Xcode UIView.init(frame:) must be used from main thread only

Question

I'm trying to render some views in background thread to not affect the main thread. That was never a problem before Xcode 9.

DispatchQueue.global(qos: .background).async {
    let customView = UIView(frame: .zero)
    DispatchQueue.main.async {
        self.view.addSubview(customView)
    }
}

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

This error occurs in the second line.

Update

The Apple UIView Documentation actually says in the Threading Considerations section:

Manipulations to your application’s user interface must occur on the main thread. Thus, you should always call the methods of the UIView class from code running in the main thread of your application. The only time this may not be strictly necessary is when creating the view object itself, but all other manipulations should occur on the main thread.

Solution 1

Xcode 9 has a new runtime Main Thread Checker that detects call to UIKit from a background thread and generate warnings.

I know its meant to generate warnings and not crash the app, but you can try disabling Main Thread Checker for your test target.

I tried this code in a sample project, the debugger paused at the issue (as it is supposed to), but the app didn't crash.

override func viewDidLoad() {
    super.viewDidLoad()

    DispatchQueue.global().async {
        let v = UIView(frame: .zero)
    }
}

Solution 2

Main Thread Entry

You can enter the main thread as follows

DispatchQueue.main.async { 
    // UIView usage
}

Solution 3

You can use this function

func downloadImage(urlstr: String, imageView: UIImageView) {
    let url = URL(string: urlstr)!
    let task = URLSession.shared.dataTask(with: url) { data, _, _ in
        guard let data = data else { return }
        DispatchQueue.main.async { // Make sure you're on the main thread here
            imageview.image = UIImage(data: data)
        }
    }
    task.resume()
}

How to use this function?

downloadImage(urlstr: "imageUrl", imageView: self.myImageView)