How can I get the Data from NSURLSession.sharedSession().dataTaskWithRequest

Question

class PostFOrData {     let url = NSURL( string: "http://210.61.209.194:8088/SmarttvWebServiceTopmsoApi/GetReadlist")     var picUrl = NSURL(string : "http://210.61.209.194:8088/SmarttvMedia/img/epi00001.png")     var responseString : NSString = ""      func forData() -> NSString {          let request = NSMutableURLRequest( URL: url!)         request.HTTPMethod = "POST"         var s : NSString = ""          let postString : String = "uid=59"         request.HTTPBody = postString.dataUsingEncoding(NSUTF8StringEncoding)          let task = NSURLSession.sharedSession().dataTaskWithRequest(request) {             data, response, error in              if error != nil {                 println("error=\(error)")                 return             } else {                 println("response = \(response!)")                 self.responseString = NSString(data: data, encoding: NSUTF8StringEncoding)!                 println("responseString = \(self.responseString)")             }          }          // I want to return NSString here, but I always get nothing          return self.responseString      } } 

Anyone know how to get the data from task?

Answer 1

You can't return data directly from an asynchronous task.

The solution with Swift 2 is to make a completion handler like this:

class PostFOrData {     // the completion closure signature is (NSString) -> ()     func forData(completion: (NSString) -> ()) {         if let url = NSURL(string: "http://210.61.209.194:8088/SmarttvWebServiceTopmsoApi/GetReadlist") {             let request = NSMutableURLRequest( URL: url)             request.HTTPMethod = "POST"             let postString : String = "uid=59"             request.HTTPBody = postString.dataUsingEncoding(NSUTF8StringEncoding)             let task = NSURLSession.sharedSession().dataTaskWithRequest(request) {                 data, response, error in                 if let data = data,                     jsonString = NSString(data: data, encoding: NSUTF8StringEncoding)                     where error == nil {                         completion(jsonString)                 } else {                     print("error=\(error!.localizedDescription)")                 }             }             task.resume()         }     } }   let pfd = PostFOrData()  // you call the method with a trailing closure pfd.forData { jsonString in     // and here you get the "returned" value from the asynchronous task     print(jsonString) } 

That way, the completion is only called when the asynchronous task is completed. It is a way to "return" the data without actually using return.

Swift 3 version

class PostFOrData {     // the completion closure signature is (String) -> ()     func forData(completion:  @escaping (String) -> ()) {         if let url = URL(string: "http://210.61.209.194:8088/SmarttvWebServiceTopmsoApi/GetReadlist") {             var request = URLRequest(url: url)             request.httpMethod = "POST"             let postString : String = "uid=59"             request.httpBody = postString.data(using: String.Encoding.utf8)             let task = URLSession.shared.dataTask(with: request) {                 data, response, error in                 if let data = data, let jsonString = String(data: data, encoding: String.Encoding.utf8), error == nil {                     completion(jsonString)                 } else {                     print("error=\(error!.localizedDescription)")                 }             }             task.resume()         }     } }   let pfd = PostFOrData()  // you call the method with a trailing closure pfd.forData { jsonString in     // and here you get the "returned" value from the asynchronous task     print(jsonString) }