Swift chaining return success and failure

//: Playground - noun: a place where people can play

import UIKit

func delay(delay:Double, closure:()->()) {
    dispatch_after(
        dispatch_time(
            DISPATCH_TIME_NOW,
            Int64(delay * Double(NSEC_PER_SEC))
        ),
        dispatch_get_main_queue(), closure)
}

class Chainable<SuccessParams, FailureParams> {
    var successClosure: ((SuccessParams) -> ())? = nil
    var failureClosure: ((FailureParams) -> ())? = nil
    
    func onSuccess(closure: (SuccessParams) -> ()) -> Chainable {
        successClosure = closure
        return self
    }
    func onFailure(closure: (FailureParams) -> ()) -> Chainable {
        failureClosure = closure
        return self
    }
    func doSuccess(params: SuccessParams) {
        if let closure = successClosure {
            closure(params)
        }
    }
    func doFailure(params: FailureParams) {
        if let closure = failureClosure {
            closure(params)
        }
    }
}

class NetworkHelper {
    func cacheImage(url: String) -> Chainable<String, Void> {
        let chains = Chainable<String, Void>()
        delay(1) {
            let outcome = rand() % 2
            if outcome > 0 {
                print("NetworkHelper: got image")
                chains.doSuccess("/dev/null")
            } else {
                print("NetworkHelper: a failure occured")
                chains.doFailure()
            }
        }
        return chains
    }
}

let helper = NetworkHelper()
helper.cacheImage("url")
helper.cacheImage("url").onSuccess({
    localURL in print("saved to local cache at \(localURL)")
})