Architecture Overview

High-level architecture and technology stack of the Divvy Android app

Overview

Divvy is built as a modern Android application using Kotlin and Jetpack Compose with a clean MVVM architecture. The app simplifies group expense splitting by bringing it to the moment of payment through bank integration and receipt scanning.

Technology Stack

Frontend

• Kotlin - Primary language
• Jetpack Compose - Modern UI toolkit
• Material 3 - Design system
• Navigation Compose - Type-safe navigation

Backend & Data

• Supabase - Database and authentication
• Postgrest - Database queries
• GoTrue - Auth provider
• Plaid - Bank integration (planned)

Dependency Injection

• Hilt - DI framework
• Dagger - Compile-time DI

Camera & Media

• CameraX - Receipt scanning
• Coil - Image loading

Project Structure

The Divvy codebase follows a feature-based organization within the MVVM pattern:

app/src/main/java/com/example/divvy/
├── backend/          # Repositories and data access layer
├── ui/               # UI layer organized by feature
│   ├── analytics/
│   ├── assignitems/
│   ├── auth/
│   ├── expenses/
│   ├── friends/
│   ├── groupdetail/
│   ├── groups/
│   ├── home/
│   ├── ledger/
│   ├── profile/
│   ├── scanreceipt/
│   ├── splitexpense/
│   ├── splitpercentage/
│   ├── navigation/   # Navigation graph and routes
│   └── theme/        # Material 3 theming
├── models/           # Domain models
├── components/       # Reusable UI components
└── di/               # Dependency injection modules

Each feature in the ui/ directory typically contains ViewModels/ and Views/ subdirectories, maintaining clear separation between business logic and presentation.

Data Flow Architecture

Divvy implements a unidirectional data flow pattern:

1. User Interaction

   User interacts with Compose UI components (buttons, text fields, etc.)

2. ViewModel Processing

   UI events are handled by ViewModels which contain business logic

3. Repository Layer

   ViewModels call repository methods for data operations

4. Backend Communication

   Repositories interact with Supabase through the SupabaseClient

5. State Updates

   Data flows back through StateFlow/Flow emissions

6. UI Recomposition

   Compose UI automatically recomposes when state changes

Dependency Injection Setup

Divvy uses Hilt for dependency injection with two main modules:

NetworkModule

Provides the Supabase client configuration:

app/src/main/java/com/example/divvy/di/NetworkModule.kt

@Module
@InstallIn(SingletonComponent::class)
object NetworkModule {

    @Provides
    @Singleton
    fun provideSupabaseClient(): SupabaseClient {
        val client = createSupabaseClient(
            supabaseUrl = BuildConfig.SUPABASE_URL,
            supabaseKey = BuildConfig.SUPABASE_ANON_KEY
        ) {
            defaultSerializer = KotlinXSerializer(Json { ignoreUnknownKeys = true })
            install(Auth) {
                scheme = "com.example.divvy"
                host = "auth"
                defaultExternalAuthAction = ExternalAuthAction.CUSTOM_TABS
            }
            install(Postgrest)
        }
        SupabaseClientProvider.setClient(client)
        return client
    }
}

AppModule

Binds repository interfaces to their Supabase implementations:

app/src/main/java/com/example/divvy/di/AppModule.kt

@Module
@InstallIn(SingletonComponent::class)
abstract class AppModule {
    @Binds @Singleton abstract fun bindAuthRepository(
        impl: SupabaseAuthRepository
    ): AuthRepository
    
    @Binds @Singleton abstract fun bindGroupRepository(
        impl: SupabaseGroupRepository
    ): GroupRepository
    
    @Binds @Singleton abstract fun bindMemberRepository(
        impl: SupabaseMemberRepository
    ): MemberRepository
    
    @Binds @Singleton abstract fun bindBalanceRepository(
        impl: SupabaseBalanceRepository
    ): BalanceRepository
    
    @Binds @Singleton abstract fun bindExpensesRepository(
        impl: SupabaseExpensesRepository
    ): ExpensesRepository
    
    @Binds @Singleton abstract fun bindProfilesRepository(
        impl: SupabaseProfilesRepository
    ): ProfilesRepository
    
    @Binds @Singleton abstract fun bindActivityRepository(
        impl: SupabaseActivityRepository
    ): ActivityRepository
}

All repositories are provided as singletons to ensure consistent state across the app and efficient resource usage.

Build Configuration

Key build configuration from app/build.gradle.kts:

Android Config

android {
    namespace = "com.example.divvy"
    compileSdk = 36

    defaultConfig {
        applicationId = "com.example.divvy"
        minSdk = 26
        targetSdk = 36
        versionCode = 1
        versionName = "1.0"
    }
    
    buildFeatures {
        compose = true
        buildConfig = true
    }
}

Plugins

plugins {
    alias(libs.plugins.android.application)
    alias(libs.plugins.kotlin.android)
    alias(libs.plugins.kotlin.serialization)
    alias(libs.plugins.kotlin.compose)
    alias(libs.plugins.ksp)
    alias(libs.plugins.hilt.android)
}

Key Dependencies

dependencies {
    // Compose
    implementation(platform(libs.androidx.compose.bom))
    implementation(libs.androidx.compose.ui)
    implementation(libs.androidx.compose.material3)
    implementation(libs.androidx.navigation.compose)
    
    // Supabase
    implementation(platform(libs.supabase.bom))
    implementation(libs.supabase.postgrest.kt)
    implementation(libs.supabase.gotrue.kt)
    
    // Hilt DI
    implementation(libs.hilt.android)
    ksp(libs.hilt.android.compiler)
    implementation(libs.androidx.hilt.navigation.compose)
    
    // CameraX
    implementation(libs.androidx.camera.core)
    implementation(libs.androidx.camera.camera2)
    implementation(libs.androidx.camera.lifecycle)
    implementation(libs.androidx.camera.view)
}

Environment Configuration

Divvy uses local.properties for environment-specific configuration:

SUPABASE_URL=https://xxxx.supabase.co
SUPABASE_ANON_KEY=eyJ...

Never commit local.properties to version control. These values are injected at build time using BuildConfig.

Authentication Flow

Supabase Auth is configured with custom URL scheme for OAuth redirects:

• Redirect URL: com.example.divvy://auth
• Auth Method: Google OAuth via Custom Tabs
• Session Management: Handled by Supabase GoTrue SDK

The redirect URL must be configured in Supabase Auth settings to enable Google sign-in.

Next Steps

MVVM Pattern

Learn how ViewModels and Views work together

Navigation

Explore the navigation architecture