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I was analyzing Android architecture components sample app (BasicSample). Why do they use MediatorLiveData in DataRepository class if there is only one source attached to it?

https://github.com/googlesamples/android-architecture-components/blob/master/BasicSample/app/src/main/java/com/example/android/persistence/DataRepository.java#L25-L31

mObservableProducts = new MediatorLiveData<>();



mObservableProducts.addSource(mDatabase.productDao().loadAllProducts(),

        productEntities -> {

            if (mDatabase.getDatabaseCreated().getValue() != null) {

                mObservableProducts.postValue(productEntities);

            }

        });

I think they should just use LiveData<List<ProductEntity> coming from DAO and facilitate it with getter method:

public LiveData<List<ProductEntity>> getProducts() {

    return mDatabase.productDao().loadAllProducts();

}

Exactly how they did with other DAO requests:

https://github.com/googlesamples/android-architecture-components/blob/master/BasicSample/app/src/main/java/com/example/android/persistence/DataRepository.java#L47-L61

public LiveData<List<ProductEntity>> getProducts() {

    return mObservableProducts;

}



public LiveData<ProductEntity> loadProduct(final int productId) {

    return mDatabase.productDao().loadProduct(productId);

}



public LiveData<List<CommentEntity>> loadComments(final int productId) {

    return mDatabase.commentDao().loadComments(productId);

}



public LiveData<List<ProductEntity>> searchProducts(String query) {

    return mDatabase.productDao().searchAllProducts(query);

}

The same question relates to ProductListViewModel. They again created MediatorLiveData only with one source.

https://github.com/googlesamples/android-architecture-components/blob/master/BasicSample/app/src/main/java/com/example/android/persistence/viewmodel/ProductListViewModel.java#L42-L46

mRepository = ((BasicApp) application).getRepository();

LiveData<List<ProductEntity>> products = mRepository.getProducts();



// observe the changes of the products from the database and forward them

mObservableProducts.addSource(products, mObservableProducts::setValue);

They are intentionally filtering the DAO emissions before the initial data is inserted into the DB. This is what the MediatorLiveData you mentioned does.

If you don't need to wait for an "initial set of data to be loaded" like they do, then you can just ditch the MediatorLiveData and use mDatabase.productDao().loadAllProducts(); directly.