Controlling tasks within Swift 2.0 - ios

Question

Working in Swift 2.0, on IOS 9.2.1 using Xcode 7.2
Learning Swift 2.0 and I have written a routine that creates a NSURL Session, gets back some JSON data and then parses it. It works great...
BUT I some help in understanding how make this work as in get the outer function, share_list_folders to wait until the task here truly completes so I can return the result?
var parsedJson:[String:AnyObject] = [:]
func shared_list_folders() {
// **** list_folders (shared) ****
let request = NSMutableURLRequest(URL: NSURL(string: "https://api.dropboxapi.com/2/sharing/list_folders")!)
let session = NSURLSession.sharedSession()
request.HTTPMethod = "POST"
request.addValue("Bearer ab-XXX", forHTTPHeaderField: "Authorization")
let task = session.dataTaskWithRequest(request, completionHandler: {data, response, error -> Void in
//print("Response: \(response)")
let strData = NSString(data: data!, encoding: NSUTF8StringEncoding)
print("Body: \(strData)\n\n")
do {
let jsonResult = try NSJSONSerialization.JSONObjectWithData(data!, options:NSJSONReadingOptions.MutableContainers);
self.jsonParser(jsonResult)
for (key, value) in self.parsedJson {
print("key2 \(key) value2 \(value)")
}
} catch {
print("Bang")
}
})
task.resume()
let string2return = parsedJson["path_lower"] as? String
return(string2return)!
}
Its not really a completion, cause task will go off and do its own thing and share_list_folders will complete? Obviously I don't get the path_lower value here, until its too late... looked at delegates? And I tried, but then I run into issued with the completion block...

Answer 1

As you noticed NSURLSession after resuming it's task go away and do it's job which requires time. It's part of asynchronous programming when you have to deal with situation when something is calculated/prepared in another thread. Under the hood NSURLSession has own thread and there is waiting for server response. Then invokes completionHandler on main thread. That's a simplified theory. Going back to your question:
It's obviously that you have to wait for server response. As in another languages code is executed line by line, so you can't return anything as you wrote. The answer is: use closures.
Function you declare could use delegate as well but let's focus on closures:
func shared_list_folders(completion: (string: String?, error: ErrorType?) -> Void) {
// **** list_folders (shared) ****
let request = NSMutableURLRequest(URL: NSURL(string: "https://api.dropboxapi.com/2/sharing/list_folders")!)
let session = NSURLSession.sharedSession()
request.HTTPMethod = "POST"
request.addValue("Bearer ab-XXX", forHTTPHeaderField: "Authorization")
let task = session.dataTaskWithRequest(request, completionHandler: {data, response, error -> Void in
//print("Response: \(response)")
if let error = error {
completion(string: nil, error: error)
return
}
let strData = NSString(data: data!, encoding: NSUTF8StringEncoding)
print("Body: \(strData)\n\n")
do {
let jsonResult = try NSJSONSerialization.JSONObjectWithData(data!, options:NSJSONReadingOptions.MutableContainers);
let parsedJson = self.jsonParser(jsonResult) // I assume this returns parsedJson?
let string2return = parsedJson["path_lower"] as? String
completion(string: string2return, error: nil)
} catch { // here we have an error
completion(string: nil, error: error)
}
})
task.resume()
}