The current file I use to smooth over API differences between AppKit and UIKit.

PlatformDifferences.swift

// Douglas Hill, April 2017
// From https://douglashill.co/reading-app/

#if os(macOS)
import AppKit

public typealias Colour = NSColor
public typealias Font = NSFont
public typealias FontDescriptor = NSFontDescriptor
public typealias Image = NSImage
public typealias KeyModifierFlags = NSEvent.ModifierFlags

let ItalicTrait = FontDescriptor.SymbolicTraits.italic
let BoldTrait = FontDescriptor.SymbolicTraits.bold

let optionKey = KeyModifierFlags.option

#elseif os(iOS) || os(tvOS)
import UIKit

public typealias Colour = UIColor
public typealias Font = UIFont
public typealias FontDescriptor = UIFontDescriptor
public typealias Image = UIImage
public typealias KeyModifierFlags = UIKeyModifierFlags

let ItalicTrait = FontDescriptor.SymbolicTraits.traitItalic
let BoldTrait = FontDescriptor.SymbolicTraits.traitBold

let optionKey = KeyModifierFlags.alternate

#endif

extension FontDescriptor {
    // Force unwraps on iOS so the result is always not optional. Somewhat unsafe. Consider your usage.
    // By the way, be very careful with this method. It discards important properties with certain inputs.
    func dh_withSymbolicTraits(_ symbolicTraits: FontDescriptor.SymbolicTraits) -> FontDescriptor {
        #if os(macOS)
        return withSymbolicTraits(symbolicTraits)
        #elseif os(iOS) || os(tvOS)
        return withSymbolicTraits(symbolicTraits)!
        #endif
    }
}

extension Font {
    // Force unwraps on Mac so the result is always not optional. Not sure when this returns nil.
    convenience init(descriptor: FontDescriptor, overrideSize size: CGFloat = 0) {
        #if os(macOS)
        self.init(descriptor: descriptor, size: size)!
        #elseif os(iOS) || os(tvOS)
        self.init(descriptor: descriptor, size: size)
        #endif
    }
}