如何使用协议扩展为类提供自动生成的标识符？

Question

I want to provide an auto-generated identifier for each custom UITableViewCell subclass. I try the following code, but the compiler says

type 'T' has no member 'autoReuseIdentifier'

protocol AutoReusable: class {
    static var autoReuseIdentifier: String { get }
}

extension AutoReusable {
    static var autoReuseIdentifier: String {
        return "\(self)"
    }
}

extension UITableViewCell: AutoReusable {}

func printClassName<T: UITableViewCell>(type type: T.Type) {
    print(T.autoReuseIdentifier)
}

It's okay to implement the protocol in an extension for UITableViewCell, but I prefer to implement it in a protocol extension. How to fix this?

Answer 1

The generic T in printClassName(...) does not know that it conforms to AutoReusable protocol (even if you, as the developer, knows that UITableViewCell does so), it only knows that it is a UITableViewCell och UITableViewCell subclass object.

You could redeem this by adding a ... where T: AutoReusable> in the generic type constraint of printClassName(...)

func printClassName<T: UITableViewCell where T: AutoReusable>(type type: T.Type) {
    print(T.autoReuseIdentifier)
}

A more common implementation of printClassName(...) would be, however, to constraint T to the protocol AutoReusable , rather than let printClassName be a function specifically for UITableViewCell objects (subclass objects)

func printClassName<T: AutoReusable>(type type: T.Type) {
    print(T.autoReuseIdentifier)
}

This generic function can then be called from any type conforming to AutoReusable , whereas you can control the default implementation of autoReuseIdentifier by different extensions to the protocol AutoReusable . Eg, as a complete example:

protocol AutoReusable: class {
    static var autoReuseIdentifier: String { get }
}

/* Default implementation */
extension AutoReusable {
    static var autoReuseIdentifier: String {
        return "\(self)"
    }
}

/* Default implementation specifically for UITableViewCell (and subclasses) */
extension AutoReusable where Self: UITableViewCell {
    static var autoReuseIdentifier: String {
        return "\(self) (UITableViewCell)"
    }
}

/* Generic function invokable by any class type conforming to printClassName */
func printClassName<T: AutoReusable>(type type: T.Type) {
    print(T.autoReuseIdentifier)
}

/* Example setup */
extension UITableViewCell: AutoReusable { }
class Foo : AutoReusable { }
class MyCell : UITableViewCell {}

/* Example usage */
let foo = Foo()
let bar = MyCell()

printClassName(type: foo.dynamicType) // Foo
printClassName(type: bar.dynamicType) // MyCell (UITableViewCell)