Multithreading in Swift 3 with GCD - multithreading

Question

I'm working on a webservice based app and I've come to a crashing hault due to sending requests too quickly to the webservice. I simply can not getting GCD working in Swift 3 and I'm scratching my head. I've decided to dumb it down and just try loading 4 web images to a web view in order. Based on everything I'm seeing online the following code should work, but it is still freezing the UI until all four images load. What am I doing wrong?
import UIKit
let imageURLs = ["http://www.planetware.com/photos-large/F/france-paris-eiffel-tower.jpg", "http://adriatic-lines.com/wp-content/uploads/2015/04/canal-of-Venice.jpg", "http://hd-wall-papers.com/images/wallpapers/hi-resolution-pictures/hi-resolution-pictures-5.jpg", "http://hd-wall-papers.com/images/wallpapers/hi-resolution-pictures/hi-resolution-pictures-1.jpg"]
class Downloader {
class func downloadImageWithURL(_ url:String) -> UIImage! {
let data = try? Data(contentsOf: URL(string: url)!)
return UIImage(data: data!)
}
}
class ViewController: UIViewController {
#IBOutlet weak var imageView1: UIImageView!
#IBOutlet weak var imageView2: UIImageView!
#IBOutlet weak var imageView3: UIImageView!
#IBOutlet weak var imageView4: UIImageView!
#IBOutlet weak var sliderValueLabel: UILabel!
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}
#IBAction func didClickOnStart(_ sender: AnyObject) {
let serialQueue = DispatchQueue(label: "syncQueue")
serialQueue.sync{
let img1 = Downloader.downloadImageWithURL(imageURLs[0])
DispatchQueue.main.async(execute: {
self.imageView1.image = img1
})
}
serialQueue.sync{
let img2 = Downloader.downloadImageWithURL(imageURLs[1])
DispatchQueue.main.async(execute: {
self.imageView2.image = img2
})
}
serialQueue.sync{
let img3 = Downloader.downloadImageWithURL(imageURLs[2])
DispatchQueue.main.async(execute: {
self.imageView3.image = img3
})
}
serialQueue.sync{
let img4 = Downloader.downloadImageWithURL(imageURLs[3])
DispatchQueue.main.async(execute: {
self.imageView4.image = img4
})
}
}
#IBAction func sliderValueChanged(_ sender: UISlider) {
self.sliderValueLabel.text = "\(sender.value * 100.0)"
}
}

Answer 1

freezing the UI
Because you are calling serialQueue.sync. You almost never want to call sync, and in this case you certainly don't. Use async instead.