Why is the view not updated on time? - ios

Question

I'm trying to change an UIImageView alpha attribute on click. However, the change of the attribute is done after all the code-lines are run, and I want it to be as soon as the click is done. To simplify the code, I changed all the code-lines for a sleep(2) as it is giving the same effect.
The click function
#IBAction func click_btn() {
imageView.alpha = 0.5
sleep(2)
}
I've tried using DispatchQueue and async methods but the result is the same.
Here's the result, as you can see, when I click the button the Image doesn't change until the 2 sleep seconds pass. How can I change the alpha property immediately after the click?

Answer 1

That is the way the event loop on iOS (and Mac OS) works. UI changes are queued up as you run through your code, and then applied once you return and the app visits the event loop. Your delay is PREVENTING the change from taking place. If you get rid of the sleep it will work as intended.
Note that normally button taps fire on "touch up inside", which means the button doesn't respond until the user releases their finger. You can attach the action to a touch down event instead if you want.
What is it you actually want to accomplish?
Edit:
Rewrite your code like this:
#IBAction func click_btn() {
imageView.alpha = 0.5
DispatchQueue.main.async() {
//Code that takes 2 seconds to run
}
}
That will cause your function to return immediately, and your alpha change to take place. On the next pass through the event loop, the system will find your async task on the main queue, pick it up and start processing it. Note that for the 2 seconds it takes to run, the UI will freeze. It might be better to run the slow code on a background queue and only update the UI once the "slow bits" are finished:
#IBAction func click_btn() {
imageView.alpha = 0.5
DispatchQueue. global().async {
// Run the code that takes 2 seconds to run on a background thread
DispatchQueue.main.async() {
// Do your UI updates that have to run after the slow code finishes on the main thread.
}
}