【发布时间】：2020-01-08 13:48:48
【问题描述】：
我是第一次尝试 Kotlin Coroutines 和 Flow，我正在尝试使用 MVI-ish 方法重现我在带有 RxJava 的 Android 上使用的某个流程，但我很难正确处理，而且我基本上被卡住了此时。
RxJava 应用程序看起来基本上是这样的：
MainActivityView.kt
object MainActivityView {
sealed class Event {
object OnViewInitialised : Event()
}
data class State(
val renderEvent: RenderEvent = RenderEvent.None
)
sealed class RenderEvent {
object None : RenderEvent()
class DisplayText(val text: String) : RenderEvent()
}
}
MainActivity.kt
MainActivity 有一个PublishSubject 的实例，其类型为Event。即MainActivityView.Event.OnViewInitialised、MainActivityView.Event.OnError 等。初始事件通过主题的.onNext(Event) 调用在onCreate() 中发送。
@MainActivityScope
class MainActivity : AppCompatActivity(R.layout.activity_main) {
@Inject
lateinit var subscriptions: CompositeDisposable
@Inject
lateinit var viewModel: MainActivityViewModel
@Inject
lateinit var onViewInitialisedSubject: PublishSubject<MainActivityView.Event.OnViewInitialised>
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setupEvents()
}
override fun onDestroy() {
super.onDestroy()
subscriptions.clear()
}
private fun setupEvents() {
if (subscriptions.size() == 0) {
Observable.mergeArray(
onViewInitialisedSubject
.toFlowable(BackpressureStrategy.BUFFER)
.toObservable()
).observeOn(
Schedulers.io()
).compose(
viewModel()
).observeOn(
AndroidSchedulers.mainThread()
).subscribe(
::render
).addTo(
subscriptions
)
onViewInitialisedSubject
.onNext(
MainActivityView
.Event
.OnViewInitialised
)
}
}
private fun render(state: MainActivityView.State) {
when (state.renderEvent) {
MainActivityView.RenderEvent.None -> Unit
is MainActivityView.RenderEvent.DisplayText -> {
mainActivityTextField.text = state.renderEvent.text
}
}
}
}
MainActivityViewModel.kt
然后这些Event 被MainActivityViewModel 类拾取，该类由.compose(viewModel()) 调用，然后通过ObservableTransformer<Event, State> 将接收到的Event 转换为一种新的State。视图模型返回一个带有renderEvent 的新状态，然后可以通过render(state: MainActivityView.State) 函数再次在MainActivity 中对其进行操作。
@MainActivityScope
class MainActivityViewModel @Inject constructor(
private var state: MainActivityView.State
) {
operator fun invoke(): ObservableTransformer<MainActivityView.Event, MainActivityView.State> = onEvent
private val onEvent = ObservableTransformer<MainActivityView.Event,
MainActivityView.State> { upstream: Observable<MainActivityView.Event> ->
upstream.publish { shared: Observable<MainActivityView.Event> ->
Observable.mergeArray(
shared.ofType(MainActivityView.Event.OnViewInitialised::class.java)
).compose(
eventToViewState
)
}
}
private val eventToViewState = ObservableTransformer<MainActivityView.Event, MainActivityView.State> { upstream ->
upstream.flatMap { event ->
when (event) {
MainActivityView.Event.OnViewInitialised -> onViewInitialisedEvent()
}
}
}
private fun onViewInitialisedEvent(): Observable<MainActivityView.State> {
val renderEvent = MainActivityView.RenderEvent.DisplayText(text = "hello world")
state = state.copy(renderEvent = renderEvent)
return state.asObservable()
}
}
我可以使用协程/流/通道实现相同的流程吗？甚至可能有点简化？
编辑：
我已经找到了适合我的解决方案，到目前为止我还没有发现任何问题。然而，这个解决方案使用ConflatedBroadcastChannel<T>，最终将被弃用，它可能会用（在撰写本文时）尚未发布的SharedFlow api 替换它（更多关于here。
它的工作方式是 Activity 和 viewmodel 共享
一个ConflatedBroadcastChannel<MainActivity.Event>，用于从Activity（或适配器）发送或提供事件。视图模型将事件减少到一个新的状态，然后发出。 Activity 正在收集由viewModel.invoke() 返回的Flow<State>，并最终呈现发出的State。
MainActivityView.kt
object MainActivityView {
sealed class Event {
object OnViewInitialised : Event()
data class OnButtonClicked(val idOfItemClicked: Int) : Event()
}
data class State(
val renderEvent: RenderEvent = RenderEvent.Idle
)
sealed class RenderEvent {
object Idle : RenderEvent()
data class DisplayText(val text: String) : RenderEvent()
}
}
MainActivity.kt
class MainActivity : AppCompatActivity(R.layout.activity_main) {
@Inject
lateinit var viewModel: MainActivityViewModel
@Inject
lateinit eventChannel: ConflatedBroadcastChannel<MainActivityView.Event>
private var isInitialised: Boolean = false
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
init()
}
private fun init() {
if (!isInitialised) {
lifecycleScope.launch {
viewModel()
.flowOn(
Dispatchers.IO
).collect(::render)
}
eventChannel
.offer(
MainActivityView.Event.OnViewInitialised
)
isInitialised = true
}
}
private suspend fun render(state: MainActivityView.State): Unit =
when (state.renderEvent) {
MainActivityView.RenderEvent.Idle -> Unit
is MainActivityView.RenderEvent.DisplayText ->
renderDisplayText(text = state.renderEvent.text)
}
private val renderDisplayText(text: String) {
// render text
}
}
MainActivityViewModel.kt
class MainActivityViewModel constructor(
private var state: MainActivityView.State = MainActivityView.State(),
private val eventChannel: ConflatedBroadcastChannel<MainActivityView.Event>,
) {
suspend fun invoke(): Flow<MainActivityView.State> =
eventChannel
.asFlow()
.flatMapLatest { event: MainActivityView.Event ->
reduce(event)
}
private fun reduce(event: MainActivityView.Event): Flow<MainActivityView.State> =
when (event) {
MainActivityView.Event.OnViewInitialised -> onViewInitialisedEvent()
MainActivityView.Event.OnButtonClicked -> onButtonClickedEvent(event.idOfItemClicked)
}
private fun onViewInitialisedEvent(): Flow<MainActivityView.State> = flow
val renderEvent = MainActivityView.RenderEvent.DisplayText(text = "hello world")
state = state.copy(renderEvent = renderEvent)
emit(state)
}
private fun onButtonClickedEvent(idOfItemClicked: Int): Flow<MainActivityView.State> = flow
// do something to handle click
println("item clicked: $idOfItemClicked")
emit(state)
}
}
类似问题：
【问题讨论】：
-
是的，您可以通过
BroadcastChannel和asFlow()来实现这一点。 -
你能举个例子吗？我无法理解 BroadcastChannel 的工作原理。
标签： android kotlin kotlin-coroutines