Return values from completion handler - swift

Question

I want to return the values from an api call.
The call to my api class (I want to get the values in res):
class ViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
let t = Api_test();
let res = t.getSomething();
print(res)
}
}
The api class:
import Foundation
class Api_test {
func getAllStations(completionHandler: (response : XMLIndexer) -> ()) {
getRequest { result in
completionHandler(response: SWXMLHash.parse(result))
};
}
func getRequest(completionHandler: (result: NSData) -> ()) {
let baseUrl = "http://api.test.com"
let request = NSMutableURLRequest(URL: NSURL(string: baseUrl)!)
let session = NSURLSession.sharedSession()
request.HTTPMethod = "GET"
let task = session.dataTaskWithRequest(request) {
(data, response, error) in
if data == nil {
print("dataTaskWithRequest error: \(error)")
return
} else {
completionHandler(result: data!)
}
}
task.resume()
}
}
Everything works as thought, but I'm stuck at the point to return the values back to the getSomething function. The data is in xml format. How can I get the result set as returned values in the res (viewDidLoad)?

Answer 1

NSURLSession is a fully asynchronous networking API so ideally your view controller should operate correctly and not wait for the data to be returned from the network.
You have three options here:
You can pass a completion block to getSomething and have it pass the result to the block:
func getSomething(completionHandler: (result: XMLIndexer) -> ()) {
getRequest { result in
completionHandler(result: SWXMLHash.parse(result))
}
}
override func viewDidLoad() {
...
t.getSomething { res in
print(res)
}
}
If you desperately need the XML data in hand before view is displayed onto screen, you can make the main thread wait till network operation finishes executing. You can use dispatch_semaphore_t:
func getSomething() -> XMLIndexer? {
var xml: XMLIndexer? = nil
let semaphore: dispatch_semaphore_t = dispatch_semaphore_create(0)
getRequest { result in
xml = SWXMLHash.parse(result)
dispatch_semaphore_signal(semaphore)
}
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER)
return xml
}
Last option is, you can use another 3rd party that does the parsing synchronously. There is a great one named Ono:
var error: NSError?
let xml = ONOXMLDocument(data: result, error: &error)