Swift default AlertViewController breaking constra

问题:

I am trying to use a default AlertViewController with style .actionSheet. For some reason, the alert causes a constraint error. As long as the alertController is not triggered (displayed) through a button, there are no constraint errors on the whole view. Could it be that this is a bug of Xcode?

The exact error I get looks like this:

2019-04-12 15:33:29.584076+0200 Appname[4688:39368] [LayoutConstraints] Unable to simultaneously satisfy constraints.
    Probably at least one of the constraints in the following list is one you don't want. 
    Try this: 
        (1) look at each constraint and try to figure out which you don't expect; 
        (2) find the code that added the unwanted constraint or constraints and fix it. 
(
    "<NSLayoutConstraint:0x6000025a1e50 UIView:0x7f88fcf6ce60.width == - 16   (active)>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x6000025a1e50 UIView:0x7f88fcf6ce60.width == - 16   (active)>

This is the code I use:

@objc func changeProfileImageTapped(){
        print("ChangeProfileImageButton tapped!")
        let alert = UIAlertController(title: "Change your profile image", message: nil, preferredStyle: .actionSheet)

        alert.addAction(UIAlertAction(title: "Photo Library", style: .default, handler: nil))
        alert.addAction(UIAlertAction(title: "Online Stock Library", style: .default, handler: nil))
        alert.addAction(UIAlertAction(title: "Cancel", style: .cancel, handler: nil))
        alert.view.tintColor = ColorCodes.logoPrimaryColor

        self.present(alert, animated: true)
    }

As you can see, it is very basic. That's why I am very confused about the strange behavior I get as this default implementation should not cause any errors, right?

Although, through breaking the constraints, the alert displays properly on all screen sizes I would be really thankful for any help I get.

回答1:

This error is not critical, seems to be unfixed bug form Apple. This constraint appears in animation style just after presenting. I tried to catch and change it (change values, relations, priority) before presenting – no success because of this dynamically added constraints.

When you turn off animation in self.present(alert, animated: false) and using alert.view.addSubview(UIView()) – the error disappears. I can't explain it, but it works!

let alert = UIAlertController(title: "Change your profile image", message: nil, preferredStyle: .actionSheet)

alert.addAction(UIAlertAction(title: "Photo Library", style: .default, handler: nil))
alert.addAction(UIAlertAction(title: "Online Stock Library", style: .default, handler: nil))
let cancel = UIAlertAction(title: "Cancel", style: .destructive, handler: nil)

alert.addAction(cancel)
alert.view.addSubview(UIView()) // I can't explain it, but it works!

self.present(alert, animated: false)

回答2:

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()

回答3:

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

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.