Swift: Block mainThread until function finished loading data / NSURLSession with completionHandler

Question

I want to create a function which loads data from an url and then returns the responseData as NSData. I want to block the mainThread until the data is finished. Here is what I have so far:

Function:

typealias CompletionBlock = (NSData!, NSURLResponse!, NSError!) -> NSData
func requestURL(targetUrl: String, httpMethod: String, httpBody: String, completion: CompletionBlock){

// REQUEST
let target = NSURL(string: targetUrl) // URL
let request = NSMutableURLRequest(URL: target!) // REQUEST

// HTTP-METHOD
var method = httpMethod
if method != "POST" { method = "GET" } // STANDARD: GET
    request.HTTPMethod = method

// HTTP-BODY
let body = httpBody
if body != "" {
    request.HTTPBody = body.dataUsingEncoding(NSUTF8StringEncoding)
}

// NSURLSession
let session = NSURLSession.sharedSession()
let task = session.dataTaskWithRequest(request, completionHandler: completion) // Compiler error!
task.resume()

}

Call:

requestURL(targetURL, "GET", "", { (responseData: NSData!, response: NSURLResponse!, error: NSError!) -> NSData in

        if responseData != nil {
            let resultString = NSString(data: responseData, encoding: NSUTF8StringEncoding) // NSASCIIStringEncoding
            println("DATA:\n\(resultString)")

        }

        if error != nil {
            println("ERROR:\n\(error)")
        }

        return responseData
    })

I get an error at within the func in line 21:

let task = session.dataTaskWithRequest(request, completionHandler: completion)

Compiler: "Cannot invoke 'dataTaskWithRequest' with an argument list of type '(NSMutableURLRequest, completionHandler: completionBlock)"

Answer 1

As for question issue: typealias CompletionBlock = (NSData!, NSURLResponse!, NSError!) -> NSData

Your completion handler returns NSData but it shouldn't return anything as in declaration:

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

This caused a type error, because you had provided a wrong closure type.

And it is quite reasonable, because dataTaskWithRequest is designed to be asynchronous. It creates an HTTP request for the specified URL request object, and calls a handler upon completion. If you really want to make a synchronous request you can use an old NSURLConnection API with sendSynchronousRequest, but you shouldn't do it, because synchronous requests are a very bad design choice: they block main UI thread and there is no way to cancel that request except when it errors on its own. That's why Apple created a new NSURLSession API, based on completion handlers and now deprecated synchronous requests in iOS 9.

Answer 2

Doing synchronous request is very bad, but this will do

let request = NSURLRequest() //set all url and method and so
var response: NSURLResponse?
var error: NSError?
NSURLConnection.sendSynchronousRequest(request, returningResponse: &response, error: &error)

important synchronous requests was removed in swift 2.0.

Answer 3

As Daniel said, this is a bad practice in general. Any time, you have to go out and get data and wait for a response, you should be handling that asynchronously. If not, your UI is going to hang up on the main thread, and the user is likely to think that your app has crashed.

Working asynchronously, you will have to keep in mind that your main thread is still working and won't have that data until it receives it in the completion block. If it's imperative, you should throw up a UI element to let the user know you are waiting ie. UIActivityIndicatorView