This document discusses architecture components for building modern Android applications. It covers common app architecture problems and principles like separation of concerns. It introduces key Architecture Components like Activities, Fragments, Services, ContentProviders, ViewModels and LiveData. It also discusses architectural patterns like MVC, MVP, MVVM and recommendations like clean architecture. The document emphasizes principles like modularity, separation of concerns, and testability. It provides an overview of alternatives like Room, Paging Library, and recommendations for legacy apps.

1. Architecture Components
Constantine Mars
Team Lead, Senior Developer @ DataArt
Построение современной архитектуры
мобильных приложений

2. Background
Software Architecture Basics

3. Components:
-Activities
-Fragments
-Services
-Content Providers
-Broadcast Receivers
Android Has “Good Bones”

4. App-hopping
OS may kill app at random time
App components lifecycle is not under control
Components should not depend on each other
Can’t rely on data, stored in components
There are Everyday Problems to Solve

5. Simply related to Activity Lifecycle

6. -Separation of concerns
-Provide solid user experience
-Keep UI lean and simple
-Keep UI free of app logic
-Drive UI from model
-Use persistent model
-Assign clear responsibilities for each model class
Common Principles for staying in mind :)

7. Image from fernandocejas.com
Separation of Concerns...

8. Image from fernandocejas.com
Clean Architecture

9. -Modular app
-Each class responsible for one well-defined function
-Should be no god objects
-The app should be testable
Remember Good Architecture Goals...

10. Android Recommended Architecture

11. Android Recommended Architecture. Another View

12. Be together. not the same

13. “It is impossible to have one way of writing apps that will be the best for
every scenario. That being said, this recommended architecture should be
a good starting point for most use cases. If you already have a good way
of writing Android apps, you don't need to change.”
Be together. not the same

14. Building blocks
Architecture Components

15. Purpose: Display data and pass on UI events
Neither contain the UI data, nor directly manipulate data
Examples: Activity, Fragment
Views = UI Controllers = LifecycleOwners

16. Lifecycle = states + events

17. Implement Lifecycle since Beta
AppCompatActivity, Fragment

18. ViewModel
Data holder for Activity/Fragment
Survives configuration changes
NEVER references View / Activity / Fragment

19. ViewModel
public class DetailActivityViewModel extends ViewModel {
private WeatherEntry mWeather;
public DetailActivityViewModel() {}
public WeatherEntry getWeather() { return mWeather; }
public void setWeather(WeatherEntry weatherEntry) { mWeather = weatherEntry; }
}

20. ViewModel and Lifecycle

21. ViewModel and LifecycleOwner
public class DetailActivity extends LifecycleActivity {
DetailActivityViewModel viewModel;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
...
viewModel = ViewModelProviders.of(this).get(DetailActivityViewModel.class);
}
...

22. Represent data needed for the UI to display
An observable data holder
Lifecycle aware
Automatic subscription management
LiveData

23. LiveData event propagation

24. LiveData sample implementation

25. Live Data
public class DetailActivityViewModel extends ViewModel {
private MutableLiveData<WeatherEntry> mWeather;
public DetailActivityViewModel() {}
public MutableLiveData<WeatherEntry> getWeather() {
return mWeather;
}
public void setWeather(WeatherEntry weatherEntry) {
mWeather.postValue(weatherEntry);
}
}

26. Live Data observing
public class DetailActivity extends LifecycleActivity {
DetailActivityViewModel viewModel;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
...
viewModel = ViewModelProviders.of(this).get(DetailActivityViewModel.class);
viewModel.getWeather().observe(this, weatherEntry -> {
if(weatherEntry!=null) { bindWeatherToUI(weatherEntry); }
});
}

27. -Single source of truth
-ViewModels simply request data
from the repository
-Is a mediator between
the different data sources
Repository
Image from fernandocejas.com

28. Manages data from a remote data
source, such as the internet
May use REST, Cloud
Remote Network Data Source

29. Manages local data stored in the
database
Model

30. Each class in the diagram only
stores a reference to the class or
classes directly "below it" and not
any classes above it
Layered architecture pattern

31. ORM by Google
Room

32. -Less boilerplate compared to the built-in APIs
-Compile-time validation of SQL queries
-Data observation via LiveData, RxJava
Room ORM purposes

33. -@Entity
-@Dao
-@Database
Room annotations

34. Entity

35. @Entity declaration
@Entity(tableName = "weather",
indices = {@Index(value = {"date"}, unique = true)})
public class WeatherEntry {
@PrimaryKey(autoGenerate = true)
private int id;
…
}

36. @Entity constructors
//Room constructor
public WeatherEntry(int id, int weatherIconId, Date date, ...) {
//Json constructor - ignored by Room
@Ignore
public WeatherEntry(int weatherIconId, Date date,
// (!) Only one constructor should be exposed to Room
...

37. Dao

38. @Dao declaration
@Dao
public interface WeatherDao {
@Query("SELECT * FROM weather") List<WeatherEntry> getAll();
@Insert(onConflict = OnConflictStrategy.REPLACE)
void insertAll(WeatherEntry... weatherEntries);
@Delete void delete(WeatherEntry weatherEntry);
}

39. Database

40. @Database declaration
@Database(entities = {WeatherEntry.class}, version = 1)
@TypeConverters(DateConverter.class)
public abstract class AppDatabase extends RoomDatabase {
public abstract WeatherDao weatherDao();
}

41. @Database - a singleton
private static final String DATABASE_NAME = "weather";
private static final Object LOCK = new Object();
private static volatile AppDatabase sInstance;
public static AppDatabase getInstance(Context context) {
if (sInstance == null) { synchronized (LOCK) {
if (sInstance == null) {
sInstance = Room.databaseBuilder(context.getApplicationContext(),
AppDatabase.class, AppDatabase.DATABASE_NAME).build();
}
}}
return sInstance;
}

42. Type converters
class DateConverter {
@TypeConverter
public static Date toDate(Long timestamp) {
return timestamp == null ? null : new Date(timestamp);
}
@TypeConverter
public static Long toTimestamp(Date date) {
return date == null ? null : date.getTime();
}
}

43. Alternatives

44. Repository

45. Repository pattern
public class WeatherRepository {
public synchronized static WeatherRepository getInstance();
public synchronized void initializeData();
private void deleteOldData();
private boolean isFetchNeeded();
private void startFetchWeatherService();
}

46. Repository - fetch data from network
mWeatherNetworkDataSource = weatherNetworkDataSource;
LiveData<WeatherEntry[]> networkData = mWeatherNetworkDataSource.getCurrentWeatherForecasts();
networkData.observeForever(newForecastsFromNetwork -> {
mExecutors.diskIO().execute(() -> {
mWeatherDao.bulkInsert(newForecastsFromNetwork);
Log.d(LOG_TAG, "New values inserted");
});
});

47. Repository - use LiveData
private WeatherNetworkDataSource(Context context, AppExecutors executors) {
mContext = context;
mExecutors = executors;
mDownloadedWeatherForecasts = new MutableLiveData<WeatherEntry[]>();
}

48. Check when to fetch
private boolean isFetchNeeded() {
Date today = CustomDateUtils.getNormalizedUtcDateForToday();
int count = mWeatherDao.countAllFutureWeather(today);
return (count < WeatherNetworkDataSource.NUM_DAYS);
}

49. Check when to fetch
public synchronized void initializeData() {
if (mInitialized) return;
mInitialized = true;
mExecutors.diskIO().execute(() -> {
if (isFetchNeeded()) {
startFetchWeatherService();
}
});
}

50. And again - observe LiveData
public class DetailActivity extends LifecycleActivity {
DetailActivityViewModel viewModel;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
...
viewModel = ViewModelProviders.of(this).get(DetailActivityViewModel.class);
viewModel.getWeather().observe(this, weatherEntry -> {
if(weatherEntry!=null) { bindWeatherToUI(weatherEntry); }
});
}

51. The whole picture for sample project

52. Paging Library

53. Paging Library flow

54. Data source
KeyedDataSource - if you need to use data from item N to fetch item N+1.
TiledDataSource - if you need to fetch pages in range of data from any location you choose in your data store

55. DAO for KeyedDataSource
@Dao
interface UserDao {
@Query("SELECT * from user ORDER BY name DESC LIMIT :limit")
public abstract List<User> userNameInitial(int limit);
@Query("SELECT * from user WHERE name < :key ORDER BY name DESC LIMIT :limit")
public abstract List<User> userNameLoadAfter(String key, int limit);
@Query("SELECT * from user WHERE name > :key ORDER BY name ASC LIMIT :limit")
public abstract List<User> userNameLoadBefore(String key, int limit);
}

56. KeyedDataSource implementation
public class KeyedUserQueryDataSource extends KeyedDataSource<String, User> {
@Override public boolean isInvalid() { return super.isInvalid(); }
@Override public String getKey(@NonNull User item) { return item.getName(); }
@Override public List<User> loadInitial(int pageSize) {
return mUserDao.userNameInitial(pageSize); }
@Override public List<User> loadBefore(@NonNull String userName, int pageSize) {
return mUserDao.userNameLoadBefore(userName, pageSize); }
@Override public List<User> loadAfter(@Nullable String userName, int pageSize) {
return mUserDao.userNameLoadAfter(userName, pageSize); }
}

57. TiledDataSource implementation
@Dao
interface UserDao {
@Query("SELECT * FROM user ORDER BY mAge DESC")
public abstract TiledDataSource<User> loadUsersByAgeDesc();
}

58. TiledDataSource - under the hood
@Dao
interface UserDao {
@Query("SELECT COUNT(*) from user")
public abstract Integer getUserCount();
@Query("SELECT * from user ORDER BY mName DESC LIMIT :limit OFFSET :offset")
public abstract List<User> userNameLimitOffset(int limit, int offset);
}

59. TiledDataSource - under the hood
public class OffsetUserQueryDataSource extends TiledDataSource<User> {
@Override public boolean isInvalid() { return super.isInvalid(); }
@Override public int countItems() { return mUserDao.getUserCount(); }
@Override public List<User> loadRange(int startPosition, int loadCount) {
return mUserDao.userNameLimitOffset(loadCount, startPosition);
}
}

60. Paging Library - PagedList
DataSource
PagedList - loads its data in chunks (pages) from a DataSource
PagedListAdapter
LivePagedListProvider

61. Paging Library - LivePagedListProvider
DataSource
PagedList
PagedListAdapter - listens to PagedList loading callbacks as pages are loaded, and uses DiffUtil on a
background thread to compute fine grained updates as new PagedLists are received.
LivePagedListProvider - provides a LiveData<PagedList>, given a
means to construct a DataSource

62. LivePagedListProvider generation by DAO
@Dao
interface UserDao {
@Query("SELECT * FROM user ORDER BY lastName ASC")
public abstract LivePagedListProvider<Integer, User> usersByLastName();
}

63. Paging Library - PagedListAdapter
DataSource
PagedList
PagedListAdapter - listens to PagedList loading callbacks as pages are
loaded, and uses DiffUtil on a background thread to compute fine grained
updates as new PagedLists are received.
LivePagedListProvider

64. PagedListAdapter usage pattern - DAO
@Dao
interface UserDao {
@Query("SELECT * FROM user ORDER BY lastName ASC")
public abstract LivePagedListProvider<Integer, User> usersByLastName();
}

65. PagedListAdapter usage pattern -
PagedList.Config
class MyViewModel extends ViewModel {
public final LiveData<PagedList<User>> usersList;
public MyViewModel(UserDao userDao) {
usersList = userDao.usersByLastName().create(
/* initial load position */ 0,
new PagedList.Config.Builder()
.setPageSize(50)
.setPrefetchDistance(50)
.build());
}
}

66. PagedListAdapter usage pattern - binding
class MyActivity extends AppCompatActivity {
@Override
public void onCreate(Bundle savedState) {
super.onCreate(savedState);
MyViewModel viewModel = ViewModelProviders.of(this).get(MyViewModel.class);
RecyclerView recyclerView = findViewById(R.id.user_list);
UserAdapter<User> adapter = new UserAdapter();
viewModel.usersList.observe(this, pagedList -> adapter.setList(pagedList));
recyclerView.setAdapter(adapter);
}
}

67. PagedListAdapter implementation
class UserAdapter extends PagedListAdapter<User, UserViewHolder> {
public UserAdapter() { super(DIFF_CALLBACK); }
@Override public void onBindViewHolder(UserViewHolder holder, int position) {
User user = getItem(position);
if (user != null) { holder.bindTo(user);
} else { holder.clear(); }
}
...

68. PagedListAdapter implementation
class UserAdapter extends PagedListAdapter<User, UserViewHolder> {
...
public static final DiffCallback<User> DIFF_CALLBACK = new DiffCallback<User>() {
@Override public boolean areItemsTheSame( @NonNull User oldUser, @NonNull User
newUser) { return oldUser.getId() == newUser.getId(); }
@Override public boolean areContentsTheSame( @NonNull User oldUser, @NonNull User
newUser) { return oldUser.equals(newUser); }
}
}

69. Paging Library flow

70. Testing
Sneak peek :)

71. UI Controller - Instrumentation

72. ViewModel - JUnit

73. Repository - JUnit, MockWebServer

74. Scalability and advices
Almost there...

75. What if...
I use RxJava?
I already have MVP?
I love Kotlin?
I’m working on legacy project?
Typical questions

76. The Answer is...

77. Read, watch and visit...
Links

78. Guide to App Architecture https://developer.android.com/topic/libraries/architecture/guide.html
Architecture Components
https://developer.android.com/topic/libraries/architecture/index.html
I/O ‘17 Architecture Components Introduction - https://youtu.be/FrteWKKVyzI
Solving the Lifecycle Problem - https://youtu.be/bEKNi1JOrNs
Persistence and Offline - https://youtu.be/MfHsPGQ6bgE
Architecture Components on GDD Europe - https://youtu.be/Ts-uxYiBEQ8
GDD Europe CodeLabs g.co/codelabs/gdd17
Google Github samples https://github.com/googlesamples/android-architecture-components
What to read and watch :)

79. Android MVP Helper https://github.com/Ufkoku/AndroidMVPHelper
Moxy https://github.com/Arello-Mobile/Moxy
Mosby https://github.com/sockeqwe/mosby
Clean Architecture https://github.com/android10/Android-CleanArchitecture
Reark https://github.com/reark/reark
MVP + Dagger2 + Rx https://android.jlelse.eu/mvp-dagger-2-rx-clean-modern-android-app-code-74f63c9a6f2f
Architecture the Lost Years by Uncle Bob https://youtu.be/WpkDN78P884
Alternatives to consider

80. Where to go :)

81. Thank you! :)
Constantine Mars
@ConstantineMars
+ConstantineMars
Mobile Architecture Club