I understand that all UI updates must be done from Main thread.
But purely for the sake of deeper understanding how GCD and dispatch main work:
I have a button that runs a network call and in its completionHandler I eventually do:
self.layer.borderColor = UIColor(red: 255/255.0, green: 59/255.0, blue: 48/255.0, alpha: 1.0).cgColor
self.layer.borderWidth = 3.0
For the color change to happen it takes 6-7 seconds. Obviously if run the above code from main thread it would change the border color immediately.
Question1 even though I don't have ANY other code to run, why doesn't the UI changes happen immediately from the background thread? What is waiting for?
Interesting though is that if I click the button to make the network call and then tap on the textField itself (before the 6-7 seconds), the border color would change immediately.
Is that happening because of:
From the background thread I've updated the model ie change the textField color which queues the UI/view to be updated...but since we're on a background queue, that UI updated could take a few seconds to happen
But then I tapped on the textField right away and forced a super quick read of the textField and all its properties which includes the border—from main thread (actual user touches are always handled through main thread)...which even though are not yet red on the screen, but since it's red on the model it will read from it and change color to red immediately.
Question2: Is that observation correct?
If I don't tap and just wait:
If I tap:
My full code is as below:
import UIKit
class ViewController: UIViewController {
@IBOutlet weak var textField: UITextField!
@IBAction func isValid(_ sender: Any) {
let userEmail = textField.text
let requestURL = NSURL(string: "https://jsonplaceholder.typicode.com")
var request = URLRequest(url: requestURL as! URL)
request.httpMethod = "POST"
let postString = "Anything"
request.httpBody = postString.data(using: .utf8)
let task = URLSession.shared.dataTask(with: request) { data, response, error in
guard let data = data, error == nil else {
print("error=\(error)")
return
}
if let httpStatus = response as? HTTPURLResponse, httpStatus.statusCode != 200 {
}
do {
let json = try? JSONSerialization.jsonObject(with: data, options: [])
if let _ = json as? [String: Any] {
self.textField.layer.borderColor = UIColor(red: 255/255.0, green: 59/255.0, blue: 48/255.0, alpha: 1.0).cgColor
self.textField.layer.borderWidth = 3.0
}
} catch let error as NSError {
print(error)
}
}
task.resume()
}
override func viewDidLoad() {
super.viewDidLoad()
}
}