Swift 4 backwards compatibility - ios

Question

I have the following code in my project in Xcode 8.3.3 (Swift 3.1):
let font = CGFont(provider!)
CTFontManagerRegisterGraphicsFont(font, &error)
But in Xcode 9 Beta (Swift 4), I get the following error:
Value of optional type 'CGFont?' not unwrapped; did you mean to use
'!' or '?'?
The error is because the initializer for CGFont that takes a CGDataProvider now returns an optional.
But when I apply the fix of:
let font = CGFont(provider)
CTFontManagerRegisterGraphicsFont(font!, &error)
The code no longer compiles in Xcode 8.3.3 with Swift 3.1 since font is not an optional and thus doesn't play nicely with the !.
Is there a way to make this work in both versions of Xcode? Is Swift 4 supposed to be backwards compatible (compile with Swift 3 compiler)?

Answer 1

This is a breaking change in Core Graphics not in Swift itself. API has changed, the initializer is now failable.
Use conditional compilation to make your code compile with both 3.1 and 4.0 compiler:
#if swift(>=4.0)
let font = CGFont(provider!)
#else
let font = CGFont(provider)!
#endif
CTFontManagerRegisterGraphicsFont(font, &error)

Answer 2

I ended up using the following method which allowed for backwards compatibility without conditional compilation (idea taken from this blog post):
func optionalize<T>(_ x: T?) -> T? {
return x
}
This way in both Xcode 8 and Xcode 9 I could use:
guard let font = optionalize(CGFont(provider)) else {
return
}
CTFontManagerRegisterGraphicsFont(font, &error)