A lifecycle-aware LiveData that sends only new updates after subscription, used for events like navigation and Snackbar messages.

SingleEventLiveData.java

/**
 * 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 SingleEventLiveData<T> extends MutableLiveData<T> {

    private static final String TAG = "SingleEventLiveData";

    private final AtomicBoolean mPending = new AtomicBoolean(false);

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

        // Observe the internal MutableLiveData
        super.observe(owner, new Observer<T>() {
            @Override
            public void onChanged(@Nullable T t) {
                if (mPending.compareAndSet(true, false)) {
                    observer.onChanged(t);
                }
            }
        });
    }

    @MainThread
    @Override
    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);
    }

}