class Librarian {
    fun getBook(id: ISBN): Book? {
        return database.getBook(id)
    }

    fun storeBook(book: Book): Boolean {
        return database.storeBook(book)
    }
}

class Librarian {

    private val database = Database()

    fun getBook(id: ISBN): Book? {
        return database.getBook(id)
    }

    fun storeBook(book: Book): Boolean {
        return database.storeBook(book)
    }
}

object Database {
    ...
}

// java
// 參考 : https://skyyen999.gitbooks.io/-study-design-pattern-in-java/content/singleton.html
public class Database {
    private static Database instance;

    private Database(){ /*setup code*/ }

    public static Database getInstance(){
        if(instance == null){
            synchronized(Database.class){
                if(instance == null){
                    instance = new Database();
                }
            }
        }
        return instance;
    }
}

class Librarian {

    fun getBook(id: ISBN): Book? {
        return Database.getBook(id)
    }

    fun storeBook(book: Book): Boolean {
        return Database.storeBook(book)
    }
}

class Librarian constructor(private val database: Database) {

    fun getBook(id: ISBN): Book? {
        return database.getBook(id)
    }

    fun storeBook(book: Book): Boolean {
        return database.storeBook(book)
    }
}

interface IDatabase {
    fun getBook(id: ISBN): Book?
    fun storeBook(book: Book): Boolean
}

class InMemoryDatabase : IDatabase {

    private val data = hashMapOf<ISBN, Book>()

    override fun getBook(id: ISBN): Book ? {
        synchronize(this) {
            return data.get(id)
        }
    }

    override fun storeBook(book: Book): Boolean {
        synchronize(this) {
            return data.put(book.id, book) == null
        }
    }
}

class ServiceLocator(applicationContext: Context) {

    // Database
    private val logsDatabase = Room.databaseBuilder(
        applicationContext,
        AppDatabase::class.java,
        "logging.db"
    ).build()

    // Database Source ，他的實作需要通過 Dao 完成。所以這裡可以使用 DI ：
    val loggerLocalDataSource = LoggerLocalDataSource(logsDatabase.logDao())

    fun provideDateFormatter() = DateFormatter()

    fun provideNavigator(activity: FragmentActivity): AppNavigator {
        return AppNavigatorImpl(activity)
    }
}

class ServiceLocator() {
    private val database by lazy {InMemoryDatabase()}

    fun getBook(id: ISBN): Book? {
        return database.getBook(id)
    }

    fun storeBook(book: Book): Boolean {
        return database.storeBook(book)
    }
}

class LogApplication : Application() {

    lateinit var serviceLocator: ServiceLocator

    override fun onCreate() {
        super.onCreate()
        serviceLocator = ServiceLocator()
    }
}

@HiltAndroidApp
class LogApplication : Application()

@AndroidEntryPoint
class ExampleActivity : AppCompatActivity() {

  private lateinit var analytics: AnalyticsAdapter
  ...
}

@AndroidEntryPoint
class ExampleActivity : AppCompatActivity() {

  @Inject lateinit var analytics: AnalyticsAdapter
  ...
}

class AnalyticsAdapter @Inject constructor(
  private val service: AnalyticsService
) { ... }

@AndroidEntryPoint
class LogsFragment : Fragment() {
    @Inject lateinit var logger: LoggerLocalDataSource
    @Inject lateinit var dateFormatter: DateFormatter
}

class DateFormatter @Inject constructor() {
    // ...
}

class LoggerLocalDataSource @Inject constructor(private val logDao: LogDao) {
    // ...
}

@AndroidEntryPoint
class MainActivity : AppCompatActivity()

@AndroidEntryPoint
class LogsFragment : Fragment() {
    @Inject lateinit var logger: LoggerLocalDataSource
    @Inject lateinit var dateFormatter: DateFormatter
}

@AndroidEntryPoint
class MainActivity : AppCompatActivity() { /* code */ }

class DateFormatter @Inject constructor() { /* code */ }

class LoggerLocalDataSource(private val logDao: LogDao) { /* code */ }

@AndroidEntryPoint
class LogsFragment : Fragment() {

    lateinit var logger: LoggerLocalDataSource
    @Inject lateinit var dateFormatter: DateFormatter
    // ...
}

android.util.Log.d("LOG_FRAG", "onAttach $dateFormatter")

@HiltAndroidApp
class LogApplication : Application() {

    lateinit var serviceLocator: ServiceLocator

    @Inject
    lateinit var dateFormatter: DateFormatter
    override fun onCreate() {
        super.onCreate()
        serviceLocator = ServiceLocator(applicationContext)

        Log.d("LOG_APP", "onCreate : $dateFormatter")
    }
}

@Singleton
class DateFormatter @Inject constructor()

@ActivityScoped
class DateFormatter @Inject constructor()

@ActivityRetainedScoped
class DateFormatter @Inject constructor()

// Default Scope
class DateFormatter @Inject constructor()

@Singleton
class LoggerLocalDataSource @Inject constructor(private val logDao: LogDao)

@InstallIn(SingletonComponent::class)
@Module
object DatabaseModule { /**/ }

private val logsDatabase = Room.databaseBuilder(
        applicationContext,
        AppDatabase::class.java,
        "logging.db"
    ).build()

val loggerLocalDataSource = LoggerLocalDataSource(logsDatabase.logDao())

@Singleton
class LoggerLocalDataSource @Inject constructor(private val logDao: LogDao)

@Database(entities = arrayOf(Log::class), version = 1, exportSchema = false)
abstract class AppDatabase : RoomDatabase() {
    abstract fun logDao(): LogDao
}

@Provides
fun {方法名稱} ({被依賴者}) : {目標對象} {
    return {目標對象}
}

@Module
@InstallIn(SingletonComponent::class)
object DatabaseModule {
    @Provides
    fun provideLogDao(database: AppDatabase): LogDao {
        return database.logDao()
    }
}

// ServiceLocator.kt
private val logsDatabase: AppDatabase = Room.databaseBuilder(
        applicationContext,
        AppDatabase::class.java,
        "logging.db"
    ).build()

@Module
@InstallIn(SingletonComponent::class)
object DatabaseModule {
    @Provides
    fun provideLogDao(database: AppDatabase): LogDao {
        return database.logDao()
    }

    @Provides
    @Singleton
    fun provideDatabase(@ApplicationContext appContext: Context): AppDatabase {
        return Room.databaseBuilder(
            appContext,
            AppDatabase::class.java,
            "logging.db"
        ).build()
    }
}

override fun onAttach(context: Context) {
    super.onAttach(context)
    populateFields(context)
}

private fun populateFields(context: Context) {

    logger = (context.applicationContext as LogApplication).serviceLocator.loggerLocalDataSource
    android.util.Log.d("LOG_FRAG", "onAttach $dateFormatter")
    dateFormatter =
        (context.applicationContext as LogApplication).serviceLocator.provideDateFormatter()
}

@Inject lateinit var logger: LoggerLocalDataSource
@Inject lateinit var dateFormatter: DateFormatter

interface AppNavigator {
    fun navigateTo(screen: Screens)
}

@AndroidEntryPoint
class MainActivity : AppCompatActivity() {
    private lateinit var navigator: AppNavigator

    /* code */

    override fun onCreate(saveInstanceState: Bundle?) {
        // ...
        navigator = (applicationContext as LogApplication).serviceLocator.provideNavigator(this)
        // ...
    }
}

fun provideNavigator(activity: FragmentActivity): AppNavigator {
    return AppNavigatorImpl(activity)
}

@InstallIn(ActivityComponent::class)
@Module
abstract class NavigationModule {
    @Binds
    abstract fun bindNavigator(impl: AppNavigatorImpl): AppNavigator
}

class AppNavigatorImpl @Inject constructor(private val activity: FragmentActivity) : AppNavigator

@Inject lateinit var navigator: AppNavigator

// and

// navigator = (applicationContext as LogApplication).serviceLocator.provideNavigator(this)

@AndroidEntryPoint
class ButtonsFragment : Fragment()

@Inject lateinit var logger: LoggerLocalDataSource
@Inject lateinit var navigator: AppNavigator

//    override fun onAttach(context: Context) {
//        super.onAttach(context)
//
//        populateFields(context)
//    }
//
//    private fun populateFields(context: Context) {
//        logger = (context.applicationContext as LogApplication).
//            serviceLocator.loggerLocalDataSource
//
//        navigator = (context.applicationContext as LogApplication).
//            serviceLocator.provideNavigator(requireActivity())
//    }

// Common interface for Logger data sources.
interface LoggerDataSource {
    fun addLog(msg: String)
    fun getAllLogs(callback: (List<Log>) -> Unit)
    fun removeLogs()
}

@ActivityScoped
class LoggerInMemoryDataSource @Inject constructor() : LoggerDataSource {

    private val logs = LinkedList<Log>()

    override fun addLog(msg: String) {
        logs.addFirst(Log(msg, System.currentTimeMillis()))
    }

    override fun getAllLogs(callback: (List<Log>) -> Unit) {
        callback(logs)
    }

    override fun removeLogs() {
        logs.clear()
    }
}

@Inject lateinit var logger: LoggerDataSource

// LoggerModule.kt
@InstallIn(SingletonComponent::class)
@Module
abstract class LoggingDatabaseModule {

    @Singleton
    @Binds
    abstract fun bindDatabaseLogger(impl: LoggerLocalDataSource): LoggerDataSource
}

@InstallIn(ActivityComponent::class)
@Module
abstract class LoggingInMemoryModule {

    @ActivityScoped
    @Binds
    abstract fun bindInMemoryLogger(impl: LoggerInMemoryDataSource): LoggerDataSource
}

@Qualifier
annotation class InMemoryLogger

@Qualifier
annotation class DatabaseLogger

@InstallIn(SingletonComponent::class)
@Module
abstract class LoggingDatabaseModule {

    @DatabaseLogger
    @Singleton
    @Binds
    abstract fun bindDatabaseLogger(impl: LoggerLocalDataSource): LoggerDataSource
}

@InstallIn(ActivityComponent::class)
@Module
abstract class LoggingInMemoryModule {

    @InMemoryLogger
    @ActivityScoped
    @Binds
    abstract fun bindInMemoryLogger(impl: LoggerInMemoryDataSource): LoggerDataSource
}

@AndroidEntryPoint
class LogsFragment : Fragment() {

    @InMemoryLogger
    @Inject lateinit var logger: LoggerDataSource
    ...
}

class CustomTestRunner : AndroidJUnitRunner() {
    override fun newApplication(cl: ClassLoader?, name: String?, context: Context?): Application {
        return super.newApplication(cl, HiltTestApplication::class.java.name, context)
    }
}

@RunWith(AndroidJUnit4::class)
class AppTest {

//    @After
//    fun tearDown() {
//        // Remove logs after the test finishes
//        ServiceLocator(getInstrumentation().targetContext).loggerLocalDataSource.removeLogs()
//    }
}

@RunWith(AndroidJUnit4::class)
@HiltAndroidTest
class AppTest

@RunWith(AndroidJUnit4::class)
@HiltAndroidTest
class AppTest {

    @get:Rule
    var hiltRule = HiltAndroidRule(this)

    ...
}