Swift System Version Checking

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.
• 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, architectures, and language versions:

#if DEBUG
     println("OTHER_SWIFT_FLAGS = -D DEBUG")
#endif

Function	Valid Arguments
os()	macOS, iOS, watchOS, tvOS, Linux
arch()	x86_64, arm, arm64, i386
swift()	>= followed by a version number

#if os(iOS)
    var image: UIImage?
#elseif os(macOS)
    var image: NSImage?
#endif

Unfortunately, os() doesn’t offer any insight into the specific version of macOS 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.

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ProcessInfo

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

Apple software releases follow semantic versioning conventions.

isOperatingSystemAtLeast

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

if ProcessInfo().isOperatingSystemAtLeast(OperatingSystemVersion(majorVersion: 9, minorVersion: 0, patchVersion: 0)) {
    print("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 = ProcessInfo().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

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

switch UIDevice.current.systemVersion.compare("8.0.0", options: .numeric) {
case .orderedSame, .orderedDescending:
    print("iOS >= 8")
case .orderedAscending:
    print("iOS < 8.0")
}

Use String.CompareOptions.numeric when comparing version number strings to ensure that, for example, "2.5" < "2.10".

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

NSAppKitVersion

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 macOS can pretty reliably check against the version of AppKit, with NSAppKitVersion:

if NSAppKitVersion.current.rawValue >= .macOS10_10.rawValue {
    println("macOS >= 10.10")
}

If you pair this with an extension to make NSAppKitVersion conform to Comparable, you can remove the .rawValues as well.

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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 macOS (AppKit) targets.
• For minimum deployment targets of iOS 8.0 or above, use ProcessInfo operatingSystemVersion or isOperatingSystemAtLeast.
• For minimum deployment targets of iOS 7.1 or below, use compare with String.CompareOptions.numeric on UIDevice systemVersion.
• For macOS deployment targets, compare NSAppKitVersion against available AppKit constants.