This document introduces Android Architecture Components and provides an overview of some key components: - Room is a persistence library that provides an abstraction layer over SQLite to allow for more robust database access while reducing boilerplate code. - Lifecycle-aware components allow classes to observe Activity and Fragment lifecycles so their behavior is linked to the lifecycle status of other components. - Additional components like ViewModel, LiveData, and Paging Library are mentioned but not described in detail. The document focuses on Room by explaining entity, DAO and database classes as well as more advanced Room features like relationships, custom types, migrations and testing. Lifecycle components are introduced to link observer behavior to owner lifec

1. ANDROID
ARCHITECTURE
COMPONENTS -
INTRODUCTION
Paulina Szklarska

2. Hello!
Nice to meet you
Paulina Szklarska
Android Developer @ DroidsOnRoids
co-organizer of Toast Wrocław meetups
cat owner
travel enthusiast
@pszklarska
@pszklarska
@p_szklarska

3. 1.
INTRO
What are
Architecture
Components?

5. November, 6
2017

6. ARCHITECTURE COMPONENTS
ROOM
LIFECYCLE COMPONENTS VIEW MODEL
LIVE DATA PAGING LIBRARY

7. ROOM
Persistence library for Android

8. Room Entity
Single database table
DAO
Defines methods to access database
Database
Holds entities and DAOs

9. Why
another
database
library?
• less boilerplate code (thanks to annotations)
• SQL statement compile-time validation (less
crashes)
• full compatibility with other Architecture
Components (LiveData) and RxJava
• easy to use
• easy to test
• easy to migrate

10. Room
Entity,
DAO,
Database

11. Cats
id
1
2
3
name
Fudge
Houdini
Scratchy
url
/fudge_photo.jpg
/houdini_photo.jpg
/scratchy_photo.jpg

12. @Entity
public class Cat {
@PrimaryKey
public int id;
public String name;
public String url;
public Cat(int id, String name, String url) {
this.id = id;
this.name = name;
this.url = url;
}a
}a

13. @Entity(tableName = „kittens”)
public class Cat {
@PrimaryKey
public int id;
public String name;
public String url;
public Cat(int id, String name, String url) {
this.id = id;
this.name = name;
this.url = url;
}a
}a

14. @Entity(tableName = „kittens”)
public class Cat {
@PrimaryKey(autoGenerate = true)
public int id;
public String name;
public String url;
public Cat(String name, String url) {
this.name = name;
this.url = url;
}a
}a

15. @Entity(tableName = „kittens”)
public class Cat {
@PrimaryKey(autoGenerate = true)
public int id;
public String name;
@ColumnInfo(name = „photo_url”)
public String url;
public Cat(String name, String url) {
this.name = name;
this.url = url;
}a
}a

16. @Dao
public interface CatDao {
@Query("SELECT * FROM kittens")
List<Cat> getAllCats();
@Insert
void insert(Cat... cats);
@Update
void update(Cat... cats);
@Delete
void delete(Cat... cats);
}a

17. @Dao
public interface CatDao {
@Insert
void insert(Cat... cats);
@Insert
void insert(Cat cat);
@Insert
void insert(List<Cat> catList);
}a

18. @Dao
public interface CatDao {
@Insert(onConflict = REPLACE)
void insert(Cat... cats);
@Insert(onConflict = IGNORE)
void insert(Cat cat);
@Insert
void insert(List<Cat> catList);
}a

19. @Dao
public interface CatDao {
@Query("SELECT * FROM kittens")
List<Cat> getAllCats();
@Query("SELECT * FROM kittens WHERE id=:id")
Cat getCatById(int id);
@Query("SELECT * FROM kittens")
Cursor getCatsCursor();
@Query("SELECT * FROM kittens WHERE name=:name LIMIT :max")
List<Cat> getCatsByName(String name, int max);
}a
!UI thread

20. @Database(entities = { Cat.class }, version = 1)
public abstract class CatDatabase extends RoomDatabase {
private static final String DB_NAME = "catsDatabase.db";
private static CatDatabase create(final Context context) {
return Room
.databaseBuilder(context, CatDatabase.class, DB_NAME)
.build();
}
public abstract CatDao getCatDao();
}a

21. CatDatabase
.getInstance(context)
.getCatDao()
.insert(new Cat(1,
"Muffin",
"/muffin.jpg"));
List<Cat> allCats = CatDatabase
.getInstance(context)
.getCatDao()
.getAllCats();

22. Cats
id
1
2
3
name
Fudge
Houdini
Scratchy
url
/fudge_photo.jpg
/houdini_photo.jpg
/scratchy_photo.jpg
Owner
id
1
2
3
login
John
Cindy
Ken
avatar_url
„/john.jpg”
„/cindy.jpg”
„/ken.jpg”
ownerId
1
1
2

23. Room
Relations -
@ForeignKey

24. @Entity
public class Owner {
@PrimaryKey
public int id;
public String login;
public String avatarUrl;
public Owner(int id, String login, String avatarUrl) {
this.id = id;
this.login = login;
this.avatarUrl = avatarUrl;
}a
}a

25. @Entity(foreignKeys = @ForeignKey(entity = Owner.class,
parentColumns = "id",
childColumns = "ownerId",
onDelete = CASCADE))
public class Cat {
@PrimaryKey
public int id;
public String name;
public String url;
public int ownerId;
public Cat(int id, String name, String url, int ownerId) {
this.id = id;
this.name = name;
this.url = url;
this.ownerId = ownerId;
}a
}a

26. @Dao
public interface CatDao {
@Query("SELECT * FROM kittens WHERE ownerId=:ownerId")
List<Cat> findCatsForOwner(int ownerId);
}a

27. Room
Relations -
@Relation

28. @Entity
public class Owner {
@PrimaryKey
public int id;
public String login;
public String avatarUrl;
public Owner(int id, String login, String avatarUrl) {
this.id = id;
this.login = login;
this.avatarUrl = avatarUrl;
}a
}a

29. @Entity
public class Cat {
@PrimaryKey
public int id;
public String name;
public String url;
public int ownerId;
public Cat(int id, String name, String url, int ownerId) {
this.id = id;
this.name = name;
this.url = url;
this.ownerId = ownerId;
}a
}a

30. public class OwnerWithCats {
@Embedded public Owner owner;
@Relation(parentColumn = "id",
entityColumn = "ownerId")
public List<Cat> catList;
}a

31. @Dao
public interface OwnerWithCatsDao {
@Query("SELECT * FROM owner")
public List<OwnerWithCats> getOwnersWithCats();
}a

32. Room
Custom Types

33. @Entity
public class Cat {
@PrimaryKey
public int id;
public String name;
public LocalDate birthday;
public Cat(int id, String name, LocalDate birthday) {
this.id = id;
this.name = name;
this.birthday = birthday;
}a
}a

34. public class DateConverter {
@TypeConverter
public LocalDate toDate(long dateLong) {
return LocalDate.ofEpochDay(dateLong);
}
@TypeConverter
public long fromDate(LocalDate date) {
return date.toEpochDay();
}
}

35. @Database(entities = { Cat.class }, version = 1)
@TypeConverters(DateConverter.class)
public abstract class CatDatabase extends RoomDatabase {
}a

36. Room
Migrations

37. @Entity
public class Cat {
@PrimaryKey public int id;
public String name;
public String url;
}a

38. @Entity
public class Cat {
@PrimaryKey public int id;
public String name;
public String url;
public String age;
}a

39. java.lang.IllegalStateException: Room cannot verify the data
integrity. Looks like you've changed schema but forgot to
update the version number. You can simply fix this by
increasing the version number.

40. @Database(entities = { Cat.class }, version = 1)a
public abstract class CatDatabase extends RoomDatabase {
private static CatDatabase create(final Context context) {
return Room
.databaseBuilder(context, CatDatabase.class, DB_NAME)
.build();
}a
}a

41. @Database(entities = { Cat.class }, version = 2)a
public abstract class CatDatabase extends RoomDatabase {
private static CatDatabase create(final Context context) {
return Room
.databaseBuilder(context, CatDatabase.class, DB_NAME)
.build();
}a
}a

42. java.lang.IllegalStateException: A migration from 1 to 2 is necessary.
Please provide a Migration in the builder or call
fallbackToDestructiveMigration in the builder in which case Room will
re-create all of the tables.

43. @Database(entities = { Cat.class }, version = 2)a
public abstract class CatDatabase extends RoomDatabase {
private static CatDatabase create(final Context context) {
return Room
.databaseBuilder(context, CatDatabase.class, DB_NAME)
.fallbackToDestructiveMigration()
.build();
}a
}a

44. @Database(entities = { Cat.class }, version = 2)a
public abstract class CatDatabase extends RoomDatabase {
private static CatDatabase create(final Context context) {
return Room
.databaseBuilder(context, CatDatabase.class, DB_NAME)
.addMigrations(Migrations.FROM_1_TO_2)
.build();
}a
}a

45. class Migrations {
static final Migration FROM_1_TO_2 = new Migration(1, 2) {
@Override
public void migrate(@NonNull final SupportSQLiteDatabase database) {
database.execSQL("ALTER TABLE kittens ADD COLUMN age TEXT");
}
};
}

46. android {
javaCompileOptions {
annotationProcessorOptions {
arguments = ["room.schemaLocation":
"$projectDir/schemas".toString()]
}
}
}
}

47. app/schemas/pl.pszklarska.kittyarchitecture.data.CatDatabase/1.json
"tableName": „kittens",
"createSql": "CREATE TABLE IF NOT EXISTS `${TABLE_NAME}` (`id` INTEGER NOT NULL, `name` TEXT,
`url` TEXT, PRIMARY KEY(`id`))",
"fields": [
{
"fieldPath": "id",
"columnName": "id",
"affinity": "INTEGER",
"notNull": true
},
{
"fieldPath": "name",
"columnName": "name",
"affinity": "TEXT",
"notNull": false
},
. . .
]

48. @Database(entities = { Cat.class }, version = 2)a
public abstract class CatDatabase extends RoomDatabase {
private static CatDatabase create(final Context context) {
return Room
.databaseBuilder(context, CatDatabase.class, DB_NAME)
.addMigrations(Migrations.FROM_1_TO_2,
Migrations.FROM_2_TO_3)
.build();
}a
}a

49. @Database(entities = { Cat.class }, version = 3)a
public abstract class CatDatabase extends RoomDatabase {
private static CatDatabase create(final Context context) {
return Room
.databaseBuilder(context, CatDatabase.class, DB_NAME)
.addMigrations(Migrations.FROM_1_TO_3)
.build();
}a
}a

50. Room
Testing

51. @RunWith(AndroidJUnit4.class)
public class CatEntityTest {
private CatDao catDao;
private CatDatabase database;
@Before
public void createDb() {
Context context = InstrumentationRegistry.getTargetContext();
database = Room.inMemoryDatabaseBuilder(context, CatDatabase.class).build();
catDao = database.getCatDao();
}
@Test
public void writeCatAndReadInList() throws Exception {
Cat cat = new Cat(1, "Jake", „fake_url");
catDao.insert(cat);
List<Cat> catsByName = catDao.getCatsByName(„Jake");
assertThat(catsByName.get(0), equalTo(cat));
}
@After
public void closeDb() throws IOException {
database.close();
}
}

52. tl;dw • less boilerplate code
• SQL statement compile-time validation
• full compatibility with other Architecture
Components and RxJava
• easy to use
• easy to test
• easy to migrate

53. LIFECYCLE
Lifecycle-aware components

54. LOLcycle

55. LIFECYCLE
COMPONENTS
LIFECYCLE
OWNER
LIFECYCLE
OBSERVER
- FragmentActivity
- Fragment (support)
- AppCompatActivity

56. INITIALIZED RESUMEDSTARTEDCREATED
DESTROYED RESUMEDSTARTEDCREATED
ON_CREATE ON_START ON_RESUME
ON_PAUSEON_STOPON_DESTROY

57. public class MainActivity extends AppCompatActivity implements LocationListener {
private LocationManager locationManager;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
locationManager = new LocationManager();
}
@Override
protected void onStart() {
super.onStart();
locationManager.addLocationListener();
}
@Override
protected void onStop() {
super.onStop();
locationManager.removeLocationListener();
}
@Override
public void onLocationChanged(final Location location) {
Log.d(„MainActivity", „Location changed: " + location.toString());
}

58. public class LocationManager implements LifecycleObserver {
public LocationManager(LifecycleOwner lifecycleOwner) {
// do something
lifecycleOwner.getLifecycle().addObserver(this);
}a
@OnLifecycleEvent(Lifecycle.Event.ON_START)
void addLocationListener() {
// register location manager listener
}a
@OnLifecycleEvent(Lifecycle.Event.ON_STOP)
void removeLocationListener() {
// remove location manager listener
}a
}a

59. public class MainActivity extends AppCompatActivity implements LocationListener {
private LocationManager locationManager;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
locationManager = new LocationManager();
}a
@Override
protected void onStart() {
super.onStart();
locationManager.addLocationListener();
}
@Override
protected void onStop() {
super.onStop();
locationManager.removeLocationListener();
}
@Override
public void onLocationChanged(final Location location) {
Log.d(„MainActivity", „Location changed: " + location.toString());
}

60. public class MainActivity extends AppCompatActivity implements LocationListener {
private LocationManager locationManager;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
locationManager = new LocationManager(this);
}a
@Override
public void onLocationChanged(final Location location) {
Log.d(„MainActivity", „Location changed: " + location.toString());
}a
}a

61. Lifecycle
Observer
It’s the LocationManager which cares
about the Lifecycle. So it should be able
to do it without the activity
babysitting itself.
I’m sure if you look at your code today,
your onStart(), onStop() methods are
like, at least 20, 30 lines of code. We
want them to be zero lines of code.
~Yigit Boyar, Software Enginner, Google,
Google I/O ‚17

62. public class GithubApplication extends Application {
@Override
public void onCreate() {
super.onCreate();
ProcessLifecycleOwner.get()
.getLifecycle()
.addObserver(new AppLifecycleObserver());
}
}

63. public class AppLifecycleObserver implements LifecycleObserver {
@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
void onFirstActivityCreated() {
// app is in the foreground now
}
@OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
void onLastActivityPaused() {
// app is in the background now
}
}

64. ViewModel

65. ACTIVITY
Rotation
ACTIVITY
RECREATED
UI Data
Holder
UI Data
Holder
?
?

66. ACTIVITY
ACTIVITY
RECREATED
ViewModel
• don’t worry
about UI
data holder
• data will
wait for us
• data will be
always
updated
even after
activity
recreating
Rotation

67. public class UsersViewModel extends ViewModel {
private List<User> userList;
public List<User> getUserList() {
if (userList == null) {
usersList = loadUsers();
}
return userList;
}
private List<User> loadUsers() {
// do something to load users
}
}

68. public class UsersViewModel extends AndroidViewModel {
public UsersViewModel(final Application application) {
super(application);
}
private List<User> userList;
public List<User> getUserList() {
if (userList == null) {
usersList = loadUsers();
}
return userList;
}
private List<User> loadUsers() {
// do something to load users
}
}

69. public class UsersActivity extends AppCompatActivity {
@Override
protected void onCreate(final Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
UsersViewModel usersViewModel =
ViewModelProviders.of(this).get(UsersViewModel.class);
showUsers(usersViewModel.getUserList());
}a
}a

70. LiveData

71. Lifecycle + Data

72. LiveData<List<User>> observes
notifies
only if activity is
at least started
List<User>

73. public class UsersViewModel extends ViewModel {
private MutableLiveData<List<User>> userList =
new MutableLiveData<>();
public List<User> getUserList() {
if (userList == null) {
usersList = loadUsers();
}a
return userList;
}a
private List<User> loadUsers() {
// do something to load users
}a
}a

74. public class UsersViewModel extends ViewModel {
private MutableLiveData<List<User>> userList =
new MutableLiveData<>();
public UsersViewModel() {
setUserListRefreshCallback(newUserList -> {
userList.postValue(newUserList);
});
}a
public LiveData<List<User>> getUserList() {
return userList;
}a
}a

75. public class UsersActivity extends AppCompatActivity {
@Override
protected void onCreate(final Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
UsersViewModel usersViewModel =
ViewModelProviders.of(this).get(UsersViewModel.class);
usersViewModel.getUserList().observe(this, this::showUsers);
}a
}a

76. tl;dr • keeps your UI always updated (follows
observer pattern)
• no memory leaks (data bound with
lifecycle)
• no data sent to stopped activity

77. @Dao
public interface CatDao {
@Query("SELECT * FROM cats WHERE name=:name")
LiveData<List<Cat>> getCatsByName(String name);
@Query("SELECT * FROM cats WHERE name=:name")
Flowable<List<Cat>> getCatsByName(String name);
}a

78. Resources Android Architecture Documentation:
https://developer.android.com/topic/libraries/
architecture/index.html
Guide To App Architecture:
https://developer.android.com/topic/libraries/
architecture/guide.html
Google Samples Repository:
https://github.com/googlesamples/android-
architecture-components
Medium series about Architecture Components:
https://medium.com/@pszklarska

79. Thanks!
Any questions?
Paulina Szklarska
@pszklarska
@pszklarska
@p_szklarska