SingleLiveEvent

一直在使用SingleLiveEvent用的很順手，今日才發現原來SingleLiveEvent不是原生的，是別人寫好extends from MutableLiveData 的物件，稍微來整理一下兩者差異。

相同：用於observer監聽LiveData改變

不同：MutableLiveData所有observe都會收到消息。
SigleLiveData只會通知給他第一個遇見的人。

舉例：
在activity中創立兩個dialog,兩個dialog都在實例化過程中訂閱某條消息，表示收到某條消息後將自己dismiss，如果將該消息放在SingleLiveEvent中發送，那麼就只會有一個dialog 執行dismiss，而如果將該消息放在MutableLiveData中發送，那麼兩個就都會dismiss。

/**
 * 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.
 */
public class SingleLiveEvent<T> extends MutableLiveData<T> {

    private static final String TAG = "SingleLiveEvent";

    private final AtomicBoolean mPending = new AtomicBoolean(false);

    @MainThread
    public void observe(@NonNull LifecycleOwner owner, @NonNull final Observer<? super T> observer) {
        if (hasActiveObservers()) {
            Log.w(TAG, "Multiple observers registered but only one will be notified of changes.");
        }

        // Observe the internal MutableLiveData
        super.observe(owner, t -> {
            if (mPending.compareAndSet(true, false)) {
                observer.onChanged(t);
            }
        });
    }

    @MainThread
    public void setValue(@Nullable T t) {
        mPending.set(true);
        super.setValue(t);
    }

    /**
     * Used for cases where T is Void, to make calls cleaner.
     */
    @MainThread
    public void call() {
        setValue(null);
    }
}