Another approach to a LiveData event bus. Where each LiveData instance is tied to a lifecycle.

LiveDataBus.java

public final class LiveDataBus {

    private static SparseArray<Map<LifecycleRegistryOwner, EventLiveData>> sSubjectMap = new SparseArray<>();

    public static final int SUBJECT_DATA_LOADED = 0, SUBJECT_DOWNLOAD_COMPLETE = 1;

    @Retention(SOURCE)
    @IntDef({SUBJECT_DATA_LOADED, SUBJECT_DOWNLOAD_COMPLETE})
    @interface Subject {
    }

    private LiveDataBus() {
        // hidden constructor
    }

    /**
     * Gets a map which has a LifecycleRegistryOwner for each EventLiveData, for a specific subject. So each lifecycle has
     * its own EventLiveData.
     */
    private static Map<LifecycleRegistryOwner, EventLiveData> getLiveDataMap(@Subject int subjectCode) {
        Map<LifecycleRegistryOwner, EventLiveData> liveDataMap = sSubjectMap.get(subjectCode);
        if (liveDataMap == null) {
            liveDataMap = new HashMap<>();
            sSubjectMap.put(subjectCode, liveDataMap);
        }

        return liveDataMap;
    }
  
    /**
     * Get the live data or create it if it's not already in memory.
     */
    @NonNull
    private static EventLiveData getLiveData(@Subject int subjectCode, @NonNull LifecycleRegistryOwner lifecycle) {
        Map<LifecycleRegistryOwner, EventLiveData> liveDataMap = getLiveDataMap(subjectCode);
        EventLiveData liveData = liveDataMap.get(lifecycle);
        if (liveData == null) {
            liveData = new EventLiveData(subjectCode, lifecycle);
            liveDataMap.put(lifecycle, liveData);
        }

        return liveData;
    }

    /**
     * Subscribe to the specified subject and listen for updates on that subject.
     */
    public static void subscribe(@Subject int subject, @NonNull LifecycleRegistryOwner lifecycle, @NonNull Observer<Object> action) {
        getLiveData(subject, lifecycle).observe(lifecycle, action);
    }

    /**
     * Removes this subject when it has no observers.
     */
    public static void unregister(@Subject int subject, @NonNull LifecycleRegistryOwner lifecycle) {
        Map<LifecycleRegistryOwner, EventLiveData> liveDataMap = getLiveDataMap(subject);
        liveDataMap.remove(lifecycle);
        if (liveDataMap.size() == 0) {
            sSubjectMap.remove(subject);
        }
    }

    /**
     * Publish an object to the specified subject for all subscribers of that subject.
     */
    public static void publish(@Subject int subject, @NonNull Object message) {
        for (EventLiveData eventLiveData : getLiveDataMap(subject).values()) {
            eventLiveData.update(message);
        }
    }
}