411 - Swift in Practice.md

Swift in Practice (411)

Availability Checking

Compiler in swift 2 will check at compile time to ensure that the apis used are available on the deployment target. The fixit for this error will add a guard for the correct #availability version

Why check based on version

• Features and users are tied to OS versions
• Checking runtime availability doesn’t actually guarantee expected behavior (ex: internal APIs)

guard makes this nice (less nesting)

guard #available(ios 9.0, *) else { return }
let stack = UIStackView()

annotation

functions can be annotated with @available(iOS 8.0, *) you ensure the same safety for your internal code and can factor out code paths that can only be called on certain versions without duplicate checks

Enforcing Application Constraints

Asset Catalog Identifiers

let fooImage = UIImage(named: “foo”)!

You have to force unwrap because your identifiers are “stringly” typed. Solution is to codify the type as an application specific enum:

extension UIImage {
	enum AssetIdentifier: String {
		case Isabella = “Isabella”
		case Foo = “Foo”
	}

convenience init!(identifier: AssetIdentifier) {
	return UIImage…
}

Benefit

• Centrally located constants
• Doesn’t pollute global namespace
• Must use one of the enum cases and results in a non-failable initializer

Constrained protocol extension

extension SegueHandlerType where Self: UIViewController {
    // can add methods only where the implementer of SegueHandlerType is a UIViewController
}