Invalid conversion from throwing function of type (_,_,_) throws -> Void to non-throwing function type (NSData?, NSURLResponse?, NSError?) -> Void

Question

I have written this code:

func getjson() {         let urlPath = "https://api.whitehouse.gov/v1/petitions.json?limit=100"         let url = NSURL(string: urlPath)         let session = NSURLSession.sharedSession()         let task = session.dataTaskWithURL(url!, completionHandler: {data, response, error -> Void in             print("Task completed")             if(error != nil) {                 print(error!.localizedDescription)             }             let err: NSError?             if let jsonResult = try NSJSONSerialization.JSONObjectWithData(data!, options: NSJSONReadingOptions.MutableContainers) as? NSDictionary {                 if(err != nil) {                     print("JSON Error \(err!.localizedDescription)")                 }                 if let results: NSArray = jsonResult["results"] as? NSArray {                     dispatch_async(dispatch_get_main_queue(), {                         self.tableData = results                         self.Indextableview.reloadData()                     })                 }             }         })          task.resume()      } 

And after update to XCode 7 it gives me this error: Invalid conversion from throwing function of type (_, _, _) throws -> Void to non-throwing function type (NSData?, NSURLResponse?, NSError?) -> Void. It is in line, where is let task.

Thanks

Answer 1

You need to implement Do Try Catch error handling as follow:

import UIKit import PlaygroundSupport PlaygroundPage.current.needsIndefiniteExecution = true  extension URL {     func asyncDownload(completion: @escaping (_ data: Data?, _ response: URLResponse?, _ error: Error?) -> ()) {         URLSession.shared             .dataTask(with: self, completionHandler: completion)             .resume()     } } 

---

let jsonURL = URL(string: "https://api.whitehouse.gov/v1/petitions.json?limit=100")! let start = Date() jsonURL.asyncDownload { data, response, error in      print("Download ended:", Date().description(with: .current))     print("Elapsed Time:", Date().timeIntervalSince(start), terminator: " seconds\n")     print("Data size:", data?.count ?? "nil", terminator: " bytes\n\n")      guard let data = data else {         print("URLSession dataTask error:", error ?? "nil")         return     }      do {         let jsonObject = try JSONSerialization.jsonObject(with: data)         if let dictionary = jsonObject as? [String: Any],             let results = dictionary["results"] as? [[String: Any]] {             DispatchQueue.main.async {                 results.forEach { print($0["body"] ?? "", terminator: "\n\n") }       //        self.tableData = results       //        self.Indextableview.reloadData()             }         }     } catch {         print("JSONSerialization error:", error)     } } print("\nDownload started:", start.description(with: .current)) 

Answer 2

As Leo suggested, your problem is that you're using try, but not within the do-try-catch construct, which means that it infers that the closure is defined to throwing the error, but since it is not defined as such, you get that error.

So, add do-try-catch:

func getjson() {     let urlPath = "https://api.whitehouse.gov/v1/petitions.json?limit=100"     let url = URL(string: urlPath)!     let session = URLSession.shared     let task = session.dataTask(with: url) { data, response, error in         print("Task completed")          guard let data = data, error == nil else {             print(error?.localizedDescription)             return         }          do {             if let jsonResult = try JSONSerialization.jsonObject(with: data) as? [String: Any] {                 if let results = jsonResult["results"] as? [Any] {                     DispatchQueue.main.async {                         self.tableData = results                         self.indexTableView.reloadData()                     }                 }             }         } catch let parseError {             print("JSON Error \(parseError.localizedDescription)")         }     }      task.resume() }