Swift System Version Checking

Written by Mattt Thompson

While it’s not accurate to say that Swift is “Objective-C without the C”, it’s for lack of resemblance to Objective-C, not the absence of C. Swift is vehemently not C.

Swift certainly draws inspiration from Haskell, Rust, Python, D, and other modern languages, but one can perhaps best understand the language as a rejection of everything that’s broken in C:

  • C is unsafe by default. Swift is safe by default (hence the unsafe naming of pointer manipulation functions).
  • C has undefined behavior. Swift has well-defined behavior (or at least theoretically; the compiler tools still have some catching up to do).
  • C uses preprocessor directives capable of unspeakable evil. Swift has a safe subset of preprocessor directives.

One could go as far to say that Swift’s type system was specifically designed out of spite for C++.

In Objective-C, checking for the availability of an API was accomplished through a combination of C preprocessor directives, conditionals on class, respondsToSelector:, and instancesRespondToSelector::

#if defined(__IPHONE_OS_VERSION_MAX_ALLOWED) && __IPHONE_OS_VERSION_MAX_ALLOWED >= 70000
if ([NSURLSession class] &&
    [NSURLSessionConfiguration respondsToSelector:@selector(backgroundSessionConfigurationWithIdentifier:)]) {
    // ...
}
#endif

However, as noted previously, Swift’s compiler directives are extremely constrained, allowing only for compiler flags and conditional compilation against specific operating systems and architectures:

#if DEBUG
     println("OTHER_SWIFT_FLAGS = -D DEBUG")
#endif
Function Valid Arguments
os() OSX, iOS
arch() x86_64, arm, arm64, i386
#if os(iOS)
    var image: UIImage?
#elseif os(OSX)
    var image: NSImage?
#endif

Unfortunately, os() does not offer any insight into the specific version of OS X or iOS, which means that checks must be made at runtime. And with Swift’s less-forgiving treatment of nil, checking for constants Objective-C-style results in a crash.

So how do you check the system version in Swift to determine API availability? Read on to find out.


NSProcessInfo

Anticipating the need for a Swift-friendly API for determining API version at runtime, iOS 8 introduces the operatingSystemVersion property and isOperatingSystemAtLeastVersion method on NSProcessInfo. Both APIs use a new NSOperatingSystemVersion value type, which contains the majorVersion, minorVersion, and patchVersion.

Apple software releases follow semantic versioning conventions.

isOperatingSystemAtLeastVersion

For a simple check, like “is this app running on iOS 9?”, isOperatingSystemAtLeastVersion is the most straightforward approach.

if NSProcessInfo().isOperatingSystemAtLeastVersion(NSOperatingSystemVersion(majorVersion: 9, minorVersion: 0, patchVersion: 0)) {
    println("iOS >= 9.0.0")
}

operatingSystemVersion

For more involved version comparison, the operatingSystemVersion can be inspected directly. Combine this with Swift pattern matching and switch statements for syntactic concision:

let os = NSProcessInfo().operatingSystemVersion
switch (os.majorVersion, os.minorVersion, os.patchVersion) {
case (8, 0, _):
    println("iOS >= 8.0.0, < 8.1.0")
case (8, _, _):
    println("iOS >= 8.1.0, < 9.0")
case (9, _, _):
    println("iOS >= 9.0.0")
default:
    // this code will have already crashed on iOS 7, so >= iOS 10.0
    println("iOS >= 10.0.0")
}

UIDevice systemVersion

Ironically, the new NSProcessInfo APIs aren’t especially useful at the time of writing, since they’re unavailable for iOS 7.

As an alternative, one can use the systemVersion property UIDevice:

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

Use NSStringCompareOptions.NumericSearch when comparing version number strings to ensure that, for example, "2.5" < "2.10".

String comparison and NSComparisonResult aren’t as sexy as a dedicated value type like NSOperatingSystemVersion, but it gets the job done all the same.

NSAppKitVersionNumber

Another approach to determining API availability is to check framework version numbers. Unfortunately, Foundation’s NSFoundationVersionNumber and Core Foundation’s kCFCoreFoundationVersionNumber have historically been out of date, missing constants for past OS releases.

This is a dead-end for iOS, but OS X can pretty reliably check against the version of AppKit, with NSAppKitVersionNumber:

if rint(NSAppKitVersionNumber) > NSAppKitVersionNumber10_9 {
    println("OS X >= 10.10")
}

Apple uses rint in sample code to round off version numbers for NSAppKitVersionNumber comparison.


To summarize, here’s what you need to know about checking the system version in Swift:

  • Use #if os(iOS) preprocessor directives to distinguish between iOS (UIKit) and OS X (AppKit) targets.
  • For minimum deployment targets of iOS 8.0 or above, use NSProcessInfo operatingSystemVersion or isOperatingSystemAtLeastVersion.
  • For minimum deployment targets of iOS 7.1 or below, use compare with NSStringCompareOptions.NumericSearch on UIDevice systemVersion.
  • For OS X deployment targets, compare NSAppKitVersionNumber against available AppKit constants.
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Questions? Corrections? Issues and pull requests are always welcome — NSHipster is made better by readers like you.

This article uses Swift version 1.0. Find status information for all articles on the status page.

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