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Adapter, Decorator, Facade, and Proxy

Adapter: translate incompatible contracts​

class FirebaseAnalyticsAdapter(
    private val firebase: FirebaseAnalytics,
) : Analytics {
    override fun record(event: AnalyticsEvent) {
        firebase.logEvent(event.name, event.toBundle())
    }
}

The application owns Analytics; adapter contains SDK types and mapping. Replacing SDK does not edit features.

Decorator: add behavior around the same contract​

class LoggingOrdersRepository(
    private val delegate: OrdersRepository,
    private val logger: Logger,
) : OrdersRepository {
    override suspend fun submit(order: Order): OrderResult {
        val started = TimeSource.Monotonic.markNow()
        return try { delegate.submit(order) }
        finally { logger.timing("submit_order", started.elapsedNow()) }
    }
}

Decorators compose caching, metrics, retries, authorization, or tracing. Order matters: retry outside metrics records one total; metrics inside retry records each attempt.

Avoid double-retry layers (OkHttp + repository + Worker) that multiply requests.

Facade: simplify a subsystem​

class AccountDeletion(
    private val remote: AccountApi,
    private val database: AppDatabase,
    private val session: SessionManager,
    private val scheduler: WorkScheduler,
) {
    suspend fun delete(): DeletionResult { /* ordered policy */ }
}

Facade provides one task-focused API over several components. It may become a god service if unrelated workflows accumulate; split by use case/capability.

Proxy: control access to another object​

A proxy may lazy-load, authorize, remote-call, cache, or stand in for another process. Retrofit creates a proxy implementation for service interfaces. Binder proxies represent remote objects.

class AuthorizedDocuments(
    private val delegate: Documents,
    private val session: Session,
) : Documents {
    override suspend fun delete(id: DocumentId) {
        check(session.permissions.canDeleteDocuments)
        delegate.delete(id)
    }
}

Adapter changes interface; decorator adds behavior while preserving it; facade simplifies several APIs; proxy controls access/represents another object. Implementations can overlap—explain intent.

Hidden semantics

A wrapper that adds caching/retry/security changes behavioral contracts. Document freshness, attempt count, authorization failure, cancellation, and observability.

Exercises​

  1. Classify Room mapper, OkHttp interceptor, Retrofit service, and feature coordinator.
  2. Determine decorator order for auth, retry, and metrics.
  3. Find the god-facade boundary in an account manager.