Here it is. So the extra two options usingSpringWithDamping and initialSpringVelocity do what they say: they give the animation a spring effect. To find more simply copy this code into xcode and option-click on either of the properties and their descriptions will pop up.
func animateLabelTransitions() {
view.layoutIfNeeded()
let screenWidth = view.frame.width
self.nextQuestionLabelCenterXConstraint.constant = 0
self.currentQuestionLabelCenterXConstraint.constant += screenWidth
UIView.animate(withDuration: 0.5, animations: {
self.currentQuestionLabel.alpha = 0
self.nextQuestionLabel.alpha = 1
})
UIView.animate(withDuration: 0.5,
delay: 0,
usingSpringWithDamping: 0.5,
initialSpringVelocity: 0.5,
options: [],
animations: {
self.currentQuestionLabel.alpha = 0
self.nextQuestionLabel.alpha = 1
self.view.layoutIfNeeded()
},
completion: { _ in
swap(&self.currentQuestionLabel, &self.nextQuestionLabel)
swap(&self.currentQuestionLabelCenterXConstraint, &self.nextQuestionLabelCenterXConstraint)
self.updateOffScreenLabel()
}
)
}