Convenient LiveData for observing click event.

ActionLiveData.kt

/**
 * LiveData that ignore current value broadcast when observe.
 * Like PulseLiveData(https://gist.github.com/niwatly/3b1ee17f25a63276e36194cd86f9b223), but ActionLiveData holds last value. so, result is defferent when getValue.
 * 
 * cited:
 * https://github.com/googlesamples/android-architecture/blob/dev-todo-mvvm-live/todoapp/app/src/main/java/com/example/android/architecture/blueprints/todoapp/SingleLiveEvent.java
 *　 https://android.jlelse.eu/android-arch-handling-clicks-and-single-actions-in-your-view-model-with-livedata-ab93d54bc9dc
 *
 * see also:
 * https://qiita.com/KazaKago/items/acce0c1a970441b44f39
 */

class ActionLiveData<T>: MediatorLiveData<T>() {
    private val pending: AtomicBoolean = AtomicBoolean(false)

    @MainThread
    fun observeSingle(owner: LifecycleOwner, callback: ((data: T) -> Unit)) {
        removeObservers(owner)

        // Observe the internal MutableLiveData
        super.observe(owner, Observer {
            if (pending.compareAndSet(true, false)) {
                it?.let {
                    callback(it)
                }
            }
        })
    }

    override fun observe(owner: LifecycleOwner, observer: Observer<T>) {
        TODO("not implemented")
    }

    @MainThread
    override fun setValue(value: T?) {
        pending.set(true)
        super.setValue(value)
    }
}