Justin Williams put together a list of what platform macros from TargetConditions.h trigger each specific platform. The results are a mess. The worst one is that TARGET_OS_MAC will target all of Apple’s platforms (macOS, iOS, watchOS, & tvOS) and TARGET_OS_IPHONE is everything except macOS.

This is one of the most underrated improvements in Swift. Instead of

#if TARGET_OS_IPHONE
  // iOS code
#else
  // macOS code
#endif

we can use something more like

#if os(macOS)
  // macOS code
#endif

The nice thing here that you don’t have to worry about if you are running on the simulator or not¹.

Even better, starting with Swift 3 you can check for the existence of a framework. This is useful for frameworks that are supported on multiple platforms.

#if canImport(UIKit)
  // iOS, tvOS, & watchOS
  import UIKit
  let color: UIColor
#elseif canImport(AppKit)
  // macOS
  import AppKit
  let color: NSColor
#endif

These are small improvements, but they make writing code that runs on multiple platforms a lot easier.