Is it possible to make one LiveData of two LiveDatas?

Question

I have two DAOs, two Repositories and two POJOs. There is some way to create one Livedata of two? I need it to make single list for Recyclerview. POJOs are similar objects.<

Answer 1

Instead of having a class to add 2 live datas, another class to add 3 live datas, etc. We can use a more abstract way where we can add as many live datas as we want.

import androidx.lifecycle.LiveData
import androidx.lifecycle.MediatorLiveData

/**
 * CombinedLiveData is a helper class to combine results from multiple LiveData sources.
 * @param liveDatas Variable number of LiveData arguments.
 * @param combine   Function reference that will be used to combine all LiveData data.
 * @param R         The type of data returned after combining all LiveData data.
 * Usage:
 * CombinedLiveData<SomeType>(
 *     getLiveData1(),
 *     getLiveData2(),
 *     ... ,
 *     getLiveDataN()
 * ) { datas: List<Any?> ->
 *     // Use datas[0], datas[1], ..., datas[N] to return a SomeType value
 * }
 */
class CombinedLiveData<R>(vararg liveDatas: LiveData<*>,
                          private val combine: (datas: List<Any?>) -> R) : MediatorLiveData<R>() {

    private val datas: MutableList<Any?> = MutableList(liveDatas.size) { null }

    init {
        for(i in liveDatas.indices){
            super.addSource(liveDatas[i]) {
                datas[i] = it
                value = combine(datas)
            }
        }
    }
}

Answer 2

I assume you want to combine them, yes? You'll need a MediatorLiveData, but the guy saying you now need Object is wrong. What you need is a MediatorLiveData<Pair<List<Expense>, List<Income>>>.

public class CombinedLiveData extends MediatorLiveData<Pair<List<Expense>, List<Income>>> {
    private List<Expense> expenses = Collections.emptyList();
    private List<Income> incomes = Collections.emptyList();

    public CombinedLiveData(LiveData<List<Expense>> ld1, LiveData<List<Income>> ld2) {
        setValue(Pair.create(expenses, incomes));

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

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

You could potentially make this generic and it'd be the implementation of combineLatest for two LiveData using tuples of 2-arity (Pair).

EDIT: like this:

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));
        });
    }
}

Beware that I lost the ability to set Collections.emptyList() as initial values of A and B with this scenario, and you WILL need to check for nulls when you access the data inside the pair.

EDIT: You can use the library https://github.com/Zhuinden/livedata-combinetuple-kt which does the same thing.

Answer 3

Let's say you want to merge LiveData<List<Expense>> & LiveData<List<Income>> then, you'll need to take MediatorLiveData<Object> (why? because, both of your live data are of different types).

So, use code like below :

LiveData liveData1 = getAllExpensesByDay();
LiveData liveData2 = getAllIncomesByDay();
MediatorLiveData liveDataMerger = new MediatorLiveData<Object>();
liveDataMerger.addSource(liveData1, value -> liveDataMerger.setValue(value));
liveDataMerger.addSource(liveData2, value -> liveDataMerger.setValue(value));

If you need to use specific data from MediatorLiveData then use Transformations on that.

Check out more here