Dark mode detector

Compose Desktop's isSystemInDarkTheme() reads the OS theme exactly once. Toggle dark mode in System Settings and your UI freezes in the previous palette until you restart. darkmode-detector replaces it with a reactive listener that triggers recomposition the instant the system theme flips.

TL;DR

• @Composable isSystemInDarkMode() — drop-in replacement, reactive.
• IDarkModeDetector.isDark: StateFlow<Boolean> — coroutine-friendly.
• macOS via Cocoa KVO, Windows via registry watch, Linux via xdg-desktop-portal.
• No polling, no AWT, no JNA.

Install

dependencies {
    implementation("dev.nucleusframework:nucleus.darkmode-detector:<version>")
}

Quickstart

import dev.nucleusframework.darkmodedetector.isSystemInDarkMode
import androidx.compose.material3.MaterialTheme

@Composable
fun App() {
    val isDark = isSystemInDarkMode()
    MaterialTheme(colorScheme = if (isDark) darkColorScheme() else lightColorScheme()) {
        // Your UI recomposes when the OS theme changes.
    }
}

How it works

Each platform plugs into the OS's own change-notification mechanism:

• macOS: Cocoa KVO on NSApp.effectiveAppearance. The bridge wakes on the AppKit notification queue and republishes to a StateFlow on a Nucleus dispatcher.
• Windows: registry watch on HKCU\Software\Microsoft\Windows\CurrentVersion\Themes\Personalize\AppsUseLightTheme.
• Linux: org.freedesktop.appearance portal (color-scheme), with a fallback to GSettings.

isSystemInDarkMode() wires that flow into a Compose State, so any reader recomposes on change.

Reference

Composable

val isDark: Boolean = isSystemInDarkMode()

Imperative / coroutines

val detector = getPlatformDarkModeDetector()
detector.start()

scope.launch {
    detector.isDark.collect { isDark ->
        println("dark mode: $isDark")
    }
}

// later
detector.stop()

Testing / fallbacks

val detector: IDarkModeDetector = NoopDarkModeDetector

Use NoopDarkModeDetector in tests or on platforms where the live detector is not desired.

Notes

• The very first read is synchronous; subsequent changes come through the StateFlow and trigger Compose recomposition.
• On Linux, ensure the portal is available (it is on every modern desktop). Headless / CI environments fall back to "light".