How to combine two live data one after the other?

Question

I have next use case: User comes to registration form, enters name, email and password and clicks on register button. After that system needs to check if email is taken or n

Answer 1

You can define a method that would combine multiple LiveDatas using a MediatorLiveData, then expose this combined result as a tuple.

public class CombinedLiveData2<A, B> extends MediatorLiveData<Pair<A, B>> {
    private A a;
    private B b;

    public CombinedLiveData2(LiveData<A> ld1, LiveData<B> ld2) {
        setValue(Pair.create(a, b));

        addSource(ld1, (a) -> { 
             if(a != null) {
                this.a = a;
             } 
             setValue(Pair.create(a, b)); 
        });

        addSource(ld2, (b) -> { 
            if(b != null) {
                this.b = b;
            } 
            setValue(Pair.create(a, b));
        });
    }
}

If you need more values, then you can create a CombinedLiveData3<A,B,C> and expose a Triple<A,B,C> instead of the Pair, etc. Just like in https://stackoverflow.com/a/54292960/2413303 .

EDIT: hey look, I even made a library for you that does that from 2 arity up to 16: https://github.com/Zhuinden/livedata-combinetuple-kt