Invocation.playground

import Foundation

func invocation<C: AnyObject, In, Out>
	(target target: C, method: C -> In -> Out)
	-> In -> Out?
{
	return {
		[weak target] input in
		if let target = target {
			return method(target)(input)
		} else {
			return nil
		}
	}
}

class Boo {
	func foo(input: Int) -> String {
		return "Foo \(input)"
	}
}

let boo = Boo()

let inv = invocation(target: boo, method: Boo.foo)