iOS adding constraints programmatically in Swift

Question

So, I'm done using the IB in Xcode and want to write all UI in Swift.

So what I've done is:

• Created a new UIView to contain the elements i want to write - lets call it "TestView"
• I've added TestView to a VC as a sub view.

• In the TestView class I've added the elements like this:

  class TestView: UIView {
        var someLabel:UILabel!
        override init(frame: CGRect) {
              super.init(frame: frame)
  
              self.someLabel = UILabel(frame: CGRect(x: self.frame.midX, y: oneSixthHeight, width: 100, height: 22))
              self.someLabel.text = "test"
  
              var constraints:[NSLayoutConstraint] = []
              self.someLabel.translatesAutoresizingMaskIntoConstraints = false
              let rightsideAnchor:NSLayoutConstraint = NSLayoutConstraint(item: self.someLabel, attribute: .Trailing, relatedBy: .Equal, toItem: self, attribute: .Trailing, multiplier: 1, constant: 1)
  
               constraints.append(rightsideAnchor)
               NSLayoutConstraint.activateConstraints(constraints)
        }
  }
  

With this I expect the UILabel to be anchored to the right side of the view.

However, I do get this error:

Terminating app due to uncaught exception 'NSGenericException', reason: 'Unable to activate constraint with items > and > because they have no common ancestor.
Does the constraint reference items in different view hierarchies? That's illegal.'

What am I doing wrong?

Answer 1

You should add constraints only after view is added to the view hierarchy. From your code it is clear that you have not added the UILabel instance to view.

Answer 2

Updated for Swift 3

import UIKit

class ViewController: UIViewController {

let redView: UIView = {

    let view = UIView()
    view.translatesAutoresizingMaskIntoConstraints = false
    view.backgroundColor = .red
    return view
}()

override func viewDidLoad() {
    super.viewDidLoad()

    setupViews()
    setupAutoLayout()
}

func setupViews() {

    view.backgroundColor = .white
    view.addSubview(redView)
}

func setupAutoLayout() {

    // Available from iOS 9 commonly known as Anchoring System for AutoLayout...
    redView.leftAnchor.constraint(equalTo: view.leftAnchor, constant: 20).isActive = true
    redView.rightAnchor.constraint(equalTo: view.rightAnchor, constant: -20).isActive = true

    redView.centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = true
    redView.heightAnchor.constraint(equalToConstant: 300).isActive = true

    // You can also modified above last two lines as follows by commenting above & uncommenting below lines...
    // redView.topAnchor.constraint(equalTo: view.topAnchor, constant: 20).isActive = true
    // redView.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
 }
}

Type of Constraints:

/*
// regular use
1.leftAnchor
2.rightAnchor
3.topAnchor
// intermediate use
4.widthAnchor
5.heightAnchor
6.bottomAnchor
7.centerXAnchor
8.centerYAnchor
// rare use
9.leadingAnchor
10.trailingAnchor
etc. (note: very project to project)
*/