Here is a high-quality blog post written for My Core Pick.
Write Once, Ship Faster: Unifying iOS and Android Logic with Kotlin Multiplatform
I remember the day I realized something was fundamentally broken in our mobile development process.
We had a bug in our checkout flow.
The Android team fixed it on Tuesday.
The iOS team fixed the exact same bug on Thursday.
We were paying two different teams to write the same logic, verify the same rules, and parse the same JSON responses.
It felt inefficient because it was.
For years, the industry solution to this was "Cross-Platform" frameworks like Flutter or React Native.
But those often meant sacrificing the native user interface (UI) or dealing with performance bridges that slowed things down.
Then came Kotlin Multiplatform (KMP).
It promised something different: share the logic, keep the UI native.
Today, Iβm going to walk you through why KMP is my core pick for modern mobile architecture, and how it allows you to write code once and ship faster than ever.
The Problem with "Write Once, Run Everywhere"
For a long time, the holy grail of mobile development was a single codebase that runs everywhere.
Frameworks like React Native or Flutter try to solve this by rendering their own UI.
They basically say, "Ignore the native iOS and Android buttons; use our buttons instead."
This works great for many apps.
But sometimes, it feels slightly off.
The animations might not be quite right, or a new iOS feature comes out that the framework doesn't support yet.
I have always been a purist when it comes to User Experience (UX).
I want my iOS app to feel 100% like an iOS app, using SwiftUI or UIKit.
I want my Android app to feel 100% like an Android app, using Jetpack Compose.
However, I don't want to write the data validation logic twice.
I donβt want to write the networking layer twice.
This is where Kotlin Multiplatform changes the game.
It doesnβt try to be a "Write Once, Run Everywhere" solution.
It is a "Write Logic Once, Run Everywhere" solution.
How KMP Actually Works (Under the Hood)
If you are new to KMP, the architecture can seem a little magical.
But it is actually grounded in very standard programming concepts.
In a KMP project, you have a Shared Module.
This module contains pure Kotlin code.
This is where you put your "business logic."
What is Business Logic?
When I say business logic, I mean the brains of your application.
This includes making API calls to your backend.
It includes caching data in a local database.
It involves validating user input (like checking if an email address is valid).
It includes parsing JSON data into objects your app can use.
The Compiler Magic
When you compile your project, KMP does something brilliant.
For the Android app, it compiles the Kotlin code into Java Bytecode, just like normal.
But for the iOS app, it compiles that same Kotlin code into an Apple framework (using LLVM).
To Xcode, your shared Kotlin code just looks like an Objective-C or Swift framework.
You import it, and you call the functions.
The iOS developer doesn't even need to know how to write Kotlin to use it.
They just see a function called fetchUser() and they call it.
The Secret Weapon: Expect and Actual
One of the most powerful features of KMP is the expect and actual mechanism.
Sometimes, you need to access platform-specific APIs in your shared code.
For example, maybe you need to generate a UUID.
Android handles this differently than iOS.
In your shared code, you can define an expect function.
It looks like this: expect fun randomUUID(): String
This is basically saying, "I expect there to be a function that returns a string, but I don't know how it works yet."
Then, inside the Android source set, you write the actual implementation using Java libraries.
Inside the iOS source set, you write the actual implementation using Apple's Foundation libraries.
This allows you to share high-level logic while still dipping into native capabilities whenever you need to.
It removes the "black box" limitation that other cross-platform frameworks often have.
My Core Pick: The KMP Tech Stack
If you decide to jump into KMP, you need the right libraries.
You can't just use Retrofit or Room, because those are Android-specific Java libraries.
You need multiplatform-compatible libraries.
Here is "My Core Pick" stack for a robust KMP application.
Networking: Ktor
Ktor is an asynchronous framework for creating microservices and web applications, but it creates amazing HTTP clients.
It is built by JetBrains (the creators of Kotlin) and it is fully multiplatform.
It handles headers, authentication, and serialization seamlessly across iOS and Android.
Database: SQLDelight
This is my favorite database tool, period.
SQLDelight generates typesafe Kotlin APIs from your SQL statements.
It verifies your schema during compilation.
If you make a typo in your SQL query, the code won't compile.
It runs on SQLite on both Android and iOS, making your data layer absolutely rock solid.
Dependency Injection: Koin
Dependency injection can be tricky in a multiplatform environment.
Koin is a lightweight injection framework that plays very nicely with KMP.
It uses a specific DSL (Domain Specific Language) that is easy to read.
It allows you to inject your database and network clients directly into your shared ViewModels.
Concurrency: Kotlin Coroutines
Dealing with threading on iOS and Android used to be a nightmare of callbacks and delegates.
Kotlin Coroutines allow you to write asynchronous code that looks like synchronous code.
You can run complex background operations on a background thread and return the result to the main thread without blocking the UI.
Crucially, this interoperates with Swift's async/await pattern, making it natural for iOS devs to consume.
The Real-World Benefits for Your Team
Why should you actually advocate for this at your company?
It comes down to three things: Speed, Consistency, and Sanity.
1. The Speed of Iteration
When you add a new feature, you write the logic once.
You write the unit tests once.
You only have to build the UI twice.
This creates a significant reduction in development time, usually around 30-40% once the team is up to speed.
2. Consistency is King
I mentioned the bug fixing earlier.
With KMP, if you fix a logic bug in the shared module, you fix it for both platforms instantly.
You guarantee that the iOS app and the Android app behave exactly the same way.
They will calculate prices the same way.
They will sort lists the same way.
This reduces the cognitive load on your QA team massively.
3. Developer Sanity
Android developers already love Kotlin.
It is a modern, expressive, safe language.
iOS developers, admittedly, are usually hesitant to leave Swift.
But because KMP leaves the UI to SwiftUI, iOS devs don't feel like they are losing control.
They still get to build beautiful, native interfaces.
They just don't have to write the boring boilerplate code for parsing JSON anymore.
Getting Started: A Strategy for Adoption
So, how do you start?
Do not rewrite your entire app tomorrow.
That is a recipe for disaster.
KMP is designed to be adopted incrementally.
Start with a small, isolated module.
Maybe it is your analytics logic.
Maybe it is a simple feature like a "News Feed."
Step 1: The Setup
Use the Kotlin Multiplatform Wizard provided by JetBrains.
It generates the folder structure and Gradle configuration for you.
It saves hours of configuration headaches.
Step 2: Move the Models
Start by moving your Data Models (your basic classes) into the shared module.
This ensures both apps are speaking the same language.
Step 3: Move the Networking
Once your models are shared, move the API calls.
Replace Retrofit (Android) and Alamofire (iOS) with Ktor in the shared module.
Pass the data back to the native UI layers.
Final Thoughts
The mobile development landscape is shifting.
We are moving away from the siloed approach where iOS and Android are completely different worlds.
We are also moving past the "compromise" of web-based cross-platform tools.
Kotlin Multiplatform hits the sweet spot.
It respects the platform differences where it matters (the UI).
It unifies the similarities where it counts (the Logic).
If you want to ship faster, reduce bugs, and keep your native look and feel, KMP is the way forward.
It is definitely a Core Pick in my developer toolkit.
Give it a try on your next feature, and I promise, you won't want to go back to writing the same code twice.