BooksViewModel_Retrofit.java

class BooksViewModel extends ViewModel {

    private GoogleBooksService service;

    public BooksViewModel() {
        // Configure Retrofit here
    }

    private MutableLiveData<List<Book>> books;

    LiveData<List<Book>> getBooks() {
        if (books == null) {
            books = new MutableLiveData<List<Book>>();
        }
        return books;
    }

    public void loadBooks(String query) {
        service.search("search+" + query)
                // enqueue runs the request on a separate thread
                .enqueue(new Callback<BookSearchResult>() {

                    // We receive a Response with the content we expect already parsed
                    @Override
                    public void onResponse(Call<BookSearchResult> call, Response<BookSearchResult> response) {
                        // Update the LiveData with fresh content
                        books.setValue(response.body().getBooks());
                    }

                    // In case of error, this method gets called
                    @Override
                    public void onFailure(Call<BookSearchResult> call, Throwable t) {
                        t.printStackTrace();
                    }
                });
    }
}