Simon k
Simon k

Reputation: 413

iOS adding constraints programmatically in Swift

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

So what I've done is:

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?

Upvotes: 4

Views: 18088

Answers (2)

iAj
iAj

Reputation: 3817

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
 }
}

enter image description here

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)
*/

Upvotes: 8

msk
msk

Reputation: 8905

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.

Upvotes: 9

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