Q:

Cannot convert value of type to expected argument type NSURLSessionDelegate

I understand it should be very simple but I am a novice with Swift and cannot get what is wrong here:
public class API : NSObject, NSURLSessionDelegate{

    public class func sendHttpRequestNew(urlPath: String, params: AnyObject, method: String)-> Int {
        var delegate = self
        let txtUserName = "admin"
        let txtPassword = "test1234"
        let PasswordString = "\(txtUserName):\(txtPassword)"
        let PasswordData = PasswordString.dataUsingEncoding(NSUTF8StringEncoding)
        let base64EncodedCredential = PasswordData!.base64EncodedStringWithOptions(NSDataBase64EncodingOptions. Encoding64CharacterLineLength)
        let url: NSURL = NSURL(string: urlPath)! 

        let request: NSMutableURLRequest = NSMutableURLRequest(URL: url)
        request.setValue("Basic \(base64EncodedCredential)", forHTTPHeaderField: "Authorization")
        request.setValue("application/json; charset=utf-8", forHTTPHeaderField: "Content-Type")
        request. HTTPMethod = method
        if method == "POST" || method == "PUT" {
            request. HTTPBody = try!  NSJSONSerialization.dataWithJSONObject(params, options: [])
        }
        let config = NSURLSessionConfiguration.backgroundSessionConfigurationWithIdentifier("unique")
        let authString = "Basic \(base64EncodedCredential)"
        config. HTTPAdditionalHeaders = ["Authorization" : authString]
        let session = NSURLSession(configuration: config, delegate: self, delegateQueue: nil)

        let downloadTask = session.downloadTaskWithRequest(request)
        downloadTask.resume()
    }
}

I got an error on this line with self:
let session = NSURLSession(configuration: config, delegate: self, delegateQueue: nil)
Cannot convert value of type 'API.type' to expected argument type NSURLSessionDelegate
What am I doing wrong? 

A:

The issue here is the method is declared with class, making it a type method.  When you refer to self inside this method, you are referring to the class API itself, and hence the error. 
public class API : NSObject, NSURLSessionDelegate
is declaring API to be a subclass of NSObject, which conforms to NSURLSessionDelegate.  
When assigning a delegate to the URL session, the object assigned has to be an instance of API. 
There are two ways to remedy this: 

Make sendHttpRequest() a normal method by getting rid of the class prefix, or,
Create the session using let session = NSURLSession(configuration: config, delegate: API(), delegateQueue: nil), which creates an instance of API and passes that as the delegate. 

In my opinion, the first option is better.  That way you can keep an instance of API around somewhere else and make your networking calls to it like that.