Modern application development often requires businesses to build applications for multiple platforms such as Android, iOS, web, and desktop.
Traditionally, developers build each platform separately. Android applications may use Kotlin, while iOS applications are commonly built with Swift. This can result in duplicated business logic, repeated development work, and additional maintenance.
Kotlin Multiplatform (KMP) provides another approach.
Kotlin Multiplatform is a technology that allows developers to share Kotlin code across multiple platforms while still building applications that can use each platform’s native capabilities.
Instead of forcing an entire application to use one shared UI or framework, KMP allows developers to decide what code should be shared and what should remain platform-specific.
In this complete beginner’s guide, you’ll learn:
- What Kotlin Multiplatform is
- How KMP works
- Why Kotlin Multiplatform is useful
- KMP vs traditional native development
- KMP vs Flutter
- Shared code and platform-specific code
commonMainand platform source setsexpectandactual- Kotlin Multiplatform project structure
- Sharing business logic
- Networking with KMP
- Database support
- KMP and Jetpack Compose
- KMP for Android and iOS
- Advantages and limitations
- Common use cases
- KMP learning roadmap
- Frequently asked questions
What Is Kotlin Multiplatform?
Kotlin Multiplatform (KMP) is a Kotlin technology that allows developers to share code across different platforms.
The primary goal is to reduce duplicated code while preserving access to native platform functionality.
For example, suppose you have an application that needs:
- Authentication
- API integration
- Data models
- Business rules
- Local database
- Validation
- Networking
Instead of implementing all of this separately for Android and iOS, you can potentially share much of the common logic.
Conceptually:
Shared Kotlin Code
│
┌────────────┼────────────┐
↓ ↓ ↓
Android iOS Desktop
The important point is that KMP does not necessarily mean sharing everything.
You can share as much or as little code as your project requires.
Why Was Kotlin Multiplatform Created?
Imagine you are developing an application for both Android and iOS.
Without code sharing, you might have:
Android
↓
Kotlin
↓
Business Logic
iOS
↓
Swift
↓
Business Logic
The same business rules may need to be implemented twice.
For example:
Login validation
API models
Authentication
Pagination
Date formatting
Data caching
Business rules
With KMP, the architecture can look like:
Shared Kotlin
│
┌──────────┴──────────┐
↓ ↓
Android UI iOS UI
↓ ↓
Android APIs iOS APIs
This allows developers to share common logic while keeping platform-specific functionality where necessary.
How Does Kotlin Multiplatform Work?
KMP organizes code into shared and platform-specific source sets.
A simplified project structure looks like:
composeApp/
│
├── src/
│ ├── commonMain/
│ │ └── kotlin/
│ │
│ ├── androidMain/
│ │ └── kotlin/
│ │
│ └── iosMain/
│ └── kotlin/
The idea is:
commonMain
↓
Shared code
androidMain
↓
Android-specific code
iosMain
↓
iOS-specific code
What Is commonMain?
commonMain contains code that can be shared between supported platforms.
For example:
class UserValidator {
fun isValidEmail(email: String): Boolean {
return email.contains("@")
}
}
This logic doesn’t need to know whether it is running on Android or iOS.
Therefore, it can live in shared code.
Common code can contain things such as:
- Data models
- Business logic
- API clients
- Repositories
- Validation
- State management
- Serialization
- Shared utilities
What Is androidMain?
androidMain contains Android-specific code.
For example, if you need an Android API that isn’t available directly from common code, you can implement it here.
androidMain
↓
Android-specific implementation
This allows you to use Android APIs when necessary.
What Is iosMain?
iosMain contains iOS-specific Kotlin code.
It can be used for functionality that needs to interact with iOS-specific APIs.
Conceptually:
iosMain
↓
iOS-specific implementation
This allows KMP applications to interact with platform-specific functionality instead of trying to abstract everything.
Shared Code vs Platform-Specific Code
One of the biggest advantages of KMP is that you control what gets shared.
For example:
Shared
├── Models
├── API
├── Repository
├── Business Logic
├── Validation
└── Data Layer
Android
├── Android Permissions
├── Android-specific APIs
└── Android UI
iOS
├── iOS Permissions
├── iOS-specific APIs
└── iOS UI
This approach gives developers flexibility.
What Is expect and actual?
Kotlin Multiplatform provides the expect and actual mechanism for platform-specific implementations.
For example:
expect fun getPlatformName(): String
This declares that each supported platform needs to provide an implementation.
Android:
actual fun getPlatformName(): String {
return "Android"
}
iOS:
actual fun getPlatformName(): String {
return "iOS"
}
The shared code can call:
val platform = getPlatformName()
The platform-specific implementation is selected for the target platform.
Why Is expect / actual Useful?
Suppose your shared business logic needs a platform-specific capability.
For example:
Shared code
↓
Need device information
↓
Android implementation
↓
iOS implementation
Instead of putting platform-specific APIs directly into common code, you can define a common contract and provide platform implementations.
Kotlin Multiplatform Project Structure
A typical KMP project can look like:
MyApp/
│
├── shared/
│ ├── src/
│ │ ├── commonMain/
│ │ ├── androidMain/
│ │ └── iosMain/
│
├── androidApp/
│
└── iosApp/
The exact structure depends on the project template and architecture.
The important concept is the separation between:
Shared Kotlin code
+
Platform-specific code
+
Platform UI
Can KMP Share UI?
This is an important question.
Yes, Kotlin Multiplatform can be used to share UI as well as non-UI logic, depending on the technologies and architecture you choose.
A popular option is Compose Multiplatform, which extends the declarative Compose approach beyond Android.
This means a project can potentially share UI code across platforms while still allowing native platform integration where needed.
However, KMP itself does not require shared UI.
You can also choose:
Shared business logic
↓
Android UI → Jetpack Compose
iOS UI → SwiftUI
This is one of the major differences between KMP and frameworks that primarily focus on cross-platform UI.
KMP With Jetpack Compose
If you already know Jetpack Compose, KMP can be particularly interesting.
A project may share Compose-based UI across supported targets using Compose Multiplatform.
Conceptually:
Shared Compose UI
│
┌──────────┴──────────┐
↓ ↓
Android iOS
This can reduce duplicated UI code.
However, you should still understand platform-specific UI and APIs because not every application requirement should necessarily be abstracted.
KMP vs Native Android and iOS Development
Let’s compare the approaches.
| Feature | Native Development | Kotlin Multiplatform |
|---|---|---|
| Android | Kotlin | Kotlin |
| iOS | Swift | Kotlin + iOS integration |
| Shared Logic | Usually limited | High potential |
| Shared UI | Separate | Optional |
| Native APIs | Excellent | Available through platform code |
| Code Duplication | Higher | Can be reduced |
| Platform Control | Very high | Very high |
| Learning | Platform-specific | Kotlin + multiplatform concepts |
KMP is not intended to completely replace native development.
Instead, it provides a way to share code where sharing makes sense.
KMP vs Flutter
Developers often compare Kotlin Multiplatform with Flutter because both can be used for cross-platform application development.
However, their approaches are different.
| Feature | Kotlin Multiplatform | Flutter |
|---|---|---|
| Main Language | Kotlin | Dart |
| Android Integration | Native Kotlin ecosystem | Platform integration available |
| iOS Integration | Native interoperability | Platform integration available |
| Shared Logic | Yes | Yes |
| Shared UI | Optional with Compose Multiplatform | Core approach |
| Native UI Option | Yes | Different rendering approach |
| Existing Kotlin Code | Can be shared | Usually requires adaptation |
| Android Ecosystem | Very strong | Strong |
| Cross-platform UI | Optional | Core feature |
The key difference is architectural philosophy.
KMP lets you decide what to share.
Flutter generally encourages building the application’s UI using Flutter widgets.
When Should You Use KMP?
KMP can be a strong option when you have:
- Android and iOS applications
- Significant shared business logic
- Kotlin expertise
- A requirement for native platform integration
- Existing Kotlin business logic
- APIs and repositories that can be shared
- A preference for native platform capabilities
For example:
E-commerce App
↓
Shared
├── API
├── Authentication
├── Models
├── Cart Logic
├── Validation
└── Repository
Android
└── Android UI
iOS
└── iOS UI
When Might KMP Not Be the Best Choice?
KMP isn’t automatically the right choice for every project.
You may prefer another technology if:
- Your team has no Kotlin experience
- You want a single UI framework and workflow
- Your application has little shared logic
- Your team is already heavily invested in another cross-platform technology
- Platform-specific requirements dominate the application
Technology selection should be based on the project’s requirements rather than simply choosing the newest framework.
Sharing Business Logic With KMP
One of the strongest KMP use cases is business logic.
Suppose you have a shopping cart.
The rules might be:
Add product
Remove product
Calculate subtotal
Apply discount
Calculate tax
Calculate total
Instead of implementing those rules twice:
Android → Kotlin
iOS → Swift
you can potentially implement the logic once:
Shared Kotlin
↓
Android + iOS
This can reduce duplication and help keep behavior consistent across platforms.
Sharing Data Models
Data models are another good candidate for shared code.
Example:
data class Product(
val id: Int,
val name: String,
val price: Double
)
The shared model can be used by multiple platform applications.
This can be particularly useful when the same API serves Android and iOS.
KMP for API Integration
KMP can share networking logic.
For example, your shared module might contain:
API Service
↓
Repository
↓
Data Models
Both Android and iOS applications can consume that shared data layer.
A simplified architecture:
Backend API
↑
↓
Shared Networking
↓
Repository
↙ ↘
Android iOS
↓ ↓
UI UI
This means API integration doesn’t necessarily need to be rewritten separately for each platform.
KMP With Coroutines
Kotlin Coroutines are highly useful in KMP applications.
They can be used for asynchronous tasks such as:
- Network requests
- Database operations
- Data processing
- Background work
Example:
class UserRepository(
private val api: UserApi
) {
suspend fun getUsers(): List<User> {
return api.getUsers()
}
}
The repository can be placed in shared code when its dependencies are multiplatform-compatible.
KMP and Kotlin Flow
Kotlin Flow can be used to represent streams of data.
For example:
val users: Flow<List<User>>
The shared layer can expose state or data streams while each platform observes them using an appropriate platform integration approach.
This is useful for:
- User state
- Authentication state
- Database updates
- Network state
- Application settings
KMP and Database
Shared persistence is another potential use case.
A KMP project can use a multiplatform-compatible database solution so that data access logic can be shared.
For example:
Shared Repository
↓
Shared Database Layer
↓
Android Database
iOS Database
The exact implementation depends on the database technology and project requirements.
KMP Architecture Example
A scalable KMP application might use:
UI
┌───────┴───────┐
↓ ↓
Android UI iOS UI
│ │
└───────┬───────┘
↓
Shared KMP
│
┌──────────┼──────────┐
↓ ↓ ↓
Repository Domain Models
│ │
└──────┬───┘
↓
Shared Data Layer
↓
Network / DB
This separation can make the application easier to maintain.
Advantages of Kotlin Multiplatform
1. Code Sharing
The biggest benefit is reducing duplicated code.
2. Native Platform Access
KMP allows developers to use platform-specific functionality when necessary.
3. Kotlin Ecosystem
Developers can use Kotlin language features and libraries that support multiplatform development.
4. Flexible Architecture
You decide what should be shared.
5. Reduced Maintenance
When common business logic changes, it can potentially be updated in one shared codebase.
6. Consistent Business Logic
Sharing important rules can help prevent differences between Android and iOS implementations.
7. Gradual Adoption
KMP can be introduced incrementally rather than requiring an entire application to be rewritten immediately.
Disadvantages of Kotlin Multiplatform
1. Learning Curve
Developers need to understand:
- Kotlin Multiplatform
- Source sets
- Platform targets
- Native interoperability
- Gradle configuration
- Shared architecture
2. Platform Differences
Android and iOS have different APIs and development models.
Not everything should or can be shared equally.
3. Build Configuration
Multiplatform projects can have more complex build configurations than a single-platform application.
4. Library Compatibility
Not every Kotlin or Android library can be used directly in common multiplatform code.
You need to choose libraries that support your target platforms.
5. iOS Knowledge May Still Be Required
If you are building for iOS, understanding Swift, SwiftUI, Xcode, and Apple’s platform concepts can still be valuable, particularly when using native iOS UI or APIs.
Common KMP Use Cases
Kotlin Multiplatform can be used for many types of applications.
E-commerce Applications
Share:
- Product models
- Cart logic
- API layer
- Authentication
- Pricing logic
Banking Applications
Share:
- Validation
- Business rules
- API models
- Networking
- Security-related application logic
Social Applications
Share:
- User models
- API integration
- Feed logic
- Pagination
- Networking
Enterprise Applications
Share:
- Business rules
- Data models
- API clients
- Repository layer
- Authentication
Productivity Applications
Share:
- Task management
- Local storage logic
- Synchronization
- Business rules
KMP for Android and iOS
One of the most common KMP scenarios is Android + iOS.
A possible architecture:
Shared KMP
/ | \
/ | \
Models API Logic
│ │ │
└─────────┼─────────┘
↓
Android + iOS
You can choose to share only the data layer or go further and share UI as well.
Is KMP the Same as Write Once, Run Everywhere?
Not exactly.
KMP is better understood as:
Share code where it makes sense while preserving native platform capabilities.
The goal isn’t necessarily to write 100% of the application once.
For example:
Shared:
70% of business/data logic
Android:
Android-specific functionality
iOS:
iOS-specific functionality
The exact percentage varies greatly from project to project.
Does KMP Compile to Native Code?
Kotlin Multiplatform uses Kotlin’s platform-specific compilation mechanisms.
For Android, shared Kotlin code can be compiled for the Android environment.
For Apple platforms, Kotlin/Native can compile Kotlin code into native binaries that integrate with iOS applications.
The important idea for beginners is that KMP is not simply running Android code inside an iOS emulator.
It provides a multiplatform compilation and interoperability approach.
How to Start Learning Kotlin Multiplatform
If you already know Kotlin, you have a strong starting point.
Step 1: Learn Kotlin
Understand:
- Variables
- Functions
- Classes
- Interfaces
- Generics
- Collections
- Coroutines
- Flow
Step 2: Learn Android Development
Understand:
- Android architecture
- ViewModel
- Repository pattern
- Jetpack Compose
- Navigation
- Networking
Step 3: Learn Multiplatform Concepts
Study:
commonMainandroidMainiosMain- Source sets
- Platform-specific implementations
expect/actual
Step 4: Learn Multiplatform Networking
Practice creating a shared API layer.
Step 5: Learn Shared State and Architecture
Explore:
- StateFlow
- Coroutines
- Repository
- Dependency injection
- UI state
Step 6: Learn Compose Multiplatform
If you want to explore shared UI, learn how Compose Multiplatform works.
Step 7: Build a Real Project
Start with something small:
KMP Notes App
↓
Shared
├── Models
├── Repository
├── Database
└── Business Logic
Android → UI
iOS → UI
Then move to larger projects.
KMP Project Ideas for Beginners
Here are some good projects to practice with:
1. Notes App
Practice:
- Shared models
- Database
- CRUD
- State
2. Weather App
Practice:
- API integration
- Networking
- JSON
- State management
3. Expense Tracker
Practice:
- Shared business logic
- Local database
- Data visualization
4. E-commerce App
Practice:
- Products
- Cart
- Authentication
- API
- Shared business rules
5. Task Management App
Practice:
- CRUD
- Authentication
- Database
- Synchronization
KMP Best Practices
Share Logic, Not Everything
Don’t force platform-specific functionality into shared code.
Share functionality that is genuinely common.
Keep Architecture Clean
Use clear boundaries:
UI
↓
ViewModel / Presentation
↓
Domain
↓
Repository
↓
Data
Minimize Platform Dependencies
Keep commonMain as platform-independent as practical.
Use Interfaces for Abstraction
Define contracts in shared code and provide platform implementations when necessary.
Keep Platform Code Isolated
Avoid spreading Android-specific or iOS-specific code throughout your shared business logic.
Test Shared Logic
One major advantage of shared business logic is that it can be tested centrally.
Frequently Asked Questions
What is Kotlin Multiplatform?
Kotlin Multiplatform is a technology that allows developers to share Kotlin code across multiple platforms such as Android and iOS while retaining access to platform-specific functionality.
Is Kotlin Multiplatform a framework?
KMP is a multiplatform development technology and set of Kotlin capabilities rather than simply a UI framework.
Is Kotlin Multiplatform the same as Flutter?
No. Flutter uses Dart and primarily provides a shared UI framework. KMP focuses on sharing Kotlin code while allowing developers to retain native platform capabilities. Shared UI is optional through technologies such as Compose Multiplatform.
Can Kotlin Multiplatform build iOS apps?
Yes. Kotlin Multiplatform can be used to share code with iOS applications, while developers can use native iOS technologies where appropriate.
Can KMP share UI?
Yes. With Compose Multiplatform, UI can also be shared across supported platforms. However, KMP can also be used purely for shared business and data logic.
Do I need Swift to learn KMP?
You can learn the shared Kotlin portion without being an expert in Swift. However, Swift and iOS knowledge are valuable when developing, integrating, and maintaining iOS applications.
Can KMP share API code?
Yes. Multiplatform-compatible networking libraries can be used to create shared API and repository layers.
Can KMP use Kotlin Coroutines?
Yes. Kotlin Coroutines are commonly used in multiplatform projects for asynchronous programming.
Is KMP good for Android developers?
Yes. Android developers who already know Kotlin can use their existing Kotlin knowledge as a strong foundation for learning multiplatform development.
Is Kotlin Multiplatform difficult?
The basic concept is straightforward, but multiplatform architecture, Gradle configuration, platform interoperability, and iOS integration require additional learning.
Conclusion
Kotlin Multiplatform provides a flexible approach to cross-platform application development.
Instead of forcing developers to completely abandon native platform development, KMP allows them to share Kotlin code where it provides value while retaining platform-specific implementations where necessary.
A simplified architecture looks like:
Shared Kotlin
│
┌─────────────┴─────────────┐
↓ ↓
Android iOS
↓ ↓
Android UI iOS UI
You can share:
- Business logic
- API integration
- Data models
- Repositories
- Validation
- Database logic
- State and other common functionality
And keep platform-specific features separate.
For developers already familiar with Kotlin, Android development, Jetpack Compose, Retrofit, Coroutines, ViewModel, and Flow, Kotlin Multiplatform is a natural next technology to explore.
The most important thing to remember is that KMP isn’t about sharing everything. It is about sharing the right code while keeping the advantages of native development.
