不需要时触发LiveData观察者

Question

我有一个视图模型，从场景中接收流作为livedata

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

在活动中，我们订阅了这个livedata，一些逻辑发生并使用了activityResult

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

当我们返回时，我们会用最后一个状态触发一个观察者，并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()
        }
    }

如何忽略返回时具有相同值的观察者触发器？

Answer 1

这个网址是帮我-https://medium.com/androiddevelopers/livedata-with-snackbar-navigation-and-other-events-the-singleliveevent-case-ac2622673150

但不适用于livedata。ktx-

我解决了这个问题，通过一个方法，我从流中收集数据，并把它放在我的可变livedata与包装从url

Answer 2

对此有一个流行的答案。您可以将您的StateInfo包装为SingleCase类：

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()
        }
}