Combine MediatorLiveData

MediatorLivedata.java

/*
https://stackoverflow.com/questions/49493772/mediatorlivedata-or-switchmap-transformation-with-multiple-parameters
https://medium.com/@BladeCoder/to-implement-a-manual-refresh-without-modifying-your-existing-livedata-logic-i-suggest-that-your-7db1b8414c0e
https://coredump.uno/questions/48769812/best-practice-runtime-filters-with-room-and-livedata
https://www.e-learn.cn/content/wangluowenzhang/1592769
https://coredump.uno/questions/49493772/mediatorlivedata-or-switchmap-transformation-with-multiple-parameters
*/
public class LiveDataUtil2<A, B, C, Y> {
    public interface Thunk2<A, B, C, Y> {
        LiveData<Y> apply(A a, B b, C c);
    }

    LiveData<Y> mSource;

    @MainThread
    public LiveData<Y> switchMap(LiveData<A> trigger1, LiveData<B> trigger2,
                                                     LiveData<C> trigger3, final Thunk2 thunk) {
        final MediatorLiveData<Y> result = new MediatorLiveData();

        result.addSource(trigger1, new Observer<A>() {
            public void onChanged(@Nullable A a) {
                LiveData<Y> newLiveData = thunk.apply(trigger1.getValue(), trigger2.getValue(), trigger3.getValue());
                if(mSource != newLiveData) {
                    if(mSource != null) {
                        result.removeSource(mSource);
                    }

                    mSource = newLiveData;
                    if(mSource != null) {
                        result.addSource(mSource, new Observer<Y>() {
                            public void onChanged(@Nullable Y y) {
                                result.setValue(y);
                            }
                        });
                    }
                }
            }
        });

        result.addSource(trigger2, new Observer<B>() {
            public void onChanged(@Nullable B b) {
                LiveData<Y> newLiveData = thunk.apply(trigger1.getValue(), trigger2.getValue(), trigger3.getValue());
                if(mSource != newLiveData) {
                    if(mSource != null) {
                        result.removeSource(mSource);
                    }

                    mSource = newLiveData;
                    if(mSource != null) {
                        result.addSource(mSource, new Observer<Y>() {
                            public void onChanged(@Nullable Y y) {
                                result.setValue(y);
                            }
                        });
                    }
                }
            }
        });

        result.addSource(trigger3, new Observer<C>() {
            public void onChanged(@Nullable C c) {
                LiveData<Y> newLiveData = thunk.apply(trigger1.getValue(), trigger2.getValue(), trigger3.getValue());
                if(mSource != newLiveData) {
                    if(mSource != null) {
                        result.removeSource(mSource);
                    }

                    mSource = newLiveData;
                    if(mSource != null) {
                        result.addSource(mSource, new Observer<Y>() {
                            public void onChanged(@Nullable Y y) {
                                result.setValue(y);
                            }
                        });
                    }
                }
            }
        });

        return result;
    }
}