Android Architecture Components: How is LiveData in the repository observed by a ViewModel

Question

i'm studying the Android Architecture Components and i'm a little bit confused. In the sample they use a repository and state that changes within the datasource of the repository are observed by the ViewModels. I don't understand how the changes within the datasource are pushed to the ViewModels, as i cannot see any code within the ViewModels that subscribes them to the repository. Analogously, the fragments observe the ViewModel's LiveData, but they actually subscribe to the LiveData:

 // Observe product data
    model.getObservableProduct().observe(this, new Observer<ProductEntity>() {
        @Override
        public void onChanged(@Nullable ProductEntity productEntity) {
            model.setProduct(productEntity);
        }
    });

I cannot see any kind of subscribing within the ViewModels to observe the Repository. Am i missing something?

Answer 1

ViewModel is not observing any data it just returning LiveData object of Product so you can observe the data in ProductFragment

This is how LiveData is observed in ProductFragment, In which the getObservableProduct() method called on ViewModel which returns LiveData<ProductEntity>

// Observe product data
    model.getObservableProduct().observe(this, new Observer<ProductEntity>() {
        @Override
        public void onChanged(@Nullable ProductEntity productEntity) {
            model.setProduct(productEntity);
        }
    });

This method in ViewModel called from ProductFragment

public LiveData<ProductEntity> getObservableProduct() {
    return mObservableProduct;
}

In constructor of that ProductViewModel the member variable mObservableProduct is initialized as follows, Which get LiveData<ProductEntity> from Repository

private final LiveData<ProductEntity> mObservableProduct;
mObservableProduct = repository.loadProduct(mProductId);

If you dig deeper, in DataRepository, LiveData<ProductEntity> is fetched from DAO

public LiveData<ProductEntity> loadProduct(final int productId) {
    return mDatabase.productDao().loadProduct(productId);
}

And in DAO its nothing but SQL query which returns the LiveData<ProductEntity> which is implemented by RoomCompiler. As you can see DAO using @Dao annotation which used by annotation processor and Write Dao implementation in ProductDao_Impl class.

@Query("select * from products where id = :productId")
LiveData<ProductEntity> loadProduct(int productId);

So In a nutshell, ViewModel holding References to all the data required by Activity or Fragment. Data get initialized in ViewModel and it can survive Activity configuration changes. Thus we are storing its references in ViewModel. In our case LiveData is just wrapper around our object which is returned by DAO implementation as an Observable object. So we can observe this in any Activity or Fragment. So as soon as the data is changed at Data Source, it called postValue() method on LiveData and we get the callback

Flow of LiveData DAO -> Repository -> ViewModel -> Fragment