ViewModel SingleLiveEvent - Wraps the MutableLiveData<ActionEvent<T>>

Event.kt

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

    var hasBeenUsed = false
        private set

    fun getContentIfNotUsed(): T? {
        return if (hasBeenUsed) {
            null
        } else {
            hasBeenUsed = true
            content
        }
    }

    fun peekContent(): T = content

}

SingleEvent.kt

open class LiveEvent<T> {

    val action: MutableLiveData<ActionEvent<T>> = MutableLiveData()

    fun observe(lifecycleOwner: LifecycleOwner, callback: (data: T) -> Unit) {
        action.observe(lifecycleOwner, Observer {
            action.value?.let {
                if (!it.hasBeenUsed) {
                    callback(it.getContentIfNotUsed() ?: it.peekContent())
                }
            }
        })
    }

}

fun <T> SingleEvent<T>.post(value: T) {
    this.action.postValue(Event(value))
}

fun SingleEvent<Unit>.post() {
    this.action.postValue(Event(Unit))
}