update text field ui in swift ios

Question

i meet a question when i'm using swift of ios SDK 8

I used a callback to fetch data background and when data fetched, i want to set a text field value and update screen.

here is my code:

@IBOutlet var txtTest: UITextField!

@IBAction func login() {
    let username = "aaa";
    let password = "bbb";
    var res = ServiceClient.login(username, password: password){
        (res:String, error:String?) -> Void in if(error == nil){
            self.txtTest.text = res;
        }
    }
}

Run this code and the data is correctly fetched and the txtTest did not updated, but when i tap txtTest, the value will be shown. So is there any way to force update UI? or send a message to UI thread?

Answer 1

Same issue that everyone is having when wanting to do UI updates in Swift: do not ever update the UI in any secondary thread. And that means anything with closures. Since Swift is using closures so much, that issue is being seen a lot more but it isn't new.

See Swift Update Label (with HTML content) takes 1min for the correct way of doing things.

Answer 2

UIKit is not thread-safe and should only be updated from the main thread. Downloads are done on the background thread, and you cannot update UI from there. Try:

For swift 3 and swift 4

DispatchQueue.main.async {
    self.txtTest.text = "Your text"
}

Related duplicate question but with specific answer

iOS label does not update text with function in Swift