How to programmatically disable constraint when animating - swift

Question

I am have a view with a fixed width anchored to the left. I want this view to animate (moving left to right) by anchoring it to the right (thus removing the left anchor). How do I do this?
I tried setting a priority but I am not sure of the syntax. I also tried to disable the constraint, but that did not work.
fileprivate func sparkle() {
let sparkleView = UIView()
sparkleView.backgroundColor = UIColor.yellow
sparkleView.alpha = 0.5
addSubview(sparkleView)
sparkleView.translatesAutoresizingMaskIntoConstraints = false
sparkleView.topAnchor.constraint(equalTo: topAnchor).isActive = true
sparkleView.bottomAnchor.constraint(equalTo: bottomAnchor).isActive = true
sparkleView.widthAnchor.constraint(equalToConstant: frame.width / 5).isActive = true
sparkleView.leftAnchor.constraint(equalTo: leftAnchor).priority = UILayoutPriorityDefaultLow // Doesn't work
sparkleView.leftAnchor.constraint(equalTo: leftAnchor).isActive = true
layoutIfNeeded()
sparkleView.leftAnchor.constraint(equalTo: leftAnchor).isActive = false // Doesn't work
sparkleView.rightAnchor.constraint(equalTo: self.rightAnchor).isActive = true
UIView.animate(withDuration: 2, animations: {
self.layoutIfNeeded()
}) { (_) in
sparkleView.removeFromSuperview()
}
}
Currently, the width constraint gets broken since we are assigning a left and right anchor.
Will attempt to recover by breaking constraint
<NSLayoutConstraint:0x6000020b8eb0 UIView:0x7fc846d551d0.width == 15.35 (active)>
-- EDIT --
I have tried creating an instance variable to hold the constraint.
let leftCons = sparkleView.leftAnchor.constraint(equalTo: leftAnchor)
leftCons.isActive = true
I then tried to modify the priority
leftCons.priority = UILayoutPriorityDefaultLow
However, this causes error
'Mutating a priority from required to not on an installed constraint (or vice-versa) is not supported. You passed priority 250 and the existing priority was 1000.'
Instead, I need to just set it as inactive (from the answers)
leftCons.active = false
-- Correct code if interested.. --
fileprivate func sparkle() {
let sparkleView = UIView()
sparkleView.backgroundColor = UIColor.yellow
sparkleView.alpha = 0.5
addSubview(sparkleView)
sparkleView.translatesAutoresizingMaskIntoConstraints = false
sparkleView.topAnchor.constraint(equalTo: topAnchor).isActive = true
sparkleView.bottomAnchor.constraint(equalTo: bottomAnchor).isActive = true
sparkleView.widthAnchor.constraint(equalToConstant: frame.width / 5).isActive = true
let leftCons = sparkleView.leftAnchor.constraint(equalTo: leftAnchor)
leftCons.isActive = true
layoutIfNeeded()
leftCons.isActive = false
sparkleView.rightAnchor.constraint(equalTo: self.rightAnchor).isActive = true
UIView.animate(withDuration: 0.5, animations: {
self.layoutIfNeeded()
}) { (_) in
sparkleView.removeFromSuperview()
}
}

Answer 1

You need to create a var
var leftCon:NSLayoutConstraint!
then alter it , you problem is every line create a new constraint
sparkleView.widthAnchor.constraint(equalToConstant: frame.width / 5).isActive = true
sparkleView.leftAnchor.constraint(equalTo: leftAnchor).isActive = true
sparkleView.rightAnchor.constraint(equalTo: self.rightAnchor).isActive = true
note: above 3 constraints create a conflict as the view can't be pinned to left and right at the same time with a width constraint that why you see the conflict break => NSLayoutConstraint:0x6000020b8eb0 UIView:0x7fc846d551d0.width == 15.35 (active)
regardless of the active assignment so those
sparkleView.leftAnchor.constraint(equalTo: leftAnchor).priority = UILayoutPriorityDefaultLow // Doesn't work
sparkleView.leftAnchor.constraint(equalTo: leftAnchor).isActive = false // Doesn't work
are on the fly and don't alter the created constraint , so to be
leftCon = sparkleView.leftAnchor.constraint(equalTo: leftAnchor)
leftCon.isActive = true
Regarding this crash
'Mutating a priority from required to not on an installed constraint (or vice-versa) is not supported
you need to set the priority before the activation
leftCon.priority = UILayoutPriorityDefaultLow
leftCon.isActive = true