Adding a subview and position it with constraints programmatically

Question

I have added a UIView using the IB on to the view controller. Let's call it the redView.

Then I have pinned its all four sides using auto layout constraints in code and when I run it, it looks like this as expected.

Now I want to add a UILabel to this view programmatically and position it in the center using auto layout constraints.

Below is the code I have so far.

import UIKit

class ViewController: UIViewController {

    @IBOutlet private var redView: UIView!

    override func viewDidLoad() {
        super.viewDidLoad()

        redView.setTranslatesAutoresizingMaskIntoConstraints(false)

        let leadingConstraint = NSLayoutConstraint(item: redView, attribute: .Leading, relatedBy: .Equal, toItem: view, attribute: .Leading, multiplier: 1, constant: 0)
        let trailingConstraint = NSLayoutConstraint(item: redView, attribute: .Trailing, relatedBy: .Equal, toItem: view, attribute: .Trailing, multiplier: 1, constant: 0)
        let topConstraint = NSLayoutConstraint(item: redView, attribute: .Top, relatedBy: .Equal, toItem: view, attribute: .Top, multiplier: 1, constant: 0)
        let bottomConstraint = NSLayoutConstraint(item: redView, attribute: .Bottom, relatedBy: .Equal, toItem: view, attribute: .Bottom, multiplier: 1, constant: 0)
        view.addConstraints([leadingConstraint, trailingConstraint, topConstraint, bottomConstraint])


        let label = UILabel()
        label.text = "Auto Layout Exercise"
        redView.addSubview(label)

        let xCenterConstraint = NSLayoutConstraint(item: label, attribute: .CenterX, relatedBy: .Equal, toItem: redView, attribute: .CenterX, multiplier: 1, constant: 0)
        let yCenterConstraint = NSLayoutConstraint(item: label, attribute: .CenterY, relatedBy: .Equal, toItem: redView, attribute: .CenterY, multiplier: 1, constant: 0)
        let leadingConstraint1 = NSLayoutConstraint(item: label, attribute: .Leading, relatedBy: .Equal, toItem: redView, attribute: .Leading, multiplier: 1, constant: 20)
        let trailingConstraint1 = NSLayoutConstraint(item: label, attribute: .Trailing, relatedBy: .Equal, toItem: redView, attribute: .Trailing, multiplier: 1, constant: -20)
        redView.addConstraints([xCenterConstraint, yCenterConstraint, leadingConstraint1, trailingConstraint1])

    }

}

The problem is the label doesn't appear on the redView. Can anyone please tell me what I'm missing here?

Thank you.

Solution 1

I'm pretty sure you need to also set:

label.setTranslatesAutoresizingMaskIntoConstraints(false)

Otherwise I don't think the constraints will apply to the label.

Solution 2

Note for Swift 2/3

It has changed to Boolean property: UIView.translatesAutoresizingMaskIntoConstraints

Instead of it previously being:

label.setTranslatesAutoresizingMaskIntoConstraints(false)

It is now:

label.translatesAutoresizingMaskIntoConstraints = false

Solution 3

You need to do this for each UIControl to assign NSConstraints

label.translatesAutoresizingMaskIntoConstraints = false