Creating a generic singleton

Question

This is a bit of a head banger (for me). Basically I want to have 2 different singletons that inherit from the same class. In either I want to use a certain class which itself is derived. So I have Utility and both AUtil:Utility and BUtil:Utility. And Singleton that is used as ASingleton using AUtility in its stomach and B respectively. I failed on all frontiers. The last attempt was a factory pattern which simply got Swift 1.2 to Segfault:

protocol Initializable { init() }
class A:Initializable {
  var x = "A"
  required init() {}
}
class B:Initializable {
  var x = "B"
  required init() {}
}

class C {
  let t:Initializable
  init(t:Initializable) {
    self.t = t
    println(t)
  }
  func factory() {
    println(t.dynamicType())
  }
}

As said I also tried to make the following pattern generic:

private let _SingletonSharedInstance = StaticClass()

class StaticClass {
  class var sharedInstance : StaticClass {
    return _SingletonSharedInstance
  }
}

let s = StaticClass.sharedInstance

(This one isn't generic as you see. But all my attempts failed and so I show my starting point.)

Anyway I seem to be lost between doom and death.

Answer 1

Do you mean something like this?

protocol Initializable: class { init() }

private var instances = [String: Initializable]()

func singletonInstance<T: Initializable>(_ ty: T.Type = T.self) -> T {
    let name = NSStringFromClass(ty)
    if let o = (instances[name] as? T) {
        return o
    }
    let o = ty()
    instances[name] = o
    return o
}

An use-side of it, for instance.

class Foo: Initializable { required init() {} }
class Bar: Initializable { required init() {} }

let foo1 = singletonInstance() as Foo // or `singletonInstance(Foo.self)`
let foo2 = singletonInstance() as Foo
assert(foo1 === foo2)

let bar1 = singletonInstance() as Bar
let bar2 = singletonInstance() as Bar
assert(bar1 === bar2)

(I've tested the code above and got it to work in Swift 1.2.)