How to use completionHandler Closure with return in Swift?

Question

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 cal

Answer 1

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
    }
}

Answer 2

The completion handler can't return anything because the closure you have to supply has to be of return type Void and not AnyObject!.

func dataTaskWithRequest(_ request: NSURLRequest!,
   completionHandler completionHandler: ((NSData!,
                              NSURLResponse!,
                              NSError!) -> Void)!) -> NSURLSessionDataTask!

Answer 3

I came across a similar issue when trying to update info in a central DB based on content was pulling from CloudKit. The patterns for pulling CK data all have these asynchronous completion handlers which allow no return value.

Since I wanted to be able to re-use similar code in different contexts, I separated the CK calls out into their own class and defined a delegate protocol that I made the core class conform to. Within the completion handlers I sent data retrieved from the CK calls back to where I want them stored via delegate methods.

Easy peasy.

Answer 4

as you see here, the dataTaskWithRequest:completionHandler: has a completion handler with no expected return value.

that means the NSURLSession instance do not expect any value from you to proceed after calling this method; with other words: you completion closure (or block, if you like) ends the procedure here.

there is not clear why you'd like sending back anything to the caller via the completion handler.