SOLID Principles in Android Development

// ❌ One class changes for UI tweaks, API changes, caching policy AND analytics events
class ProfileViewModel(/* api, dao, prefs, analytics */) : ViewModel() { /* 600 lines */ }

// ✅ Each concern has an owner
class ProfileRepository(/* api, dao */)            // data access + caching
class ProfileValidator                              // business rules for edits
class ProfileViewModel(                             // screen state + coordination
    private val repository: ProfileRepository,
    private val validator: ProfileValidator,
    private val analytics: Analytics,
) : ViewModel()

interface PaymentMethodHandler {
    val id: String
    suspend fun pay(orderId: String, amountPaise: Long): PaymentResult
}

class UpiHandler @Inject constructor(/* ... */) : PaymentMethodHandler { override val id = "upi" /* ... */ }
class CardHandler @Inject constructor(/* ... */) : PaymentMethodHandler { override val id = "card" /* ... */ }

class PaymentProcessor @Inject constructor(
    private val handlers: Set<@JvmSuppressWildcards PaymentMethodHandler>,   // Hilt multibinding
) {
    suspend fun pay(methodId: String, orderId: String, amount: Long) =
        handlers.first { it.id == methodId }.pay(orderId, amount)
}
// Adding Wallet = new class + @IntoSet binding; PaymentProcessor never changes.

interface ArticleRepository {
    /** Emits cached articles immediately (possibly empty); never throws for network failures. */
    fun observeArticles(): Flow<List<Article>>
}

// ❌ The bookmarks screen depends on admin operations it never calls
interface ArticleService {
    fun observe(): Flow<List<Article>>
    suspend fun bookmark(id: String)
    suspend fun publish(article: Article)
    suspend fun delete(id: String)
}

// ✅ Focused interfaces; one class can implement several
interface ArticleReader { fun observe(): Flow<List<Article>> }
interface BookmarkWriter { suspend fun bookmark(id: String) }
interface ArticleAdmin { suspend fun publish(article: Article); suspend fun delete(id: String) }

class WeatherViewModel @Inject constructor(private val repo: WeatherRepository) : ViewModel()
// Production: DefaultWeatherRepository(Retrofit, Room). Tests: FakeWeatherRepository.