The closure in Swift

Question

I just started to use Swift in my project. When reading the instruction of Alamofire library, I found some code like:

extension Request {
    class func XMLResponseSerializer() -> Serializer {
        return { (request, response, data) in
            if data == nil {
                return (nil, nil)
            }

            var XMLSerializationError: NSError?
            let XML = ONOXMLDocument.XMLDocumentWithData(data, &XMLSerializationError)

            return (XML, XMLSerializationError)
        }
    }

    func responseXMLDocument(completionHandler: (NSURLRequest, NSHTTPURLResponse?, OnoXMLDocument?, NSError?) -> Void) -> Self {
        return response(serializer: Request.XMLResponseSerializer(), completionHandler: { (request, response, XML, error) in
            completionHandler(request, response, XML, error)
        })
    }
}

In the second function responseXMLDocument I wonder why we have to rewrite the closure. If the parameter of the function is already a closure, why cannot we just write like:

return response(serializer: Request.XMLResponseSerializer(), completionHandler:completionHandler)

I tried it but it does not compile.

Answer 1

It occurred to me just after I posted my question... The problem is that the type of the parameter does not match. More specific, it is the XML parameter, which should be AnyObject?, is OnoXMLDocument? in the inner handler.