如何以编程方式更新UILabel的约束

Question

I am creating a UILabel in stroyboard which frame is (15, 55, 250, 20) . According to my requirement I want to change that label frame to (15, 10, 250, 20) through coding. But there is no affect on that label.

So I update constraint through coding like below:

mylabel.frame = CGRectMake(15, 10, 250, 20);
[mylabel needsUpdateConstraints];

But it's not working. Please anybody help me. I am stuck on this from last 2 days.

I am adding these constraint through storyboard

Answer 1

Updating frames does not work well with AutoLayout. Create and animate constraints instead.

Here's how you can create a NSLayoutConstraint and add it to the view:

var myLabelConstraint = NSLayoutConstraint (item: myLabel,
        attribute: NSLayoutAttribute.Top, // You can also choose Width, Height, Bottom, Leading, Trailing
        relatedBy: NSLayoutRelation.Equal,
        toItem: self.view
        attribute: NSLayoutAttribute.Top,
        multiplier: 1,
        constant: 10)
    self.view.addConstraint(myLabelConstraint)

If you are creating a constraint for width and height, remember to change toItem into nil and attribute into NSLayoutAttribute.NotAnAttribute .

If you want to animate a constraint just change the constant of it:

myConstraint.constant = 15
self.view.layoutIfNeeded()

Answer 2

First, you should delete the one constraint to the right edge of the screen (45); you're over constraining your view.

When you want to change a constraint you made in the storyboard, you should make an IBOutlet to it, and modify its constant value in code. Since you want to change the y position, make an outlet to the vertical spacing constraint (the 10 one), I'll call it yCon for this example. You only need to do this,

self.yCon.constant = 55;
[self.view layoutIfNeeded];