sgleadow/Swift4Generics

Sample code to support a short talk on generics and typing in Swift 4 (mostly available before Swift 4 too)

Swift 4 Generics Demo

This is some sample code I used in a live coding demo of some properties of generics in Swift 4.

Most of these features were already available in Swift 3, so it just touched on a couple of items, like:

• adding generics to a subscript methpod (new in Swift 4)
• conforming to multiple protocols (already existed before Swift 4)

Video

Jesse does a great job of recording the presentations each month at Melbourne Cocoaheads, including this partocular talk about extracting strongly typed values.

Requirements

The early parts of the demo use Xcode 8, but the later parts that require Swift 4, you'll need to be running Xcode 9.

Topics

Generics on subscripts

In Xcode 8:

extension InfoDictionary {
    // This couldn't have a generic type constraint
    subscript(key: Key) -> Any? {
        let value = infoDictionary?[key.rawValue]
        return value
    }
}

// So you had to cast on the way out
let value: String? = bundle.info[.name] as? String

In Xcode 9:

extension InfoDictionary {
    // Can define a generic type constraint here
    subscript<T: Convertible & Defaulting>(key: Key) -> T {
        let value = infoDictionary?[key.rawValue] as? String
        return value.flatMap(T.convert) ?? T.defaultValue
    }
}

// So you don't need to cast on the way out
let value: String? = bundle.info[.name]

Extracting defined values

The demo uses a little wrapper around Bundle to extract values based on pre-defined kets.

struct InfoDictionary {
    enum Key: String {
        case name = "CFBundleName"
        case version = "CFBundleShortVersionString"
        case build = "CFBundleVersion"
    }

    let infoDictionary: [String : Any]?
}

protocol BundleType {
    var infoDictionary: [String : Any]? { get }
}

extension Bundle: BundleType {}

extension BundleType {
    var info: InfoDictionary {
        return InfoDictionary(infoDictionary: infoDictionary)
    }
}

Extra useful protocols

The demo defines a couple of extra protocols used to convert values to the correct type, and to define default values if anything is missing or fails.

I've defined implementations of these for both String and Int.

protocol Convertible {
    static func convert(_ value: String) -> Self?
}

protocol Defaulting {
    static var defaultValue: Self { get }
}