[swift-evolution] Proposal: Access control for protocols

[Adrian Zubarev]

I hope markdown is supported in these mails. :)

A few month ago I build a singleton generator with the help of protocols (just for fun). As you all know a true singleton will hide its init(), but right know there is no Access control for protocols. This might be a good feature for other code abstraction too.

Any feedback is welcome.

Here is my little implementation.

public protocol SingletonType { /* private */ init() } // wait for this feature to create a true singleton

private var singletonInstances = [String: SingletonType]()

public extension SingletonType {
     
    typealias SingletonInstance = Self
    typealias SingletonMetatype = Self.Type
     
    public static var getSingleton: SingletonInstance { return setSingleton { $0 } }
     
    public static var setSingleton: SingletonMetatype { return self }
     
    public static func setSingleton(setter: (_: SingletonInstance) -> SingletonInstance) -> SingletonInstance {
         
        guard let instance = singletonInstances["\(self)"] as? Self else {
             
            return setInstance(self.init(), withSetter: setter, overridable: true)
        }
        return setInstance(instance, withSetter: setter, overridable: false)
    }
     
    private static func setInstance(var instance: Self, withSetter setter: (_: Self) -> Self, overridable: Bool) -> Self {
         
        // I will have to fix `var instance` in Swift 3.0 :/
         
        instance = restoreInstanceIfNeeded(instance1: instance, instance2: setter(instance), overridable: overridable)
         
        singletonInstances["\(self)"] = instance
         
        return instance
    }
     
    private static func restoreInstanceIfNeeded(instance1 i1: Self, instance2 i2: Self, overridable: Bool) -> Self {

        guard i1.dynamicType is AnyClass else { return i2 }
         
        return ((i1 as! AnyObject) !== (i2 as! AnyObject)) && !overridable ? i1 : i2
    }
}

infix operator « { associativity none }

func « <Instance: SingletonType>(type: Instance.Type, newInstance: Instance) -> Instance {
     
    return type.setSingleton { (_) -> Instance in newInstance }
}


— 
Regards Adrian
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20151204/4ea76c78/attachment.html>