How to switch between two auto-layout constraints?

Question

I have two UI layout constraints that are conflicting with each other by design. Only one of could be active at a time.

In UIViewController's method updateConstraintsIfNeeded, I have the following code which toggles between the two constraints, depending on the state of a data model.

override func updateConstraintsIfNeeded() {
    super.updateConstraintsIfNeeded()

    if question?.thumbURL != nil {
        showAttachmentConstraint.active = true
        hideAttachmentConstraint.active = false        
    } else {
        showAttachmentConstraint.active = false
        hideAttachmentConstraint.active = true
    }
}

This work as intended, but I got this familiar warning in the debug output:

Unable to simultaneously satisfy constraints. Probably at least one of the constraints in the following list is one you don't want. ...

Apparently when the statement showAttachmentConstraint.active = true is executed, it temporarily conflicts with hideAttachmentConstraint which is still active at that time.

Is it possible to make this toggle operation atomic? I'm hoping there is something like beginUpdate and endUpdate in UITableView.

Answer 1

you could change the priority of one of the conflicting constraints to 999 instead of 1000. so you do not even have any problems at design time.