefinition(event: EVENT): EventDefinition<SCREEN, EVENT>? =
            eventDefinitions
                .filterKeys { eventClass -> eventClass.isInstance(event) }
                .values
                .firstOrNull()

        data class EventDefinition<SCREEN, EVENT>(val navigateFunc: (SCREEN, EVENT) -> SCREEN) {
            fun navigate(fromScreen: SCREEN, event: EVENT): SCREEN = navigateFunc(fromScreen, event)
        }
    }

    class Builder<SCREEN, EVENT> {
        val screenDefinitions = mutableMapOf<Class<out SCREEN>, ScreenDefinition<SCREEN, EVENT>>()

        inline fun <reified S : SCREEN> screen(noinline init: ScreenDefinitionBuilder<S>.() -> Unit) {
            require(!screenDefinitions.containsKey(S::class.java)) { "${S::class.java.simpleName}は既に定義されています" }
            screenDefinitions[S::class.java] = ScreenDefinitionBuilder<S>().apply(init).build()
        }

        fun build(): ScreenNavigator<SCREEN, EVENT> = ScreenNavigator(screenDefinitions.toMap())

        inner class ScreenDefinitionBuilder<S : SCREEN> {
            val eventDefinitions = mutableMapOf<Class<out EVENT>, ScreenDefinition.EventDefinition<SCREEN, EVENT>>()

            inline fun <reified E : EVENT> on(noinline navigateFunc: S.(E) -> SCREEN) {
                require(!eventDefinitions.containsKey(E::class.java)) { "${E::class.java.simpleName}は既に定義されています" }
                @Suppress("UNCHECKED_CAST")
                eventDefinitions[E::class.java] = ScreenDefinition.EventDefinition { fromScreen, event -> navigateFunc((fromScreen as S), event as E) }
            }

            fun build() = ScreenDefinition(eventDefinitions)
        }
    }

    companion object {
        /** [SCREEN]と[EVENT]、それらに対応する遷移関数を登録していく */
        fun <SCREEN, EVENT> navigator(init: Builder<SCREEN, EVENT>.() -> Unit): ScreenNavigator<SCREEN, EVENT> =
            Builder<SCREEN, EVENT>().apply(init).build()
    }
}

class NavigationNotFoundException(msg: String) : Exception(msg)