Initialising UIView in main thread from a background thread

Question

In a global, async closure, I'd like to initialise a UIView on the main thread. I thought this code would possibly do it, but I get an Analyser warning:

UIView.init(frame:) must by used from the main thread only.

let view: UIView = { // Line with the warning
      DispatchQueue.main.sync {
          return UIView()
      }
}()

What's the purpose of doing such a thing? could you please mention what's your general goal? — Ahmad F, Oct 30 '17 at 14:35
I'm generating a PDF from some xibs. I need a container for them. This is the only line that gives an warning. — Josh Paradroid, Oct 30 '17 at 14:56
I think this is odd! Are you tried without `DispatchQueue.main.sync`? Why do you need this? — Mannopson, Oct 30 '17 at 15:08
I haven't because that would still be on the global queue and that's what I'm trying to avoid. — Josh Paradroid, Oct 30 '17 at 15:11
I have code later that depends upon the contents of view. Wouldn't async cause that value to be possibly unset at that point in time? — Josh Paradroid, Oct 30 '17 at 15:18
I'd like to know what the down-vote was for. I'm not disputing it, I'm just interested. — Josh Paradroid, Oct 30 '17 at 15:20
@JoshParadroid You're right! Honestly. It's not me. Good luck mate! — Mannopson, Oct 30 '17 at 15:22

score 3 · Accepted Answer · answered Oct 30 '17 at 15:08

I'll be helpful if u post more of the code so we could understand the context better. Anyway, this should work nicely:

class SomeViewController: UIViewController {

    private var customView: UIView!

    override func viewDidLoad() {
        super.viewDidLoad()
        setupCustomView()
    }

    private func setupCustomView() {
        DispatchQueue.global().async {
            /*
             1. Do some preperation / data init' on background thread.
             2. When ready, perform UI dependent initialization on main thread.
             */
            DispatchQueue.main.async {

                /* Initialize `customView` on the main queue. for example: */
                self.customView = UIView()
                self.view.addSubview(self.customView)
                self.customView.backgroundColor = .red
                self.customView.frame = CGRect(x: 0, y: 0, width: 100, height: 100)
            }
        }
    }
}

I hope that helps.

Using a completion handler would be cleaner. Technically, I would recommend split it into two functions anyway. — Sulthan, Oct 30 '17 at 15:14
@Sulthan, I agree. My code wasn't by 'best practice' standards but it should be executed as expected. — DanielH, Oct 30 '17 at 15:17

score 0 · Answer 2 · answered Oct 31 '17 at 05:37

Try something like that:

You can keep using your own implementation, without DispatchQueue.main.sync

let view: UIView = { 
      return UIView()
}()

However you can go back to the main thread anytime:

DispatchQueue.main.async {
  self.view.addSubview(view) 
}

Use the .async, it's gives you an independent thread

Initialising UIView in main thread from a background thread

2 Answers2