Architecture Overview

MyTwin follows MVVM (Model–View–ViewModel) with a lightweight manual Service Locator for dependency wiring. No Hilt or Dagger — the trade-off was deliberate for a 36-hour hackathon: fast setup, zero annotation processing overhead, trivially inspectable.

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High-Level Diagram

graph TD
    subgraph UI["UI Layer"]
        S[Screens<br/>Composable]
        VM[ViewModels]
    end

    subgraph Domain["Domain / Simulation"]
        SR[SimulationRepository]
        SE[SimulationEngine]
        CR[ChatRepository]
        CB[ClaudeContextBuilder]
        CA[ClaudeApiService]
    end

    subgraph Data["Data Layer"]
        UPR[UserProfileRepository]
        HR[HealthRepository]
        OR[OnboardingRepository]
        UPD[UserProfileDataSource<br/>DataStore JSON]
        SHD[SamsungHealthDataSource<br/>Samsung Health SDK]
        OD[OnboardingDataSource<br/>DataStore Preferences]
    end

    SL[ServiceLocator]

    S -->|observes state| VM
    VM --> SR
    VM --> CR
    VM --> UPR
    VM --> OR
    SR --> UPR
    SR --> HR
    SR --> SE
    CR --> UPR
    CR --> HR
    CR --> SR
    CR --> CB
    CR --> CA
    HR --> SHD
    UPR --> UPD
    OR --> OD
    SL -.->|lazy init| VM
    SL -.->|lazy init| SR
    SL -.->|lazy init| CR

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Layer Responsibilities

Layer	Package	Purpose
Screens	ui/screens	Composable UI, no business logic
ViewModels	ui/viewmodels	UI state holders, coroutine scope owners
Repositories	data/repository, chat, simulation	Business logic, data aggregation
Data Sources	data/datasource	Raw access to DataStore and Samsung Health SDK
Models	data/model, simulation, chat	Pure data classes and enums
Service Locator	di/ServiceLocator	Singleton DI container

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Key Design Decisions

Manual Service Locator over Hilt

ServiceLocator is an object initialized once in MyTwinApplication.onCreate(). Every dependency is a lazy val — created on first access. This avoids generated code, annotation processing, and the learning curve of Hilt, which matters when you have 36 hours.

No XML layouts

Every screen is a Jetpack Compose @Composable. MainActivity hosts a single AppNavGraph. There are no fragments.

DataStore for persistence

Two DataStore instances:

• Onboarding flag — a single boolean Preference
• User profile — the full UserProfile data class serialized to JSON via kotlinx.serialization

Streaming chat

ClaudeApiService opens a raw HttpURLConnection to api.anthropic.com/v1/messages with Accept: text/event-stream. Tokens arrive as SSE content_block_delta events and are emitted as a Kotlin Flow<String>, which ChatViewModel collects into the current message bubble in real time.