HTTP POST error Handling in Swift 2 - http

Question

I'm new here, and this is my first question...
I try to write an App in Swift 2 that makes an HTTP POST request but I can't figure out how to use the new error handling of Swift 2. Can anyone tell me please how to implement the "do-try-catch" error handling of Swift 2 to the code snippet below?
(This code snippet uses the old error handling of swift 1.2)
func post(params : Dictionary<String, String>, url : String) {
var request = NSMutableURLRequest(URL: NSURL(string: url)!)
var session = NSURLSession.sharedSession()
request.HTTPMethod = "POST"
var err: NSError?
request.HTTPBody = NSJSONSerialization.dataWithJSONObject(params, options: nil/*, error: &err*/)
request.addValue("application/json", forHTTPHeaderField: "Content-Type")
request.addValue("application/json", forHTTPHeaderField: "Accept")
var task = session.dataTaskWithRequest(request, completionHandler: {data, response, error -> Void in
print("Response: \(response)")
var strData = NSString(data: data!, encoding: NSUTF8StringEncoding)
print("Body: \(strData)")
var err: NSError?
var json = NSJSONSerialization.JSONObjectWithData(data!, options: .MutableLeaves/*, error: &err*/) as? NSDictionary
// Did the JSONObjectWithData constructor return an error? If so, log the error to the console
if(err != nil) {
print(err!.localizedDescription)
let jsonStr = NSString(data: data!, encoding: NSUTF8StringEncoding)
print("Error could not parse JSON: '\(jsonStr)'")
}
else {
// The JSONObjectWithData constructor didn't return an error. But, we should still
// check and make sure that json has a value using optional binding.
if let parseJSON = json {
// Okay, the parsedJSON is here, let's get the value for 'success' out of it
var success = parseJSON["success"] as? Int
print("Succes: \(success)")
}
else {
// Woa, okay the json object was nil, something went worng. Maybe the server isn't running?
let jsonStr = NSString(data: data!, encoding: NSUTF8StringEncoding)
print("Error could not parse JSON: \(jsonStr)")
}
}
})
task!.resume()
}

Answer 1

You presumably want to wrap your NSJSONSerialization calls in do/try/catch logic as shown below.
In Swift 3:
var request = URLRequest(url: URL(string: urlString)!)
let session = URLSession.shared
request.httpMethod = "POST"
request.addValue("application/json", forHTTPHeaderField: "Content-Type")
request.addValue("application/json", forHTTPHeaderField: "Accept")
request.httpBody = try! JSONSerialization.data(withJSONObject: parameters)
// or if you think the conversion might actually fail (which is unlikely if you built `parameters` yourself)
//
// do {
// request.httpBody = try JSONSerialization.data(withJSONObject: parameters)
// } catch {
// print(error)
// }
let task = session.dataTask(with: request) { data, response, error in
guard let data = data, error == nil else {
print("error: \(error)")
return
}
// this, on the other hand, can quite easily fail if there's a server error, so you definitely
// want to wrap this in `do`-`try`-`catch`:
do {
if let json = try JSONSerialization.jsonObject(with: data) as? [String: Any] {
let success = json["success"] as? Int // Okay, the `json` is here, let's get the value for 'success' out of it
print("Success: \(success)")
} else {
let jsonStr = String(data: data, encoding: .utf8) // No error thrown, but not dictionary
print("Error could not parse JSON: \(jsonStr)")
}
} catch let parseError {
print(parseError) // Log the error thrown by `JSONObjectWithData`
let jsonStr = String(data: data, encoding: .utf8)
print("Error could not parse JSON: '\(jsonStr)'")
}
}
task.resume()
Or, in Swift 2
let request = NSMutableURLRequest(URL: NSURL(string: urlString)!)
let session = NSURLSession.sharedSession()
request.HTTPMethod = "POST"
request.addValue("application/json", forHTTPHeaderField: "Content-Type")
request.addValue("application/json", forHTTPHeaderField: "Accept")
request.HTTPBody = try! NSJSONSerialization.dataWithJSONObject(parameters, options: [])
// or if you think the conversion might actually fail (which is unlikely if you built `parameters` yourself)
//
// do {
// request.HTTPBody = try NSJSONSerialization.dataWithJSONObject(params, options: [])
// } catch {
// print(error)
// }
let task = session.dataTaskWithRequest(request) { data, response, error in
guard let data = data where error == nil else {
print("error: \(error)")
return
}
// this, on the other hand, can quite easily fail if there's a server error, so you definitely
// want to wrap this in `do`-`try`-`catch`:
do {
if let json = try NSJSONSerialization.JSONObjectWithData(data, options: []) as? NSDictionary {
let success = json["success"] as? Int // Okay, the `json` is here, let's get the value for 'success' out of it
print("Success: \(success)")
} else {
let jsonStr = String(data: data, encoding: NSUTF8StringEncoding) // No error thrown, but not NSDictionary
print("Error could not parse JSON: \(jsonStr)")
}
} catch let parseError {
print(parseError) // Log the error thrown by `JSONObjectWithData`
let jsonStr = String(data: data, encoding: NSUTF8StringEncoding)
print("Error could not parse JSON: '\(jsonStr)'")
}
}
task.resume()
I'd also suggest being a little more careful about forced unwrapping of data, because you want to detect/handle the errors, not crash. For example, above I use a guard statement to unwrap it.

Answer 2

In general if a function throws you have to write it inside a do catch block or just mark the outer scope function (in this case post) as throws:
func post(params : Dictionary<String, String>, url : String) {
let request = NSMutableURLRequest(URL: NSURL(string: url)!)
let session = NSURLSession.sharedSession()
request.HTTPMethod = "POST"
do {
request.HTTPBody = try NSJSONSerialization.dataWithJSONObject(params, options: .PrettyPrinted)
} catch {
//handle error. Probably return or mark function as throws
print(error)
return
}
request.addValue("application/json", forHTTPHeaderField: "Content-Type")
request.addValue("application/json", forHTTPHeaderField: "Accept")
let task = session.dataTaskWithRequest(request, completionHandler: {data, response, error -> Void in
// handle error
guard error == nil else { return }
print("Response: \(response)")
let strData = NSString(data: data!, encoding: NSUTF8StringEncoding)
print("Body: \(strData)")
let json: NSDictionary?
do {
json = try NSJSONSerialization.JSONObjectWithData(data!, options: .MutableLeaves) as? NSDictionary
} catch let dataError {
// Did the JSONObjectWithData constructor return an error? If so, log the error to the console
print(dataError)
let jsonStr = NSString(data: data!, encoding: NSUTF8StringEncoding)
print("Error could not parse JSON: '\(jsonStr)'")
// return or throw?
return
}
// The JSONObjectWithData constructor didn't return an error. But, we should still
// check and make sure that json has a value using optional binding.
if let parseJSON = json {
// Okay, the parsedJSON is here, let's get the value for 'success' out of it
let success = parseJSON["success"] as? Int
print("Succes: \(success)")
}
else {
// Woa, okay the json object was nil, something went worng. Maybe the server isn't running?
let jsonStr = NSString(data: data!, encoding: NSUTF8StringEncoding)
print("Error could not parse JSON: \(jsonStr)")
}
})
task!.resume()
}
If you don't want to handle these errors right in your post function you can just declare it as throws than you don't have to use do catch at all