Loading custom fonts programmatically in Swift 3

How do you include custom fonts in your application, if they don't reside in the main bundle? In this case, loading them through the conventional PLIST method doesn't work, and you will need to resort to CoreGraphics to register them programmatically. 

Typefaces are an essential part of good design. On iOS, custom fonts can be added to apps through a somewhat convoluted process. Chris Ching has a good write-up on the topic, but in short: ensure the font files are properly included in your app's target and declare them in your app's Info.plist ("Fonts provided by application"). This works well in the straightforward case where the fonts are simply embedded into the main app bundle.

You quickly run into the limits of the PLIST-based registration method when they live outside of the main bundle, e.g. in a separate .framework. In this case, simply register the fonts using the older CoreGraphics APIs.

Marco Arment described the approach on his blog back in 2012, but the example code he lists is Objective-C. Fortunately, the Swift version of this code is much less cumbersome than its Obj-C counterpart, as less C-trickery is required:

// Load bundle which hosts the font files. Bundle has various ways of locating bundles.
// This one uses the bundle's identifier
guard let bundle = Bundle(identifier: "some.framework.identifier")
else { return }

// List the fonts by name and extension, relative to the bundle
let fonts = [
    bundle.url(forResource: "gotham_book", withExtension: "ttf"),
    bundle.url(forResource: "gotham_medium", withExtension: "ttf")
]

// Iterate over the resulting urls, filtering out nil-values with .flatMap()
for url in fonts.flatMap({ $0 }) {
  // Create a CGDataPRovider and a CGFont from the URL.
  // Register the font with the system.
    if let dataProvider = CGDataProvider(url: url as CFURL) {
        let font = CGFont(dataProvider)
        CTFontManagerRegisterGraphicsFont(font, nil)
    }
}

Just make sure you don't forget to remove any duplicate PLIST declarations, or your app will throw an exception at startup.

Update

@sveinhal in the comments suggested this functional rewrite of the above code, which is way more succinct if that's your cup of tea:

let fonts = [
bundle.url(forResource: "gotham_book", withExtension: "ttf"),
bundle.url(forResource: "gotham_medium", withExtension: "ttf"),
]

fonts
.flatMap { $0 }
.flatMap { CGDataProvider(url: $0 as CFURL) }
.map(CGFont.init)
.forEach { CTFontManagerRegisterGraphicsFont($0, nil) }