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Module Boundary Rules

Based on NowInAndroid's multi-module structure with Feature API/Impl separation.

Module Graph

:app
├─→ :feature:*/api + :feature:*/impl
├─→ :core:designsystem, :core:ui, :core:navigation
├─→ :core:data, :core:analytics
└─→ :sync:work

:feature:*/impl
├─→ :feature:*/api (own API only)
├─→ :core:ui, :core:designsystem, :core:navigation
├─→ :core:domain, :core:data, :core:model
└─→ :core:analytics

:feature:*/api
└─→ :core:navigation (NavKey only)

:core:data → :core:database, :core:datastore, :core:network, :core:model
:core:domain → :core:data, :core:model (NO Android imports)
:core:designsystem → (standalone, Material 3 components)

Feature API/Impl Separation

  • MUST split each feature into :feature:{name}/api and :feature:{name}/impl
  • MUST export only NavKey (serializable) from API module
  • MUST keep ViewModel, Screen, Repository observers in impl module
  • MUST NOT depend on another feature's impl module
// :feature:topic/api — minimal exports
@Serializable
data class TopicNavKey(val topicId: String) : NavKey

// :feature:topic/impl — all implementation
@HiltViewModel
class TopicViewModel @Inject constructor(/*...*/) : ViewModel()

@Composable
fun TopicScreen(/*...*/) { /*...*/ }

Module Responsibilities

Module Contains MUST NOT contain
:core:model Domain data classes, enums Android imports, annotations
:core:domain UseCases Android framework, Room, Retrofit
:core:data Repository impl, Syncable UI code, Composables
:core:database Room DAOs, Entities Network code, UI code
:core:network Retrofit services, DTOs Database code, UI code
:core:datastore Proto DataStore Network, Database
:core:designsystem Theme, Material 3 components Business logic, data access
:core:ui Shared screen-level composables Feature-specific logic
:core:navigation Navigator, NavigationState, NavKey Feature implementations

Convention Plugins (build-logic/)

  • MUST use convention plugins for shared build configuration
  • MUST NOT duplicate Gradle config across modules
Plugin Applies To
nowinandroid.android.feature.api Feature API modules (minimal deps)
nowinandroid.android.feature.impl Feature impl modules (Hilt, Lifecycle, Navigation)
nowinandroid.android.library Core library modules
nowinandroid.android.library.compose Compose-enabled libraries
nowinandroid.hilt Any module needing DI
nowinandroid.android.room Database modules

Visibility

  • MUST use internal for module-scoped classes (Repository impl, DAO, DataSource)
  • MUST use public only for cross-module contracts (interfaces, NavKeys, models)
  • MUST NOT expose Room Entity, Retrofit DTO, or DataStore proto outside their modules

Gradle Dependencies

  • MUST use implementation for internal dependencies
  • MUST use api only when transitive exposure is intentional
  • Feature impl modules auto-include :core:ui, :core:designsystem via convention plugin