Changing the constant of a NSLayoutConstraint - ios

Question

Background
I'm trying to do a fairly standard NSLayoutConstraint constant update, with both an animated and non-animated option. To give you some background, I have a UIView subclass called ProgressView. Within the ProgressView, there is the Progress Bar UIView. The Progress Bar UIView is connected as a referencing outlet progressBar to the ProgressView class. Here's how the fairly simple hierarchy looks within the Storyboard:
As you might have guessed, I'm building a custom progress bar. The ProgressView UIView is the "track," and the Progress Bar UIView is the "fill." The plan is to use the trailing constraint of the Progress Bar UIView to make changes to the progress.
Existing configuration
The constraints for the Progress Bar UIView are set up to be flush with the superview (Progress View):
The trailing constraint of the Progress Bar UIView is connected to the ProgressView class as a strong outlet: #IBOutlet var trailingConstraint: NSLayoutConstraint!.
Within the ProgressView class, there is a setProgress function. This function is called from the ViewController which contains the ProgressView. Here's the function with comments explaining how it works:
public func setProgress(_ progress: Double, animationDuration: Double) {
// Pre-conditions
guard progress >= 0.0 && progress <= 1.0 && animationDuration >= 0.0 else {
return
}
// Calculate the new constraint constant based on the progress
let newConstant = CGFloat(1 - progress) * self.bounds.size.width
// If the update should be made without animation, just make the change and return
guard animationDuration > 0.0 else {
trailingConstraint.constant = newConstant
return
}
// Set the constraint first
self.trailingConstraint.constant = newConstant
// Animate!
UIView.animate(withDuration: animationDuration) {
self.layoutIfNeeded()
}
}
The problem
As you can see, this function can update the progress with and without animation. However, when called from the ViewController, neither works. The trailing constraint constant appears to remain 0, and the progress bar fills the entire track.
Attempted solutions
I have tried calling layoutIfNeeded() and setNeedsLayout() in every position and configuration where it makes sense (on both self [where self is the ProgressView] and on progressBar). I have tried putting the code explicitly on the main thread with DispatchQueue.main.async. As a ground truth test, I tried connecting the trailingConstraint outlet to the ViewController itself and updating the constant from the viewDidAppear() and viewDidLayoutSubviews() methods. Nothing works.
A possible hint in the right direction
Here's the weird part. The progress bar appears filled on the screen, but when I debug the view hierarchy, the progress bar looks correct. The trailingConstraint constant seems to have been set correctly.
I was testing on an iOS 10 device and running Xcode 8.3.3. This issue seems really odd to me, and I'm not quite sure what to do from here. Thanks for your help.

Answer 1

I faced the same problem.My problem was solved by changing constant in animation .Try this out:
UIView.animate(withDuration: 0.2) {
self.trailingConstraint.constant = newConstant
self.layoutIfNeeded()
}