Swift callback in dataTaskWithRequest - swift2

Question

New to Swift and trying to write a login screen for my app.
My controller:
#IBAction func loginButton(sender: UIButton) {
let emailAddress = emailTextField.text
let password = passwordTextField.text
let api = Api()
api.authenticate(emailAddress!, password: password!)
}
Api class:
class Api {
func authenticate(username: String, password: String) -> Bool {
var params = [String: String]()
params["username"] = username
params["password"] = password
params["grant_type"] = "password"
var headers = [String: String]()
headers["Content-Type"] = "application/x-www-form-urlencoded"
post("authenticate", params: params, headers: headers) {data, response in
// do something
}
}
func post(location: String, params: [String: String], headers: [String: String], callback: ((data: NSString!, response: NSHTTPURLResponse) -> Void)) {
let request = NSMutableURLRequest()
let session = NSURLSession.sharedSession()
request.URL = NSURL(string: Configuration.apiBaseUrl + location)
request.HTTPMethod = "POST"
request.HTTPBody = NSString(string: getPostBody(params)).dataUsingEncoding(NSUTF8StringEncoding)
request.addValue("Basic " + Configuration.apiAuthorization, forHTTPHeaderField: "Authorization")
for (key, value) in headers {
request.addValue(value, forHTTPHeaderField: key)
}
let task = session.dataTaskWithRequest(request, completionHandler: {data, response, error -> Void in
let data = NSString(data: data!, encoding: NSUTF8StringEncoding)
let httpResponse = response as! NSHTTPURLResponse
callback(data: data, response: httpResponse)
return
})
task.resume()
}
}
As I plan to add more API methods after authentication, I want generic POST/PUT/GET/DELETE methods. I am trying to understand how I can best set up the callbacks in Swift so the post call in the authenticate method can return true/false and show like a dialog on the UI thread. Returning a boolean in the post call in the authenticate method is not allowed now, because it expects Void.
I am familiar with callbacks in nodejs and trying to grasp this now. Any hints what the best approach would be?