Update the constant property of a constraint programmatically in Swift?

Question

I want to animate an object, so I declare a constraint and add it to the view. I then update the constant property of the constraint inside an UIView animation. Why doesn't this code move the object?

UIView.animateWithDuration(1, animations: {
     myConstraint.constant = 0   
     self.view.updateConstraints(myConstraint)
})

Solution

In order to declare an animation, you cannot re-define the constraint and call updateConstraints. You are supposed to change the constant of your constraint and follow the format below:

self.view.layoutIfNeeded()
UIView.animate(withDuration: 1) {
    self.sampleConstraint.constant = 20
    self.view.layoutIfNeeded()
}

Comments

[Mudit Jaju]

self.view.layoutIfNeeded() was exactly what I was looking for! Thanks a lot for this. Just a doubt .. does it have some kind of performance hit as well ?

[duan]

@MuditJaju If its not in UITableView，usually there's no performance problem.