How does multithreading interact with UIKit?

UIKit is not thread-safe. All user interface updates (for example, updating UILabel.text or hiding a UIView) must be performed on the main thread.

Attempting to modify the UI from a background thread will lead to errors or unpredictable app behavior.

For executing long or blocking operations (such as network requests or image processing), background threads should be used (for example, via DispatchQueue.global()) and then return to the main thread to update the UI using DispatchQueue.main.async.

Example:

// Simulating a long operation on a background thread
DispatchQueue.global(qos: .userInitiated).async {
    // Perform heavy work...
    Thread.sleep(forTimeInterval: 2)

    // Get the result
    let newText = "Updated!"

    // Return to the main thread to update UI
    DispatchQueue.main.async {
        // Update UILabel on the main thread
        self.myLabel.text = newText
    }
}

Using OperationQueue also allows managing task execution on background threads and specifying which thread to use for completion operations.