How to change bottom layout constraint in iOS, Swift

Question

I have scroll view as @IBOutlet

@IBOutlet weak var mainScrollView: UIScrollView!

I want to change the

"Bottom space to: Bottom Layout Guide" 

constraint programmatically.

First Item : Bottom Layout Guide.Top
Relation : Equal
Second Item: Scroll View.Bottom
Constant: 0 -> 50 // (I want to change this programmatically)
Priority: 1000
Multiplier: 1

How can I do this?

Answer 1

Take the constraint as IBOutlet of NSLayoutConstraint.

Set the constraint outlets and change constant value by :

self.sampleConstraint.constant = 20
self.view.layoutIfNeeded()

Answer 2

If you are adding constraint programatically like this:

var constraintButton = NSLayoutConstraint (item: buttonPlay, 
                                           attribute: NSLayoutAttribute.Bottom, 
                                           relatedBy: NSLayoutRelation.Equal, 
                                           toItem: self.view, 
                                           attribute: NSLayoutAttribute.Bottom, 
                                           multiplier: 1,
                                           constant: 0)
// Add the constraint to the view
self.view.addConstraint(constraintButton)

Then you can update it this way:

self.constraintButton.constant = 50
self.view.layoutIfNeeded()

And if you want that with animation you can do it this way:

self.view.layoutIfNeeded()
UIView.animateWithDuration(1, animations: {
    self.constraintButton.constant = 50
    self.view.layoutIfNeeded()
})

Hope it helps.