Change Once, See It on Both Apps
Edit shared logic and watch both platforms update.
Change Once, See It on Both Apps is a free Kotlin Multiplatform Academy lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Kotlin Multiplatform Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
The Real Payoff
Now comes the magic of KMP: edit the shared function once and both apps change together. No copy-paste, no drift.
Tweak the Greeting
Open greet in commonMain and change the wording. This is the only place you touch, even though two apps depend on it.
fun greet(name: String): String {
return "Welcome, $name!"
}Rebuild Android
Run the Android app again. Because it links the shared module, it instantly shows the new Welcome text with zero extra edits.
Rebuild iOS
Rebuild the iOS app and the same new line appears. The shared framework was recompiled from the one source you changed.
Add Logic, Not Just Text
Change behavior too, like trimming spaces. Both platforms gain the fix at once, because the logic truly lives in one place.
fun greet(name: String): String {
return "Welcome, " + name.trim() + "!"
}One Bug, One Fix
If greet had a bug, you fix it once and both apps are healed. That is the consistency KMP is built to give you.
Why Builds Are Needed
Each platform compiles the shared code into its own artifact. So after an edit, both apps need a fresh build to pick it up.
Tests Guard the Logic
Write one test for greet in commonTest and it protects every platform. A single safety net covers all your apps.
Less Code to Maintain
Sharing logic shrinks your surface area. Fewer lines, fewer places for bugs, and behavior that can never disagree across apps.
Keep UI Native
The text changed everywhere, but each app still styles it with its own UI. You share the brain and keep platform-native looks. 💡
You Did It End to End
You wrote shared code, called it from both apps, and proved a single edit updates them together. That is the whole KMP loop. 🎉
Quick Check
What happens after you edit shared logic?
Recap
One edit in commonMain flowed to both apps after a rebuild. Share the logic, test it once, and keep each UI native. 🎉
Frequently asked questions
Is the “Change Once, See It on Both Apps” lesson free?
Yes — the full text of “Change Once, See It on Both Apps” is free to read here on the web, and the Kotlin Multiplatform Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Kotlin Multiplatform Academy course, upgrade to CoddyKit PRO.
What will I learn in “Change Once, See It on Both Apps”?
Edit shared logic and watch both platforms update. You practise Kotlin Multiplatform Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Kotlin Multiplatform Academy?
No prior experience is required. Kotlin Multiplatform Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Change Once, See It on Both Apps” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Kotlin Multiplatform Academy lesson?
Yes. Every Kotlin Multiplatform Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- A Greeting Function in commonMain
- Call Shared Code from the Android App
- Call Shared Code from the iOS App
- Change Once, See It on Both Apps