Add OsVersion (#732)

skiko/src/commonMain/kotlin/org/jetbrains/skiko/OsVersion.kt

+package org.jetbrains.skiko
+
+data class OSVersion(
+    val major: Int,
+    val minor: Int = 0,
+    val patch: Int = 0
+) : Comparable<OSVersion> {
+
+    init {
+        require(major >= 0) { "Major version must be a positive number" }
+        require(minor >= 0) { "Minor version must be a positive number" }
+        require(patch >= 0) { "Patch version must be a positive number" }
+    }
+
+    override fun toString(): String = "$major.$minor.$patch"
+
+    override fun compareTo(other: OSVersion): Int {
+        if (major > other.major) return 1
+        if (major < other.major) return -1
+        if (minor > other.minor) return 1
+        if (minor < other.minor) return -1
+        if (patch > other.patch) return 1
+        if (patch < other.patch) return -1
+
+        return 0
+    }
+}

skiko/src/darwinMain/kotlin/org/jetbrains/skiko/OsVersion.apple.kt

+package org.jetbrains.skiko
+
+import kotlinx.cinterop.useContents
+import platform.Foundation.NSProcessInfo
+
+val hostOSVersion: OSVersion
+    get() = NSProcessInfo.processInfo.operatingSystemVersion.useContents {
+        OSVersion(
+            majorVersion.toInt(),
+            minorVersion.toInt(),
+            patchVersion.toInt()
+        )
+    }
+
+fun available(vararg pairs: Pair<OS, OSVersion>) =
+    pairs.find { it.first == hostOs }?.let { it.second <= hostOSVersion } ?: false

skiko/src/iosMain/kotlin/org/jetbrains/skiko/SystemTheme.ios.kt

 package org.jetbrains.skiko
 
-import kotlinx.cinterop.useContents
-import platform.Foundation.NSProcessInfo
-import platform.UIKit.*
-import platform.UIKit.UIUserInterfaceStyle.*
+import platform.UIKit.UITraitCollection
+import platform.UIKit.UIUserInterfaceStyle.UIUserInterfaceStyleDark
+import platform.UIKit.UIUserInterfaceStyle.UIUserInterfaceStyleLight
+import platform.UIKit.currentTraitCollection
 
 actual val currentSystemTheme: SystemTheme
-    get() = if (supportsCurrentTraitCollectionApi) {
+    get() = if (available(OS.Ios to OSVersion(13))) {
+        /*
+         * Getting trait collection for the current execution context supports only on iOS 13.0+
+         * https://developer.apple.com/documentation/uikit/uitraitcollection/3238080-currenttraitcollection?language=objc
+         */
         when (UITraitCollection.currentTraitCollection.userInterfaceStyle) {
             UIUserInterfaceStyleDark -> SystemTheme.DARK
             UIUserInterfaceStyleLight -> SystemTheme.LIGHT
@@ -15,11 +19,3 @@ actual val currentSystemTheme: SystemTheme
     } else {
         SystemTheme.LIGHT
     }
-
-/*
- * Getting trait collection for the current execution context supports only on iOS 13.0+
- * https://developer.apple.com/documentation/uikit/uitraitcollection/3238080-currenttraitcollection?language=objc
- */
-private val supportsCurrentTraitCollectionApi get() = NSProcessInfo.processInfo.operatingSystemVersion.useContents {
-    majorVersion >= 13
-}