Design Patterns Every Android Developer Should Know

class CartStore {
    private val _items = MutableStateFlow<List<CartItem>>(emptyList())
    val items: StateFlow<List<CartItem>> = _items.asStateFlow()     // observers subscribe here
    fun add(item: CartItem) = _items.update { it + item }          // all observers notified
}

// Java-style builder
val request = Request.Builder().url(url).header("Accept", "application/json").build()

// Kotlin: named + default arguments do the same for your own classes
data class UploadRequest(val uri: Uri, val compress: Boolean = true, val maxSizeMb: Int = 10)
val upload = UploadRequest(uri = photo, maxSizeMb = 5)

interface FeedParser { fun parse(raw: String): List<FeedItem> }

object FeedParserFactory {
    fun create(contentType: String): FeedParser = when {
        contentType.contains("json") -> JsonFeedParser()
        contentType.contains("xml") -> RssFeedParser()
        else -> error("Unsupported content type: $contentType")
    }
}

suspend fun LegacyAuthClient.signInSuspending(email: String, password: String): Session =
    suspendCancellableCoroutine { cont ->
        signIn(email, password, object : AuthCallback {
            override fun onSuccess(session: Session) = cont.resume(session)
            override fun onError(error: Exception) = cont.resumeWithException(error)
        })
    }

class AuthInterceptor(private val tokens: TokenProvider) : Interceptor {
    override fun intercept(chain: Interceptor.Chain): Response {
        val request = chain.request().newBuilder()
            .header("Authorization", "Bearer ${tokens.current()}")
            .build()
        return chain.proceed(request)
    }
}

enum class SortOption(val comparator: Comparator<Product>) {
    PRICE_LOW(compareBy { it.pricePaise }),
    PRICE_HIGH(compareByDescending { it.pricePaise }),
    RATING(compareByDescending<Product> { it.rating }.thenBy { it.name }),
}
fun List<Product>.sorted(option: SortOption) = sortedWith(option.comparator)

interface Analytics {
    fun screen(name: String)
    fun event(name: String, params: Map<String, Any> = emptyMap())
}
// One implementation talks to the analytics SDK(s); the app sees only these two methods.