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Architecture Guide

This document describes the high-level architecture of the Heart Rate Monitor app.

High-Level Diagram

graph TD
    subgraph UI_Layer
        Compose[Jetpack Compose UI]
        VM[ViewModels]
    end

    subgraph Domain_Core_Layer
        HRE[HeartRate Engine]
        CSI[CameraSupport Interface]
        PS[Processing Engine]
    end

    subgraph Data_Layer
        Repo[HistoryRepository]
        Room[Room Database]
    end

    subgraph Hardware
        Cam[Device Camera]
    end

    %% UI to ViewModel
    Compose --> VM
    
    %% ViewModel to Repository and Engine
    VM --> Repo
    VM --> HRE
    
    %% Engine to Camera and Processing
    HRE --> CSI
    CSI --> PS
    CSI --> Cam
    
    %% Repository to Database
    Repo --> Room
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Layers Description

1. UI Layer

  • Jetpack Compose: Uses a modern declarative approach to build the UI.
  • ViewModels: Use StateFlow to expose state to the UI and viewModelScope for asynchronous operations.
  • Hilt: Handles dependency injection for ViewModels.

2. Domain / Core Layer

  • HeartRate Engine: Manages the lifecycle of a measurement (start, stop, timer). It implements the PreviewListener to receive data from the camera.
  • CameraSupport: A hardware abstraction layer. Currently implemented using CameraX, but can be swapped easily due to its interface-based design.
  • Processing Engine: Specialized logic that converts camera frames into "redness" values used for pulse detection.

3. Data Layer

  • HistoryRepository: The single source of truth for heart rate history data.
  • Room Database: Handles local persistence of history models.

Data Flow (Unidirectional Data Flow)

  1. User Action: User taps "Start" in the UI.
  2. ViewModel: Updates state to isStarted = true and calls HeartRate.startPulseCheck().
  3. Engine: Opens the camera and starts a timer.
  4. Hardware/Core: Camera frames are processed; the engine calculates BPM and emits results via a listener.
  5. ViewModel: Receives BPM updates and pushes them to the UI via beatsPerMinute StateFlow.
  6. Persistence: Upon completion, the user can save the result, which flows through the ViewModel to the Repository and into the Room database.