Swift 3: GCD and URLSessionDataTask with a completionHandler

Swift3GCDURLSessionDataTask.swift

//: # Swift 3: CGD and URLSessionDataTask
import Foundation
import PlaygroundSupport


//: ### Create background queue
let queue = DispatchQueue.global(qos: .background)

//: ### Computed variable
var time:DispatchTime! {
    return DispatchTime.now() + 1.0 // seconds
}

//: ### URLSession initialization
let sessionConfiguration = URLSessionConfiguration.default
let session = URLSession(configuration: sessionConfiguration)
//: ### JSON Parsing
func parseJSON(from data: Data) -> Any? {
    do {
        let json = try JSONSerialization.jsonObject(with: data, options: .mutableContainers)
        return json
    } catch {
        return nil
    }
}


//: ### Call a closure on a queue
queue.async(execute: {
    print(">>> [\(queue.label)]: At the queue start")


    //: ### Create a URLDataTask for an URL
    //: Pass a `completionHandler` to (optionally) receive the raw data, response details and error
    if let url = URL(string: "http://www.sthoughts.com/dl/categories_data.json") {
        let task = session.dataTask(with: url, completionHandler: {
            data, response, error in

            if let e = error {
                print(">>>>>> [URLDataTask completion]: \(e.localizedDescription)")
                return
            }

            if let d = data, let dataStr = String(data: d, encoding: .utf8) {
                print(">>> [URLDataTask completion]: Got data, parsed result comes in a second, errors are shown immediately")
                if let json = parseJSON(from: d) {

                    //: ### DispatchQueue delay
                    //: pass a `DispatchTime` object and a closure to get executed when the time arrives
                    DispatchQueue.main.asyncAfter(deadline: time, execute: {
                        print(">>> [Main Queue]: Here's the JSON")
                        print("\n    \(dataStr)")
                        print(">>> [Main Queue]: Here's the parsed object\n")
                        debugPrint(json)
                        PlaygroundPage.current.finishExecution()
                    })
                } else {
                    print(">>> [URLDataTask completion]: Error parsing JSON")
                    debugPrint(response)
                    print(dataStr)
                    PlaygroundPage.current.finishExecution()
                }


            } else {

                //: ### Show the NSURLResponse data
                //: In case the URL returns a document without a body
                print(">>> \(queue.label): No data. Here's the response")
                debugPrint(response)
                PlaygroundPage.current.finishExecution()
            }
        })

        //: ### Start the request
        task.resume()



    } else {
        print(">>> [\(queue.label)]: Error: Cannot compose the URL")
        PlaygroundPage.current.finishExecution()
    }
    
    print(">>> [\(queue.label)]: At the queue end. Goodbye.")
})


PlaygroundPage.current.needsIndefiniteExecution = true