LiveData Observer 在不需要时触发

Question

我有一个视图模型从场景中接收流作为实时数据

val state get () = syncScenario.state.asLiveData ()

在activity中，我们订阅了这个实时数据，发生了一些逻辑并使用了activityResult

private val resultLauncher = registerForActivityResult (activityResult ()) {result ->
         when (result.resultCode) {
             Activity.RESULT_OK -> sync()
             Activity.RESULT_CANCELED -> return
         }
     }

当我们 return 时，我们有一个用最后一个状态触发的观察者，并且再次执行之前的导航逻辑

private val syncStateObserver = Observer<StateInfo?> {
        it?: return@Observer

        when (it) {
            is Guest -> doWhenUserIsGuest()
            is Authorized -> doWhenUserIsAuthorized()
        }
    }

如何忽略 return 上具有相同值的观察者触发器？

Answer 1

对此有一个流行的答案。你可以用 SingleEvent class:

包装你的 StateInfo

open class SingleEvent<out T>(private val content: T) {

    var hasBeenHandled = false
        private set // Allow external read but not write

    /**
     * Returns the content and prevents its use again.
     */
    fun getContentIfNotHandled(): T? {
        return if (hasBeenHandled) {
            null
        } else {
            hasBeenHandled = true
            content
        }
    }

    /**
     * Returns the content, even if it's already been handled.
     */
    fun peekContent(): T = content
}

因此您的观察者如下所示：

private val syncStateObserver = Observer<SingleEvent<StateInfo>> {
        it.getContentIfNotHandled()?: return@Observer

        when (it.peek()) {
            is Guest -> doWhenUserIsGuest()
            is Authorized -> doWhenUserIsAuthorized()
        }
}

Answer 2

这个url对我有帮助-https://medium.com/androiddevelopers/livedata-with-snackbar-navigation-and-other-events-the-singleliveevent-case-ac2622673150

但不适用于 livedata.ktx -> liveData{ syncScenario.state.collect { emit(Wrapper(it))} }

我通过制作一种方法解决了这个问题，在该方法中，我从流程中收集数据并将其放入带有来自 url

的包装器的可变实时数据中

LiveData Observer 在不需要时触发

LiveData Observer triggers when not needed

android

mvvm

android-livedata

kotlin-flow