How to parse JSON in Swift using NSURLSession

Don’t declare an AnyObject type for your decoded object since you want it to be an NSDictionary and you’re performing a conversion to do this.

Also it’s better to use zero options for NSJSONSerialization instead of random ones.

In my example I’ve also used a custom error type just for demonstration.

Note, if you’re using a custom error type, you have to also include a generic catch to be exhaustive (in this example, with a simple downcasting to NSError).

enum JSONError: String, ErrorType {
    case NoData = "ERROR: no data"
    case ConversionFailed = "ERROR: conversion from JSON failed"
}

func jsonParser() {
    let urlPath = "http://headers.jsontest.com/"
    guard let endpoint = NSURL(string: urlPath) else {
        print("Error creating endpoint")
        return
    }
    let request = NSMutableURLRequest(URL:endpoint)
    NSURLSession.sharedSession().dataTaskWithRequest(request) { (data, response, error) in
        do {
            guard let data = data else {
                throw JSONError.NoData
            }
            guard let json = try NSJSONSerialization.JSONObjectWithData(data, options: []) as? NSDictionary else {
                throw JSONError.ConversionFailed
            }
            print(json)
        } catch let error as JSONError {
            print(error.rawValue)
        } catch let error as NSError {
            print(error.debugDescription)
        }
   }.resume()
}

The same with Swift 3.0.2:

enum JSONError: String, Error {
    case NoData = "ERROR: no data"
    case ConversionFailed = "ERROR: conversion from JSON failed"
}

func jsonParser() {
    let urlPath = "http://headers.jsontest.com/"
    guard let endpoint = URL(string: urlPath) else {
        print("Error creating endpoint")
        return
    }
    URLSession.shared.dataTask(with: endpoint) { (data, response, error) in
        do {
            guard let data = data else {
                throw JSONError.NoData
            }
            guard let json = try JSONSerialization.jsonObject(with: data, options: []) as? NSDictionary else {
                throw JSONError.ConversionFailed
            }
            print(json)
        } catch let error as JSONError {
            print(error.rawValue)
        } catch let error as NSError {
            print(error.debugDescription)
        }
    }.resume()
}