为什么存在 expect/actual

Most of your Kotlin runs everywhere, but some tasks need each platform's own tools. That gap is exactly what expect/actual was built to close.

Getting the OS version, a file path, or current time relies on Android or iOS APIs that simply do not exist in shared code. You still want to call them from commonMain. 🤔

Think of expect as a promise: common code declares an API exists. Then each platform fulfills that promise with a real implementation.

In commonMain you write a declaration with no body. The expect keyword means the implementation lives elsewhere, per platform.

Each target supplies the body with actual. Android returns one value, iOS returns another, but the signature must match exactly.

Your shared logic just calls the function and never sees the difference. The compiler picks the right actual for each build.

An interface needs an object to implement it at runtime. expect/actual is resolved at compile time, so there is zero runtime cost.

The expect goes in commonMain. Every actual lives in its matching source set, like androidMain or iosMain, side by side.

If you ship to Android and iOS, both need an actual. A missing one is a compile error, not a surprise crash later.

Reach for expect/actual only for the thin layer that truly differs. The more code stays in common, the more you share. ✨

expect/actual is a clean bridge to native power without splitting your project. You keep one API while each platform speaks its own language.

You learned why expect/actual exists: it lets common code call platform-specific behavior through one API, resolved at compile time with no runtime cost. 🎉