Dagger multibinding and plugin architectures

Multibindings assemble independently contributed implementations into Set<T> or Map<K, V>.

interface AnalyticsEnricher { fun enrich(event: Event): Event }

@Module
interface AnalyticsContributions {
    @Binds @IntoSet
    fun deviceEnricher(impl: DeviceEnricher): AnalyticsEnricher

    @Binds @IntoSet
    fun sessionEnricher(impl: SessionEnricher): AnalyticsEnricher
}

class AnalyticsPipeline @Inject constructor(
    private val enrichers: Set<@JvmSuppressWildcards AnalyticsEnricher>,
)

Feature modules can contribute without editing a central list. This supports validators, serializers, navigation destinations, startup tasks, debug tools, and command handlers.

Map binding for keyed dispatch

@MapKey annotation class PaymentTypeKey(val value: PaymentType)

@Binds @IntoMap @PaymentTypeKey(PaymentType.Card)
fun card(handler: CardPaymentHandler): PaymentHandler

Map keys must be unique at compile time. Lookup failure still needs a product policy for unsupported server-configured types.

Ordering is not guaranteed

A Set is not a workflow sequence. If order matters, contribute objects with explicit priority and sort, or model a chain centrally. Equal priorities need deterministic tie-breaking.

Plugin failure isolation

One analytics enricher should not prevent core event delivery unless contract says so. Decide exception isolation, timeout, threading, observability, and whether plugins may mutate shared objects. Prefer immutable input/output.

Avoid hidden magic

Multibinding can make “who runs?” difficult to discover. Provide documentation/graph tests, meaningful map keys, ownership rules, and duplicate-policy checks. For three fixed handlers in one module, an explicit list may be clearer.

Dynamic features

Compile-time Dagger graph cannot automatically know code absent from the base build. Dynamic feature integration may need component dependencies/entry points and runtime registration; design availability and uninstall/update behavior explicitly.

Architect takeaway

Use multibinding when independent contributors are a real organizational extension point. Do not replace understandable control flow with DI-driven discovery by default.

Interview exercises

1. Design an ordered validation plugin pipeline.
2. Isolate a crashing analytics plugin.
3. Choose explicit list versus Set/Map multibinding for four scenarios.