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Overview

Divvy uses the Repository Pattern to abstract data access logic from the UI and business logic layers. All repositories follow a consistent interface-based design with Supabase implementations.

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Architecture

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Interface-Based Abstractions

Each repository is defined as a Kotlin interface that declares the contract for data operations: 
interface GroupRepository {
    fun listGroups(): Flow<DataResult<List<Group>>>
    fun getGroup(groupId: String): Flow<Group>
    suspend fun createGroup(name: String, icon: GroupIcon): Group
    suspend fun updateGroup(groupId: String, name: String, icon: GroupIcon)
    suspend fun deleteGroup(groupId: String)
    suspend fun refreshGroups()
}

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Supabase Implementations

Each interface has a corresponding Supabase implementation:
  • GroupRepository → SupabaseGroupRepository
  • ExpensesRepository → SupabaseExpensesRepository
  • AuthRepository → SupabaseAuthRepository
  • ProfilesRepository → SupabaseProfilesRepository
Implementations are marked with @Singleton and injected via Hilt: 
@Singleton
class SupabaseGroupRepository @Inject constructor(
    private val supabaseClient: SupabaseClient
) : GroupRepository {
    // Implementation...
}

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DataResult Wrapper

Divvy uses a DataResult sealed class to handle loading states, success, and errors uniformly across the app.

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Definition

Source: backend/DataResult.kt:3 
sealed class DataResult<out T> {
    data class Success<T>(val data: T) : DataResult<T>()
    data class Error(val message: String, val cause: Throwable? = null) : DataResult<Nothing>()
    data object Loading : DataResult<Nothing>()
}

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Usage Pattern

Repositories return DataResult for operations that may fail: 
val groups: Flow<DataResult<List<Group>>> = groupRepository.listGroups()

groups.collect { result ->
    when (result) {
        is DataResult.Loading -> showLoadingSpinner()
        is DataResult.Success -> displayGroups(result.data)
        is DataResult.Error -> showError(result.message)
    }
}

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Error Handling

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Try-Catch Pattern

Repository implementations wrap Supabase calls in try-catch blocks: 
override suspend fun refreshGroups() {
    try {
        val rows = supabaseClient.postgrest
            .rpc("get_my_groups_summary")
            .decodeList<GroupSummaryRow>()
        
        _groups.value = DataResult.Success(rows.map { /* ... */ })
    } catch (e: Exception) {
        _groups.value = DataResult.Error("Failed to load groups", e)
    }
}

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Error States

Repositories maintain internal MutableStateFlow with DataResult states: 
private val _groups = MutableStateFlow<DataResult<List<Group>>>(DataResult.Loading)

override fun listGroups(): Flow<DataResult<List<Group>>> = _groups

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Dependency Injection with Hilt

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Repository Binding

Repositories are injected as singletons: 
@Singleton
class SupabaseGroupRepository @Inject constructor(
    private val supabaseClient: SupabaseClient
) : GroupRepository

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ViewModel Injection

ViewModels receive repository instances via constructor injection: 
@HiltViewModel
class GroupsViewModel @Inject constructor(
    private val groupRepository: GroupRepository
) : ViewModel()

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Reactive Data with Flow

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Observing Data Changes

Repositories expose Flow<DataResult<T>> for reactive UI updates: 
// In Repository
override fun listGroups(): Flow<DataResult<List<Group>>> = _groups

// In ViewModel
val groupsState: StateFlow<DataResult<List<Group>>> = 
    groupRepository.listGroups()
        .stateIn(viewModelScope, SharingStarted.Lazily, DataResult.Loading)

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Data Refresh

Repositories provide explicit refresh methods: 
suspend fun refreshGroups() {
    try {
        val data = fetchFromSupabase()
        _groups.value = DataResult.Success(data)
    } catch (e: Exception) {
        _groups.value = DataResult.Error("Refresh failed", e)
    }
}

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Suspend Functions

All data mutations use Kotlin coroutines with suspend functions: 
suspend fun createGroup(name: String, icon: GroupIcon): Group
suspend fun updateGroup(groupId: String, name: String, icon: GroupIcon)
suspend fun deleteGroup(groupId: String)
This ensures operations are non-blocking and can be safely called from coroutine scopes.

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Repository List

GroupRepository

Manage groups, members, and group metadata

ExpensesRepository

Create, update, and track expenses with splits

AuthRepository

Handle user authentication and sessions

ProfilesRepository

Manage user profiles and preferences

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Best Practices

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1. Always Use Interfaces

Depend on interfaces, not concrete implementations: 
// Good
class MyViewModel @Inject constructor(
    private val groupRepo: GroupRepository
)

// Bad
class MyViewModel @Inject constructor(
    private val groupRepo: SupabaseGroupRepository
)

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2. Handle All DataResult States

Always handle Loading, Success, and Error states: 
when (result) {
    is DataResult.Loading -> { /* Show loading UI */ }
    is DataResult.Success -> { /* Display data */ }
    is DataResult.Error -> { /* Show error message */ }
}

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3. Use Coroutine Scopes

Call suspend functions from appropriate coroutine scopes: 
viewModelScope.launch {
    val group = groupRepository.createGroup("Trip", GroupIcon.Flight)
}

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4. Local State Updates

Update local state optimistically, then sync with backend: 
override suspend fun deleteGroup(groupId: String) {
    // Call backend
    supabaseClient.postgrest.rpc("delete_group_cascade", params)
    
    // Update local state
    _groups.update { result ->
        val current = (result as? DataResult.Success)?.data ?: return@update result
        DataResult.Success(current.filter { g -> g.id != groupId })
    }
}

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