A wrapper class to hold and pass around a Swift function in ObjC Code.

FuncBox.swift

import Foundation

@objc
public final class FuncBox<T,U> {
    
    private var _fn: [T->U]
    
    public var fn: T->U { return _fn[0] }
    
    // A trick to make callable types by (ab)using subscript.
    public subscript(param:T) -> U { return _fn[0](param) }
    
    public init(_ fn: T->U) { _fn = [fn] }
    
    public var signature: UInt { return ObjectIdentifier(self.dynamicType.self).uintValue() }
    
    public var parameterSignature: UInt { return FuncBox<T,Void>({ (p:T)->Void in }).signature }

    public var returnSignature: UInt { return FuncBox<U,Void>({ (p:U)->Void in }).signature }
    
    public func accepts(#signature: UInt) -> Bool { return parameterSignature == signature }
    
    public func returns(#signature: UInt) -> Bool { return returnSignature == signature }
}

//public let voidSignature = FuncBox<Void,Void>({}).signature

typealias RenderFunction = (String) -> String
typealias RenderLambda = (String, RenderFunction) -> String

func renderText(text: String) -> String { return text.uppercaseString }

let lambda: RenderLambda = { (text, render) -> String in render(text) }


func f() { println("Void->Void") }
func f(x: Double, y: Double) -> Double { return x * y }


class Prefix {
    let p: String
    init(_ p: String) { self.p = p }
    
    func prefixed(n: String) -> String { return p + " " + n }
}

let fn = FuncBox(renderText)

let fn2 = FuncBox(lambda)

let fn3 = FuncBox { (p:String)->String in p }

let fn4 = FuncBox(Prefix("Mr.").prefixed)

let fn5 = FuncBox<(Double,Double),Double>(f)

let fn6 = FuncBox<Void,Void>(f)

let fn7 = FuncBox<Void,String>({ "The is a test!" })

let fn8 = FuncBox(Prefix.prefixed)

fn.signature

fn.signature == fn2.signature // False
fn.signature == fn3.signature // True
fn.signature == fn4.signature // True

fn.parameterSignature == fn.returnSignature

fn6.returnSignature == fn7.parameterSignature

fn6.signature == fn6.parameterSignature

fn4.signature == fn8.signature

fn4.signature
fn8.returnSignature

fn.signature

fn["test"]
fn2["test", renderText]
fn3["me"]
fn4["Mehr"]
fn5[3,4]
fn6[]