NSLayoutConstraint not animating - animation

Question

Everything animates fine from this code except for the mainViewConstraint. I am trying to make the mainView slide in from the top as it appears by transforming the mainViewConstraint coordinate from -195 to 0. Unfortunately, it is not moving from -195 to 0. It just starts to appear at 0.
import UIKit
class FirstViewController: UIViewController {
#IBOutlet weak var bgImage: UIImageView!
#IBOutlet weak var mainView: UIView!
#IBOutlet weak var titleLabel: UILabel!
#IBOutlet weak var findButton: UIButton!
#IBOutlet weak var mainViewConstraint: NSLayoutConstraint!
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
mainViewConstraint.constant = -195
for i in [mainView, titleLabel, findButton] {
i?.alpha = 0
}
UIView.animate(withDuration: 1, animations: {
}) { (true) in
self.animateView()
}
}
func animateView() {
UIView.animate(withDuration: 2, animations: {
self.mainView.alpha = 1
self.mainViewConstraint.constant = 0
self.view.layoutIfNeeded()
}) { (true) in
self.animateLbl()
}
}
func animateLbl() {
UIView.animate(withDuration: 1, animations: {
self.titleLabel.alpha = 1
}) { (true) in
self.animateBtn()
}
}
func animateBtn() {
UIView.animate(withDuration: 1) {
self.findButton.alpha = 1
}
}
}

Answer 1

In your code, you're calling animateView() inside an animation block, which seems to be the problem.
Moving it out of the animation block should fix the problem.
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
mainViewConstraint.constant = -195
for i in [mainView, titleLabel, findButton] {
i?.alpha = 0
}
self.animateView()
}

Answer 2

To fix this, I changed the location of the mainViewConstraint to -195 in Storyboards and made the following changes to the code:
mainViewConstraint.constant = 0 //-195
for i in [mainView, titleLbl, findBtn] {
i?.alpha = 0
}
UIView.animate(withDuration: 1, animations: {
self.bgImage.alpha = 1
}) { (true) in
self.animateView()
}
}
func animateView() {
UIView.animate(withDuration: 2, animations: {
self.mainView.alpha = 1
//self.mainViewConstraint.constant = 0
self.view.layoutIfNeeded()