Android Architecture Components Basic Sample

This sample showcases the following Architecture Components:

• Room
• ViewModels
• LiveData

Features

This sample contains two screens: a list of products and a detail view, that shows product reviews.

Presentation layer

The presentation layer consists of the following components:

• A main activity that handles navigation.
• A fragment to display the list of products.
• A fragment to display a product review.

The app uses a Model-View-ViewModel (MVVM) architecture for the presentation layer. Each of the fragments corresponds to a MVVM View. The View and ViewModel communicate using LiveData and the following design principles:

• ViewModel objects don't have references to activities, fragments, or Android views. That would cause leaks on configuration changes, such as a screen rotation, because the system retains a ViewModel across the entire lifecycle of the corresponding view.

• ViewModel objects expose data using LiveData objects. LiveData allows you to observe changes to data across multiple components of your app without creating explicit and rigid dependency paths between them.

• Views, including the fragments used in this sample, subscribe to corresponding LiveData objects. Because LiveData is lifecycle-aware, it doesn’t push changes to the underlying data if the observer is not in an active state, and this helps to avoid many common bugs. This is an example of a subscription:

        // Update the list of products when the underlying data changes.
        viewModel.getProducts().observe(this, new Observer<List<ProductEntity>>() {
            @Override
            public void onChanged(@Nullable List<ProductEntity> myProducts) {
                if (myProducts != null) {
                    mBinding.setIsLoading(false);
                    mProductAdapter.setProductList(myProducts);
                } else {
                    mBinding.setIsLoading(true);
                }
            }
        });

Data layer

The database is created using Room and it has two entities: a ProductEntity and a CommentEntity that generate corresponding SQLite tables at runtime.

Room populates the database asynchronously on first use. The DatabaseCreator class is responsible for creating the database and tables, and populating them with sample product and review data. This is done on the first use of the database, with the help of an AsyncTask. To simulate low-performance, an artificial delay is added. To let other components know when the data has finished populating, the DatabaseCreator exposes a LiveData object..

To access the data and execute queries, you use a Data Access Object (DAO). For example, a product is loaded with the following query:

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

Queries that return a LiveData object can be observed, so when a change in one of the affected tables is detected, LiveData delivers a notification of that change to the registered observers.

Transformations

Fragments don't observe the database directly, they only interact with ViewModel objects. A ViewModel observes database queries as well as the DatabaseCreator, which exposes whether the database is created or not.

For the purpose of the sample, the database is deleted and re-populated each time the app is started, so the app needs to wait until this process is finished. This is solved with a Transformation:

        mObservableProducts = Transformations.switchMap(databaseCreated,
                new Function<Boolean, LiveData<List<ProductEntity>>>() {
            @Override
            public LiveData<List<ProductEntity>> apply(Boolean isDbCreated) {
                if (!isDbCreated) {
                    return ABSENT;
                } else {
                    return databaseCreator.getDatabase().productDao().loadAllProducts();
                }
            }
        });

Whenever databaseCreated changes, mObservableProducts will get a new value, either an ABSENT LiveData or the list of products. The database will be observed with the same scope as mObservableProducts.

Note that the first time a LiveData object is observed, the current value is emitted and onChanged is called.

The following diagram shows the general structure of the sample:

Exercise for the reader: try to apply a transformation to the list of products in the ViewModel before they are delivered to the fragment. (hint: Transformations.Map).

License

---

Copyright 2015 The Android Open Source Project, Inc.

Licensed to the Apache Software Foundation (ASF) under one or more contributor license agreements. See the NOTICE file distributed with this work for additional information regarding copyright ownership. The ASF licenses this file to you under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.