Write shared code first, platform code last

battery.lucent.ts

import { PLATFORM } from "lucent:platform";
import { UIDevice } from "lucent:ios/UIKit";
import { BatteryManager } from "lucent:android/android.os";
import { appContext } from "lucent:android";
import { main } from "lucent:thread";

// Shared: plain Lucent, the same on both platforms.
export function describe(level: number): string {
  if (level < 0) return "unknown";
  return level < 0.2 ? "low" : "ok";
}

// Platform code: only the SDK call differs.
export async function level(): Promise<number> {
  if (PLATFORM === "ios") {
    return main(() => {
      UIDevice.current.isBatteryMonitoringEnabled = true;
      return UIDevice.current.batteryLevel;
    });
  } else {
    const manager = appContext().getSystemService(BatteryManager);
    const percent = manager?.getIntProperty(BatteryManager.BATTERY_PROPERTY_CAPACITY) ?? -1;
    return percent < 0 ? -1 : percent / 100;
  }
}

Put as much as possible in shared code: it is one piece of logic, tested once. Keep platform code to the SDK calls, and turn their results into plain values as soon as you can.

Where each piece goes

• No SDK call: shared code, in any module. It also runs in tests on your computer.
• A few SDK calls in a function: a platform branch, if (PLATFORM === "ios") { … } else { … }, in the same module.
• Helpers, delegates or state that use one SDK: top-level declarations of the same module. They belong to that platform, and compile only there.
• Two halves that share nothing: platform files, x.ios.lucent.ts and x.android.lucent.ts, behind a declaration file x.lucent.ts. This is the rare case.

Caution

A branch needs its else. After if (PLATFORM === "ios") return …;, the following code is still shared, so an Android call there fails with LUCENT3004.

Both branches are type-checked on every build. A platform whose SDK isn't installed is untyped, and its code is skipped.