Swift default AlertViewController breaking constraints

Question

I am trying to use a default AlertViewController with style .actionSheet. For some reason, the alert causes a constraint error. As long as

Answer 1

The solution for Objective-C:

1. Subclass your own Alert Controller from UIAlertController

2. Define prune-function like in previous reply

   @implementation TemplateAlertController
   
   -(void) viewDidLoad {
   
       [super viewDidLoad];
       [self mPruneNegativeWithConstraints];
   }
   
   -(void) mPruneNegativeWithConstraints {
   
       for (UIView* iSubview in [self.view subviews]) {
           for (NSLayoutConstraint* iConstraint in [iSubview constraints]) {
               if ([iConstraint.debugDescription containsString:@"width == - 16"]) {
                   [iSubview removeConstraint:iConstraint];
               }
           }
       }
   }
   
   @end
   

Answer 2

It's a new bug in iOS versions:

• 12.2
• 12.3
• 12.4
• 13.0
• 13.1
• 13.2
• 13.2.3
• 13.3
• 13.4
• 13.4.1
• 13.5
• 13.6
• 14.0

The only thing we can do is to file a bug report to Apple (I just did that and you should too).

I'll try to update answer for a new version(s) of iOS when it come out.

Answer 3

Safe Solution

You should not remove the constraint because it is used in the future with a correct value.

As an alternative, you can change its constant to a positive value:

class PXAlertController: UIAlertController {
    override func viewDidLoad() {
        super.viewDidLoad()

        for subview in self.view.subviews {
            for constraint in subview.constraints {
                if constraint.firstAttribute == .width && constraint.constant == -16 {
                    constraint.constant = 10 // Any positive value
                }
            }
        }
    }
}

And then to initialize your controller use:

let controller = PXAlertController(title: "Title", message: "Message", preferredStyle: .actionSheet)

Answer 4

Interesting ideas here. Personally I don't like the idea of deleting the constraint or changing it's value (size).

As the issue hinges on the constraint resolution being forced into a position where it must break a mandated (priority 1000) constraint, a less brutal approach is just to tell the framework that this constraint could be broken if needed.

So (based on Josh's "Safe" class):

class PXAlertController: UIAlertController {
    override func viewDidLoad() {
        super.viewDidLoad()
        tweakProblemWidthConstraints()
    }
    
    func tweakProblemWidthConstraints() {
        for subView in self.view.subviews {
            for constraint in subView.constraints {
                // Identify the problem constraint
                // Check that it's priority 1000 - which is the cause of the conflict.
                if constraint.firstAttribute == .width &&
                    constraint.constant == -16 &&
                    constraint.priority.rawValue == 1000 {
                    // Let the framework know it's okay to break this constraint
                    constraint.priority = UILayoutPriority(rawValue: 999)
                }
            }
        }
    }
}

This has the advantages that it doesn't change any layout dimensions, it also stands a good chance of being well behaved in the event of a fix in the framework.

Tested in iPhone SE simulator (which was giving me my original problem) - constraint related debug has gone.

Answer 5

An alternative way of getting away from the NSLayoutConstraint bug, is to use preferredStyle: .alert instead of preferredStyle: .actionSheet. This works without generating warnings, but it will display the menu modally.

Answer 6

The following removes the warning without needing to disable animation. And assuming Apple eventually fixes the root cause of the warning, it shouldn't break anything else.

extension UIAlertController {
    func pruneNegativeWidthConstraints() {
        for subView in self.view.subviews {
            for constraint in subView.constraints where constraint.debugDescription.contains("width == - 16") {
                subView.removeConstraint(constraint)
            }
        }
    }
}

This can then be used like this:

// After all addActions(...), just before calling present(...)
alertController.pruneNegativeWidthConstraints()