LiveData Observer 在不需要时触发

Question

我有一个视图模型，它从场景中接收流作为实时数据

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

在活动中，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
         }
     }

当我们返回时，我们有一个观察者被最后一个 state 触发，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()
        }
    }

如何忽略返回值相同的观察者触发器？

Answer 1

对此有一个流行的答案。 您可以使用SingleEvent class 包装您的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
}

所以你的观察者看起来像下面这样：

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，在该方法中我从流中收集数据并将其放入带有包装器的可变实时数据中 url