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Android Architecure Components - introduction

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Paulina Szklarska

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

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ANDROID ARCHITECTURE COMPONENTS - INTRODUCTION Paulina Szklarska
Hello! Nice to meet you Paulina Szklarska Android Developer @ DroidsOnRoids co-organizer of Toast Wrocław meetups cat owner travel enthusiast @pszklarska @pszklarska @p_szklarska
1. INTRO What are Architecture Components?
Android Architecure Components - introduction
November, 6 2017
ARCHITECTURE COMPONENTS ROOM LIFECYCLE COMPONENTS VIEW MODEL LIVE DATA PAGING LIBRARY
ROOM Persistence library for Android
Room Entity Single database table DAO Defines methods to access database Database Holds entities and DAOs
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
Room Entity, DAO, Database
Cats id 1 2 3 name Fudge Houdini Scratchy url /fudge_photo.jpg /houdini_photo.jpg /scratchy_photo.jpg
@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
@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
@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
@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
@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
@Dao
 public interface CatDao {
 
 @Insert
 void insert(Cat... cats);
 
 @Insert
 void insert(Cat cat);
 
 @Insert
 void insert(List<Cat> catList);
 
 }a
@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
@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
@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
CatDatabase
 .getInstance(context)
 .getCatDao()
 .insert(new Cat(1, 
 "Muffin", 
 "/muffin.jpg")); List<Cat> allCats = CatDatabase
 .getInstance(context)
 .getCatDao()
 .getAllCats();
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
Room Relations - @ForeignKey
@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
@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
@Dao
 public interface CatDao {
 
 @Query("SELECT * FROM kittens WHERE ownerId=:ownerId")
 List<Cat> findCatsForOwner(int ownerId);
 
 }a

Room Relations - @Relation
@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
@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
public class OwnerWithCats {
 @Embedded public Owner owner;
 
 @Relation(parentColumn = "id",
 entityColumn = "ownerId") public List<Cat> catList;
 }a
@Dao
 public interface OwnerWithCatsDao {
 
 @Query("SELECT * FROM owner")
 public List<OwnerWithCats> getOwnersWithCats();
 
 }a
Room Custom Types
@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
public class DateConverter {
 
 @TypeConverter public LocalDate toDate(long dateLong) { return LocalDate.ofEpochDay(dateLong); } @TypeConverter public long fromDate(LocalDate date) { return date.toEpochDay(); }
 }
@Database(entities = { Cat.class }, version = 1)
 @TypeConverters(DateConverter.class)
 public abstract class CatDatabase extends RoomDatabase {
 
 }a
Room Migrations
@Entity
 public class Cat {
 @PrimaryKey public int id;
 public String name;
 public String url;
 }a
@Entity
 public class Cat {
 @PrimaryKey public int id;
 public String name;
 public String url;
 public String age;
 }a
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.
@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
@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
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.
@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
@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
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");
 }
 };
 }
android { javaCompileOptions {
 annotationProcessorOptions {
 arguments = ["room.schemaLocation":
 "$projectDir/schemas".toString()]
 }
 }
 } }
app/schemas/pl.pszklarska.kittyarchitecture.data.CatDatabase/1.json "tableName": „kittens",
 "createSql": "CREATE TABLE IF NOT EXISTS `${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
 }, . . . ]
@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
@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
Room Testing
@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();
 }
 }
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
LIFECYCLE Lifecycle-aware components
LOLcycle
LIFECYCLE COMPONENTS LIFECYCLE OWNER LIFECYCLE OBSERVER - FragmentActivity - Fragment (support) - AppCompatActivity
INITIALIZED RESUMEDSTARTEDCREATED DESTROYED RESUMEDSTARTEDCREATED ON_CREATE ON_START ON_RESUME ON_PAUSEON_STOPON_DESTROY
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());
 }

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
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());
 }

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
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
public class GithubApplication extends Application { @Override public void onCreate() { super.onCreate(); ProcessLifecycleOwner.get() .getLifecycle() .addObserver(new AppLifecycleObserver()); } }
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 } }
ViewModel
ACTIVITY Rotation ACTIVITY RECREATED UI Data Holder UI Data Holder ? ?
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
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
 }
 }
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
 }
 }
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
LiveData
Lifecycle + Data
LiveData<List<User>> observes notifies only if activity is at least started List<User>
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
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
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
tl;dr • keeps your UI always updated (follows observer pattern) • no memory leaks (data bound with lifecycle) • no data sent to stopped activity
@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

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

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Android Architecure Components - introduction

  • 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
  • 6. ARCHITECTURE COMPONENTS ROOM LIFECYCLE COMPONENTS VIEW MODEL LIVE DATA PAGING LIBRARY
  • 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
  • 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();
  • 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

  • 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
  • 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
  • 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
  • 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
  • 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 } }
  • 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
  • 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