BMI Calculator – Kotlin Multiplatform (KMP)

A simple BMI (Body Mass Index) Calculator built using Kotlin Multiplatform (KMP) with:

• Jetpack Compose for Android
• SwiftUI for iOS
• Shared MVVM architecture using StateFlow

This project demonstrates how to build a real-world cross‑platform app with clean architecture and proper state management.

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📱 Features

• Input weight (kg) and height (cm)

• BMI calculation using standard formula

• Validation for empty and invalid inputs

• BMI category detection:

  • Underweight
  • Normal
  • Overweight
  • Obese

• Color‑coded BMI result

• Shared business logic between Android & iOS

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🏗 Architecture Overview

This app follows MVVM with Unidirectional Data Flow.

UI (Compose / SwiftUI)
        ↓ Intent
Shared ViewModel (KMP)
        ↓
StateFlow<BmiUiState>
        ↓
UI observes & renders

Why this architecture?

• Immutable state
• Predictable UI behavior
• Easy testing
• Platform‑independent business logic

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🧩 Modules

BMI/
 ├── androidApp/        → Android app (Jetpack Compose)
 ├── iosApp/            → iOS app (SwiftUI)
 └── shared/            → Shared KMP module

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🔁 Shared Module (KMP)

Key Components

• BmiViewModel – handles logic & state
• BmiUiState – immutable UI state
• StateFlow – reactive state updates

Example State

data class BmiUiState(
    val weight: String = "",
    val height: String = "",
    val bmiResult: BmiResult? = null,
    val error: String? = null
)

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🤖 Android (Jetpack Compose)

• Observes StateFlow using collectAsState()
• Fully declarative UI
• Color‑coded BMI output

val state by viewModel.uiState.collectAsState()

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🍎 iOS (SwiftUI)

Important Concepts Used

• ObservableObject for ViewModel wrapper
• @Published for UI updates
• @MainActor for thread safety
• Manual Binding to avoid mutating Kotlin state directly

@Published var state: BmiUiState?

State updates are dispatched via ViewModel functions, not direct mutation.

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🔐 Validation Rules

• Weight must be > 0
• Height must be > 0
• Non‑numeric input is rejected
• Error messages shown in UI

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🎨 BMI Color Coding

Category	Color
Underweight	Blue
Normal	Green
Overweight	Orange
Obese	Red

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🧪 Formula Used

BMI = weight (kg) / (height (m) * height (m))

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📦 Dependencies

Shared Module

• Kotlin Coroutines
• StateFlow

Android

• Jetpack Compose

iOS

• SwiftUI

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🚀 How to Run

Android

1. Open project in Android Studio
2. Select androidApp
3. Run on emulator or device

iOS

1. Open iosApp.xcodeproj in Xcode
2. Select simulator
3. Run

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🎯 Learning Outcomes

This project helps you understand:

• Kotlin Multiplatform basics
• Sharing ViewModels across platforms
• StateFlow ↔ SwiftUI integration
• Immutable state handling
• Clean MVVM architecture

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🔮 Possible Enhancements

• Unit conversion (kg/lbs, cm/ft)
• Save BMI history
• Charts & analytics
• Dependency Injection (Koin)
• Unit tests

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📄 License

MIT License

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👨‍💻 Author

Manabendu

Built as a learning‑oriented KMP sample project 🚀