Programmatically Add Centerx/Centery Constraints

Programmatically Add CenterX/CenterY Constraints

Update for Swift 3/Swift 4:

As of iOS 8, you can and should activate your constraints by setting their isActive property to true. This enables the constraints to add themselves to the proper views. You can activate multiple constraints at once by passing an array containing the constraints to NSLayoutConstraint.activate()

let label = UILabel(frame: CGRect.zero)
label.text = "Nothing to show"
label.textAlignment = .center
label.backgroundColor = .red  // Set background color to see if label is centered
label.translatesAutoresizingMaskIntoConstraints = false
self.tableView.addSubview(label)

let widthConstraint = NSLayoutConstraint(item: label, attribute: .width, relatedBy: .equal,
                                         toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 250)

let heightConstraint = NSLayoutConstraint(item: label, attribute: .height, relatedBy: .equal,
                                          toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 100)

let xConstraint = NSLayoutConstraint(item: label, attribute: .centerX, relatedBy: .equal, toItem: self.tableView, attribute: .centerX, multiplier: 1, constant: 0)

let yConstraint = NSLayoutConstraint(item: label, attribute: .centerY, relatedBy: .equal, toItem: self.tableView, attribute: .centerY, multiplier: 1, constant: 0)

NSLayoutConstraint.activate([widthConstraint, heightConstraint, xConstraint, yConstraint])

Better Solution:

Since this question was originally answered, layout anchors were introduced making it much easier to create the constraints. In this example I create the constraints and immediately activate them:

label.widthAnchor.constraint(equalToConstant: 250).isActive = true
label.heightAnchor.constraint(equalToConstant: 100).isActive = true
label.centerXAnchor.constraint(equalTo: self.tableView.centerXAnchor).isActive = true
label.centerYAnchor.constraint(equalTo: self.tableView.centerYAnchor).isActive = true

or the same using NSLayoutConstraint.activate():

NSLayoutConstraint.activate([
    label.widthAnchor.constraint(equalToConstant: 250),
    label.heightAnchor.constraint(equalToConstant: 100),
    label.centerXAnchor.constraint(equalTo: self.tableView.centerXAnchor),
    label.centerYAnchor.constraint(equalTo: self.tableView.centerYAnchor)
])

Note: Always add your subviews to the view hierarchy before creating and activating the constraints.

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Original Answer:

The constraints make reference to self.tableView. Since you are adding the label as a subview of self.tableView, the constraints need to be added to the "common ancestor":

   self.tableView.addConstraint(xConstraint)
   self.tableView.addConstraint(yConstraint)

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As @mustafa and @kcstricks pointed out in the comments, you need to set label.translatesAutoresizingMaskIntoConstraints to false. When you do this, you also need to specify the width and height of the label with constraints because the frame no longer is used. Finally, you also should set the textAlignment to .Center so that your text is centered in your label.

    var  label = UILabel(frame: CGRectZero)
    label.text = "Nothing to show"
    label.textAlignment = .Center
    label.backgroundColor = UIColor.redColor()  // Set background color to see if label is centered
    label.translatesAutoresizingMaskIntoConstraints = false
    self.tableView.addSubview(label)

    let widthConstraint = NSLayoutConstraint(item: label, attribute: .Width, relatedBy: .Equal,
        toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: 250)
    label.addConstraint(widthConstraint)

    let heightConstraint = NSLayoutConstraint(item: label, attribute: .Height, relatedBy: .Equal,
        toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: 100)
    label.addConstraint(heightConstraint)

    let xConstraint = NSLayoutConstraint(item: label, attribute: .CenterX, relatedBy: .Equal, toItem: self.tableView, attribute: .CenterX, multiplier: 1, constant: 0)

    let yConstraint = NSLayoutConstraint(item: label, attribute: .CenterY, relatedBy: .Equal, toItem: self.tableView, attribute: .CenterY, multiplier: 1, constant: 0)

    self.tableView.addConstraint(xConstraint)
    self.tableView.addConstraint(yConstraint)

How to programmatically add constraint center with multiplier

So it's possible, I missused the centerXAnchor instead of using .centerX

Also the order in which I called each item was not correct:

// Not Working
NSLayoutConstraint(item: self, attribute: centerXAnchor, relatedBy: .equal, toItem: buttonLeft, attribute: centerXAnchor, multiplier: 0.5, constant: 0)

// Working
NSLayoutConstraint(item: buttonLeft, attribute: .centerX, relatedBy: .equal, toItem: self, attribute: .centerX, multiplier: 0.5, constant: 0)

Though I could not find any way to create the constraint using the anchors methods.

Setting centerXAnchor constraint with multiplier programmatically doesn't work

You can't use multipliers using helpers functions, try this way

 let center = NSLayoutConstraint(item: newView, attribute: .centerX, relatedBy: .equal, toItem: view, attribute: .centerX, multiplier: 0.5, constant: 0)

Refer to answer

How to set an element center of view programmatically in iOS Swift 3?

I would suggest trying to use Anchors which are more convenient and easy to understand. This code centered my view:

    let someView = UIView()
    someView.backgroundColor = .red
    someView.translatesAutoresizingMaskIntoConstraints = false
    view.addSubview(someView)
    NSLayoutConstraint.activate([
        someView.centerXAnchor.constraint(equalTo: view.centerXAnchor),
        someView.centerYAnchor.constraint(equalTo: view.centerYAnchor),
        someView.heightAnchor.constraint(equalToConstant: 50),
        someView.widthAnchor.constraint(equalToConstant: 50)
    ])

Autolayout not working on centerX- centerY

Remove the addConstraint lines and add .active = YES after each instantiation of the constraint. Also change [image superview] for cell

Can't add constraints to video programmatically

Here's the code, with explanation.

• Always remember that if you are using auto layout constraints, do not set frames. The layout engine will walk all over them. If you are instantiating your view in code, don't set a frame, or if necessary, it communicates things best if you set the frame to CGRect.zero.
• Understand the view life cycle. Specifically, you can set your constraints in viewDidLoad, where they should be created only once.
• Remember to set the auto resizing mask to false. This is the most common error when you learning auto layout in code.
• There are actually three ways to create constraints, and a few ways to activate them. In your question, I think the easiest way is to use anchors.

Here's an example of centering a view (any view) with a width of 343 and a height of 264:

let myView = UIView()    // note, I'm not setting any frame

override func viewDidLoad() {
    super.viewDidLoad()
    view.translatesAutoresizingMaskIntoConstraints = false
    myView.translatesAutoresizingMaskIntoConstraints = false
    myView.widthAnchor.constraint(equalToConstant: 343.0).isActive = true
    myView.heightAnchor.constraint(equalToConstant: 264.0).isActive = true
    myView.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
    myView.centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = true
}

That's all there is to it! BUT....

I'd suggest one more thing. Don't use constants in setting the height and width. That's not being "adaptive". Your 4 inch iPhone SE has a screen size of 568x320, where this may look centered and large enough. But on an iPhone Plus with a screen size of 736x414 it may be pretty small. (To say nothing of a 12.9 inch iPad Pro!)

Notice how my code uses the superview for the centerX/centerY anchors. (And instead of equalToConstant it's equalTo.) Do the same with the width and height. Through the use of multiplier and constant, along with UILayoutGuides, you can make your layouts adapt to whatever screen size Apple throws at you.

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