How to use completionHandler Closure with return in Swift?

closures, swift

Solution

func getSomething(callback: (Array<AnyObject>) -> ()) {
    var dataTask = NSURLSessionDataTask()
    dataTask = session.dataTaskWithRequest(request) { (data, response, error) in
        if (error == nil) {
            var callbackArray = Array<MyObject>()
            let responseDict = NSJSONSerialization.JSONObjectWithData(data, options: .MutableContainers, error: nil) as NSDictionary
            let response = responseDict.objectForKey("response_key") as NSDictionary
            let array = response.objectForKey("array_key") as NSArray

            for item: AnyObject in array {
                var arrayItem = MyObject(dict: item as NSDictionary)
                callbackArray.append(arrayItem)
            }

            callback(callbackArray)
        } else {
            // handle an error
        }
    }
    dataTask.resume()
}

Then you could do something like:

getSomething() { (response) in
    if let responseArray = response as? Array<MyObject> {
        self.somethings = responseArray
    }
}

Problem

I am trying to us a RESTful API that returns some json data. I want to encapsulate the code that creates the HTTP Request and sets the headers in its own method so I can call it by entering a url String and then have the method return a JSON object. In the following snippet of code, I have already created the request object and set the headers, and I call that variable "req". I have not declared any objects named data, response, or error. I have the following code that correctly prints out a JSON object ``` let sesh = NSURLSession.sharedSession() let dataTask = sesh.dataTaskWithRequest(req, completionHandler: {(data, response, error) in var jsonError : NSError? let jsonBlob = NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions.MutableLeaves, error: &jsonError) println(jsonBlob) }); dataTask.resume() ``` So here's my question. How do I make it so that this completionHandler block is able to return the jsonBlob, which is of type "AnyObject!"? If I modify the code slightly to be the following: ``` let sesh = NSURLSession.sharedSession() let dataTask = sesh.dataTaskWithRequest(req, completionHandler: {(data, response, error) -> AnyObject! in var jsonError : NSError? let jsonBlob : AnyObject! = NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions.MutableLeaves, error: &jsonError) return jsonBlob }); dataTask.resume() ``` then the program will not compile as the call to dataTaskWithRequest:completionHandler gives a compiler warning saying: ``` Could not find an overload for 'dataTaskWithRequest' that accepts the supplied arguments ``` I don't understand this. I'm using the correct syntax for returning closures, as is given in this page of the Swift Docs: https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Closures.html

Original source