用于防止使用livedata时出现数据倒灌问题

public class SingleLiveEvent<T> extends MutableLiveData<T> {
    private final AtomicBoolean mPending = new AtomicBoolean(false);
    @Override
    public void observe(@NonNull LifecycleOwner owner, @NonNull final Observer<? super T> observer) {
        super.observe(owner, new Observer<T>() {
            @Override
            public void onChanged(@Nullable T 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);
    }
}