Animate Constraint Relative to Another One - ios

Question

I setup an animation to hide one switch/label when another one is turned on. Simultaneously the switch that was just turned on move up. This works great with the simple explanation here.
However, when I try to move the switch/label back down after it is turned off it doesn't budge. The other switch reappears fine, but the top constraint change doesn't fire.
I'm relatively new to doing this type of setup and animating all programmatically and after spending an hour on this I'm stumped. Is it because I'm animating a top constraint relative to another one? How does that matter if it works the first time around? Even though the alpha of the hidden switch is set to zero, its frame is still there, right? Or am I doing something simple stupidly?
// Works Perfectly!
func hideVeg() {
self.view.layoutIfNeeded()
UIView.animate(withDuration: 1, delay: 0, options: [.curveEaseIn], animations: {
self.vegetarianSwitch.alpha = 0
self.vegetarianLabel.alpha = 0
self.veganSwitch.topAnchor.constraint(equalTo: self.vegetarianSwitch.bottomAnchor, constant: -30).isActive = true
self.view.layoutIfNeeded()
})
}
// Showing the label and switch works, but the topAnchor constraint never changes!
func showVeg() {
self.view.layoutIfNeeded()
UIView.animate(withDuration: 1, delay: 0, options: [.curveEaseIn], animations: {
self.vegetarianSwitch.alpha = 1
self.vegetarianLabel.alpha = 1
// This is the constraint that doesn't change.
// This is exactly what it was set to before the other hideVeg() runs.
self.veganSwitch.topAnchor.constraint(equalTo: self.vegetarianSwitch.bottomAnchor, constant: 40).isActive = true
self.view.layoutIfNeeded()
})
}

Answer 1

The issue here is that you are not modifying the constraints but actually creating new constraints with each animation. What you want to do instead is just create the constraint once (you can do it in code or in Interface Builder and drag and outlet). You can then just change the .constant field of the existing constraint in your animation block.

Answer 2

The constant needs to be changed with the animation, not creating an entirely new constraint. The old constraint still exists causing the issue.
var veganTopConstraint = NSLayoutConstraint()
// Top Constraint set up this way so it can be animated later.
veganTopConstraint = veganSwitch.topAnchor.constraint(equalTo: vegetarianSwitch.bottomAnchor, constant: 40)
veganTopConstraint.isActive = true
func hideVeg() {
UIView.animate(withDuration: 1, delay: 0, options: [.curveEaseIn], animations: {
self.vegetarianSwitch.alpha = 0
self.vegetarianLabel.alpha = 0
self.veganTopConstraint.constant = -30
self.view.layoutIfNeeded()
})
}
func showVeg() {
self.view.layoutIfNeeded()
UIView.animate(withDuration: 1, delay: 0, options: [.curveEaseIn], animations: {
self.vegetarianSwitch.alpha = 1
self.vegetarianLabel.alpha = 1
self.veganTopConstraint.constant = 40
self.view.layoutIfNeeded()
})
}