Programmatically Change multiplier of Alignment Constraint of Center X / Y

Question

How can I programmatically change the multiplier in the simplest way?

For Swift 2.0

Answer 1

So for Y, if you set the top of the image equal with a constant of 0 to the top of the superView. then enter this code:

@IBOutlet weak var topc: NSLayoutConstraint!


let heightOfSuperview = self.view.bounds.height
topc.constant = heightOfSuperview * 0.90 // this has the same effect as multiplier  

This is equivalent of Center.y multiplier = 0.9:1

Answer 2

I try to use extension but it not work, but change to the global utility function and it work for me:

public class func changeMultiplier(constraint: NSLayoutConstraint, multiplier: CGFloat) -> NSLayoutConstraint {
    let newConstraint = NSLayoutConstraint(
        item: constraint.firstItem,
        attribute: constraint.firstAttribute,
        relatedBy: constraint.relation,
        toItem: constraint.secondItem,
        attribute: constraint.secondAttribute,
        multiplier: multiplier,
        constant: constraint.constant)

    newConstraint.priority = constraint.priority
    NSLayoutConstraint.deactivateConstraints([constraint])
    NSLayoutConstraint.activateConstraints([newConstraint])
    return newConstraint
}

Answer 3

Simple answer, no extensions required. I tried for my case, worked fine for me.

So as multiplier is a get only property, we can simply set multiplier in the following way :

yourConstraintOutlet.setValue(yourDesiredMultiplierValue, forKey: "multiplier")

yourConstraintOutlet.setValue(0.75, forKey: "multiplier")

No need to write extensions or multiply by height of superview.