SingleLiveEvent.kt

/*
 * Copyright (c) 2017 Halcyon Mobile
 * http://www.halcyonmobile.com
 * All rights reserved.
 */

import android.arch.lifecycle.LifecycleOwner
import android.arch.lifecycle.MutableLiveData
import android.arch.lifecycle.Observer
import android.support.annotation.MainThread
import java.util.concurrent.atomic.AtomicBoolean

/**
 * A lifecycle-aware observable that sends only new updates after subscription, used for events like
 * navigation and Snackbar messages.
 * <p>
 * This avoids a common problem with events: on configuration change (like rotation) an update
 * can be emitted if the observer is active. This LiveData only calls the observable if there's an
 * explicit call to setValue() or call().
 * <p>
 * Note that only one observer is going to be notified of changes.
 */
class SingleLiveEvent<T> : MutableLiveData<T>() {
    
    private val pendingEvent = AtomicBoolean(false)

    @MainThread override fun observe(owner: LifecycleOwner, observer: Observer<T>) {
        // Observe the internal MutableLiveData
        super.observe(owner, Observer<T> { t ->
            if (pendingEvent.compareAndSet(true, false)) {
                observer.onChanged(t)
            }
        })
    }

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

    /**
     * Used for cases where T is Void, to make calls cleaner.
     */
    @MainThread
    fun call() {
        value = null
    }
}