flow-mvvm.kt

class StateHolder<T : Any>(state: T) {
    private val _liveData = MutableLiveData(state)
    val liveData: LiveData<T> get() = _liveData

    private var _value: T = state
    val value: T get() = _value

    fun update(notify: Boolean = true, block: T.() -> T) {
        synchronized(LOCK) {
            _value = block(value)
            if (notify) _liveData.postValue(value)
        }
    }

    private companion object {
        val LOCK = Object()
    }
}

class VM : ViewModel() {
    private val _state = StateHolder(State())
    val state: LiveData<State> get() = _state.liveData
    private val _actions = Channel<Action>(Channel.CONFLATED)
    val actions: Flow<Action> = flow { for (e in _actions) emit(e) }
    private val _viewActions = Channel<ViewAction>(Channel.CONFLATED)
    val viewActions: Flow<ViewAction> = flow { for (e in _viewActions) emit(e) }

    sealed class Action {
        data class Navigate(val directions: NavDirections) : Action()
    }

    sealed class ViewAction {
        data class VisitPage(val url: String) : ViewAction()
    }

    data class State(
            val isLoading: Boolean = false
    )
}

class Fragment : Fragment() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        lifecycleScope.launchWhenStarted {
            vm.actions.collect { onActionRequested(it) }
        }
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        viewLifecycleOwner.lifecycleScope.launchWhenCreated {
            vm.viewActions.collect { onViewActionRequested(it) }
        }
        vm.state.observe(viewLifecycleOwner) {
            onViewStateChanged(it)
        }
    }

    private fun onActionRequested(action: VM.Action) {
        when (action) {
            is VM.Action.Navigate -> findNavController().navigate(action.directions)
        }
    }

    private fun onViewActionRequested(action: VM.ViewAction) {
        when (action) {
            is VM.ViewAction.VisitPage -> binding.web.loadUrl(action.url)
        }
    }

    private fun onViewStateChanged(state: VM.State) {
        progress?.isVisible = state.isLoading
        binding.web.isVisible = state.isWebViewVisible
        binding.finishCheckInHint.isVisible = state.isFinishButtonVisible
    }
}