如果你想查看WatchOS。

斯威夫特

let watchOSVersion = WKInterfaceDevice.currentDevice().systemVersion
print("WatchOS version: \(watchOSVersion)")

objective - c

NSString *watchOSVersion = [[WKInterfaceDevice currentDevice] systemVersion];
NSLog(@"WatchOS version: %@", watchOSVersion);

let Device = UIDevice.currentDevice()
let iosVersion = NSString(string: Device.systemVersion).doubleValue

let iOS8 = iosVersion >= 8
let iOS7 = iosVersion >= 7 && iosVersion < 8

检查为

if(iOS8)
{

}
else 
{
}  

注:iOS 8.0及以上版本支持。OS X v10.10及以上版本

var majorVersion: Int    { return NSProcessInfo.processInfo().operatingSystemVersion.majorVersion }
var minorVersion: Int    { return NSProcessInfo.processInfo().operatingSystemVersion.minorVersion }
var patchVersion: Int    { return NSProcessInfo.processInfo().operatingSystemVersion.patchVersion }
var myOSVersion:  String { return NSProcessInfo.processInfo().operatingSystemVersionString        }

在Swift 2及更高版本中，检查系统版本(以及许多其他版本)最简单和最简单的方法是:

if #available(iOS 9.0, *) { // check for iOS 9.0 and later

}

此外，使用#available你可以检查这些版本:

iOS
iOSApplicationExtension
macOS
macOSApplicationExtension
watchOS
watchOSApplicationExtension
tvOS
tvOSApplicationExtension
swift

马特·汤普森分享了一种非常简便的方法

switch UIDevice.currentDevice().systemVersion.compare("8.0.0", options: NSStringCompareOptions.NumericSearch) {
case .OrderedSame, .OrderedDescending:
    println("iOS >= 8.0")
case .OrderedAscending:
    println("iOS < 8.0")
}

let osVersion = NSProcessInfo.processInfo().operatingSystemVersion
let versionString = osVersion.majorVersion.description + "." + osVersion.minorVersion.description + "." + osVersion.patchVersion.description
print(versionString)