Different type to create constraint programmatically

Do you plan to have a squared UIView of width: 100 and Height: 100 centered inside the UIViewof an UIViewController? If so, you may try one of the 6 following Auto Layout styles (Swift 4 / iOS 11):

---

1. Using NSLayoutConstraint initializer

override func viewDidLoad() {
    let newView = UIView()
    newView.backgroundColor = UIColor.red
    view.addSubview(newView)

    newView.translatesAutoresizingMaskIntoConstraints = false
    let horizontalConstraint = NSLayoutConstraint(item: newView, attribute: NSLayoutAttribute.centerX, relatedBy: NSLayoutRelation.equal, toItem: view, attribute: NSLayoutAttribute.centerX, multiplier: 1, constant: 0)
    let verticalConstraint = NSLayoutConstraint(item: newView, attribute: NSLayoutAttribute.centerY, relatedBy: NSLayoutRelation.equal, toItem: view, attribute: NSLayoutAttribute.centerY, multiplier: 1, constant: 0)
    let widthConstraint = NSLayoutConstraint(item: newView, attribute: NSLayoutAttribute.width, relatedBy: NSLayoutRelation.equal, toItem: nil, attribute: NSLayoutAttribute.notAnAttribute, multiplier: 1, constant: 100)
    let heightConstraint = NSLayoutConstraint(item: newView, attribute: NSLayoutAttribute.height, relatedBy: NSLayoutRelation.equal, toItem: nil, attribute: NSLayoutAttribute.notAnAttribute, multiplier: 1, constant: 100)
    view.addConstraints([horizontalConstraint, verticalConstraint, widthConstraint, heightConstraint])
}

override func viewDidLoad() {
    let newView = UIView()
    newView.backgroundColor = UIColor.red
    view.addSubview(newView)

    newView.translatesAutoresizingMaskIntoConstraints = false
    let horizontalConstraint = NSLayoutConstraint(item: newView, attribute: NSLayoutAttribute.centerX, relatedBy: NSLayoutRelation.equal, toItem: view, attribute: NSLayoutAttribute.centerX, multiplier: 1, constant: 0)
    let verticalConstraint = NSLayoutConstraint(item: newView, attribute: NSLayoutAttribute.centerY, relatedBy: NSLayoutRelation.equal, toItem: view, attribute: NSLayoutAttribute.centerY, multiplier: 1, constant: 0)
    let widthConstraint = NSLayoutConstraint(item: newView, attribute: NSLayoutAttribute.width, relatedBy: NSLayoutRelation.equal, toItem: nil, attribute: NSLayoutAttribute.notAnAttribute, multiplier: 1, constant: 100)
    let heightConstraint = NSLayoutConstraint(item: newView, attribute: NSLayoutAttribute.height, relatedBy: NSLayoutRelation.equal, toItem: nil, attribute: NSLayoutAttribute.notAnAttribute, multiplier: 1, constant: 100)
    NSLayoutConstraint.activate([horizontalConstraint, verticalConstraint, widthConstraint, heightConstraint])
}

override func viewDidLoad() {
    let newView = UIView()
    newView.backgroundColor = UIColor.red
    view.addSubview(newView)

    newView.translatesAutoresizingMaskIntoConstraints = false
    NSLayoutConstraint(item: newView, attribute: NSLayoutAttribute.centerX, relatedBy: NSLayoutRelation.equal, toItem: view, attribute: NSLayoutAttribute.centerX, multiplier: 1, constant: 0).isActive = true
    NSLayoutConstraint(item: newView, attribute: NSLayoutAttribute.centerY, relatedBy: NSLayoutRelation.equal, toItem: view, attribute: NSLayoutAttribute.centerY, multiplier: 1, constant: 0).isActive = true
    NSLayoutConstraint(item: newView, attribute: NSLayoutAttribute.width, relatedBy: NSLayoutRelation.equal, toItem: nil, attribute: NSLayoutAttribute.notAnAttribute, multiplier: 1, constant: 100).isActive = true
    NSLayoutConstraint(item: newView, attribute: NSLayoutAttribute.height, relatedBy: NSLayoutRelation.equal, toItem: nil, attribute: NSLayoutAttribute.notAnAttribute, multiplier: 1, constant: 100).isActive = true
}

---

2. Using Visual Format Language

override func viewDidLoad() {
    let newView = UIView()
    newView.backgroundColor = UIColor.red
    view.addSubview(newView)

    newView.translatesAutoresizingMaskIntoConstraints = false
    let views = ["view": view!, "newView": newView]
    let horizontalConstraints = NSLayoutConstraint.constraints(withVisualFormat: "H:[view]-(<=0)-[newView(100)]", options: NSLayoutFormatOptions.alignAllCenterY, metrics: nil, views: views)
    let verticalConstraints = NSLayoutConstraint.constraints(withVisualFormat: "V:[view]-(<=0)-[newView(100)]", options: NSLayoutFormatOptions.alignAllCenterX, metrics: nil, views: views)
    view.addConstraints(horizontalConstraints)
    view.addConstraints(verticalConstraints)
}

override func viewDidLoad() {
    let newView = UIView()
    newView.backgroundColor = UIColor.red
    view.addSubview(newView)

    newView.translatesAutoresizingMaskIntoConstraints = false
    let views = ["view": view!, "newView": newView]
    let horizontalConstraints = NSLayoutConstraint.constraints(withVisualFormat: "H:[view]-(<=0)-[newView(100)]", options: NSLayoutFormatOptions.alignAllCenterY, metrics: nil, views: views)
    let verticalConstraints = NSLayoutConstraint.constraints(withVisualFormat: "V:[view]-(<=0)-[newView(100)]", options: NSLayoutFormatOptions.alignAllCenterX, metrics: nil, views: views)
    NSLayoutConstraint.activate(horizontalConstraints)
    NSLayoutConstraint.activate(verticalConstraints)
}

---

3. Using a mix of NSLayoutConstraint initializer and Visual Format Language

override func viewDidLoad() {
    let newView = UIView()
    newView.backgroundColor = UIColor.red
    view.addSubview(newView)

    newView.translatesAutoresizingMaskIntoConstraints = false
    let views = ["newView": newView]
    let widthConstraints = NSLayoutConstraint.constraints(withVisualFormat: "H:[newView(100)]", options: NSLayoutFormatOptions(rawValue: 0), metrics: nil, views: views)
    let heightConstraints = NSLayoutConstraint.constraints(withVisualFormat: "V:[newView(100)]", options: NSLayoutFormatOptions(rawValue: 0), metrics: nil, views: views)
    let horizontalConstraint = NSLayoutConstraint(item: newView, attribute: NSLayoutAttribute.centerX, relatedBy: NSLayoutRelation.equal, toItem: view, attribute: NSLayoutAttribute.centerX, multiplier: 1, constant: 0)
    let verticalConstraint = NSLayoutConstraint(item: newView, attribute: NSLayoutAttribute.centerY, relatedBy: NSLayoutRelation.equal, toItem: view, attribute: NSLayoutAttribute.centerY, multiplier: 1, constant: 0)
    view.addConstraints(widthConstraints)
    view.addConstraints(heightConstraints)
    view.addConstraints([horizontalConstraint, verticalConstraint])
}

override func viewDidLoad() {
    let newView = UIView()
    newView.backgroundColor = UIColor.red
    view.addSubview(newView)

    newView.translatesAutoresizingMaskIntoConstraints = false
    let views = ["newView": newView]
    let widthConstraints = NSLayoutConstraint.constraints(withVisualFormat: "H:[newView(100)]", options: NSLayoutFormatOptions(rawValue: 0), metrics: nil, views: views)
    let heightConstraints = NSLayoutConstraint.constraints(withVisualFormat: "V:[newView(100)]", options: NSLayoutFormatOptions(rawValue: 0), metrics: nil, views: views)
    let horizontalConstraint = NSLayoutConstraint(item: newView, attribute: NSLayoutAttribute.centerX, relatedBy: NSLayoutRelation.equal, toItem: view, attribute: NSLayoutAttribute.centerX, multiplier: 1, constant: 0)
    let verticalConstraint = NSLayoutConstraint(item: newView, attribute: NSLayoutAttribute.centerY, relatedBy: NSLayoutRelation.equal, toItem: view, attribute: NSLayoutAttribute.centerY, multiplier: 1, constant: 0)
    NSLayoutConstraint.activate(widthConstraints)
    NSLayoutConstraint.activate(heightConstraints)
    NSLayoutConstraint.activate([horizontalConstraint, verticalConstraint])
}

override func viewDidLoad() {
    let newView = UIView()
    newView.backgroundColor = UIColor.red
    view.addSubview(newView)

    newView.translatesAutoresizingMaskIntoConstraints = false
    let views = ["newView": newView]
    let widthConstraints = NSLayoutConstraint.constraints(withVisualFormat: "H:[newView(100)]", options: NSLayoutFormatOptions(rawValue: 0), metrics: nil, views: views)
    let heightConstraints = NSLayoutConstraint.constraints(withVisualFormat: "V:[newView(100)]", options: NSLayoutFormatOptions(rawValue: 0), metrics: nil, views: views)
    NSLayoutConstraint.activate(widthConstraints)
    NSLayoutConstraint.activate(heightConstraints)
    NSLayoutConstraint(item: newView, attribute: NSLayoutAttribute.centerX, relatedBy: NSLayoutRelation.equal, toItem: view, attribute: NSLayoutAttribute.centerX, multiplier: 1, constant: 0).isActive = true
    NSLayoutConstraint(item: newView, attribute: NSLayoutAttribute.centerY, relatedBy: NSLayoutRelation.equal, toItem: view, attribute: NSLayoutAttribute.centerY, multiplier: 1, constant: 0).isActive = true
}

---

4. Using UIViewAutoresizing

Note: Springs and Struts will be translated into corresponding auto layout constraints at runtime.

override func viewDidLoad() {
    let newView = UIView(frame: CGRect(x: 0, y: 0, width: 100, height: 100))
    newView.backgroundColor = UIColor.red
    view.addSubview(newView)

    newView.translatesAutoresizingMaskIntoConstraints = true
    newView.center = CGPoint(x: view.bounds.midX, y: view.bounds.midY)
    newView.autoresizingMask = [UIViewAutoresizing.flexibleLeftMargin, UIViewAutoresizing.flexibleRightMargin, UIViewAutoresizing.flexibleTopMargin, UIViewAutoresizing.flexibleBottomMargin]
}

---

5. Using NSLayoutAnchor

override func viewDidLoad() {
    let newView = UIView()
    newView.backgroundColor = UIColor.red
    view.addSubview(newView)

    newView.translatesAutoresizingMaskIntoConstraints = false
    let horizontalConstraint = newView.centerXAnchor.constraint(equalTo: view.centerXAnchor)
    let verticalConstraint = newView.centerYAnchor.constraint(equalTo: view.centerYAnchor)
    let widthConstraint = newView.widthAnchor.constraint(equalToConstant: 100)
    let heightConstraint = newView.heightAnchor.constraint(equalToConstant: 100)
    view.addConstraints([horizontalConstraint, verticalConstraint, widthConstraint, heightConstraint])
}

override func viewDidLoad() {
    let newView = UIView()
    newView.backgroundColor = UIColor.red
    view.addSubview(newView)

    newView.translatesAutoresizingMaskIntoConstraints = false
    let horizontalConstraint = newView.centerXAnchor.constraint(equalTo: view.centerXAnchor)
    let verticalConstraint = newView.centerYAnchor.constraint(equalTo: view.centerYAnchor)
    let widthConstraint = newView.widthAnchor.constraint(equalToConstant: 100)
    let heightConstraint = newView.heightAnchor.constraint(equalToConstant: 100)
    NSLayoutConstraint.activate([horizontalConstraint, verticalConstraint, widthConstraint, heightConstraint])
}

override func viewDidLoad() {
    let newView = UIView()
    newView.backgroundColor = UIColor.red
    view.addSubview(newView)

    newView.translatesAutoresizingMaskIntoConstraints = false
    newView.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
    newView.centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = true
    newView.widthAnchor.constraint(equalToConstant: 100).isActive = true
    newView.heightAnchor.constraint(equalToConstant: 100).isActive = true
}

---

6. Using intrinsicContentSize and NSLayoutAnchor

import UIKit

class CustomView: UIView {

    override var intrinsicContentSize: CGSize {
        return CGSize(width: 100, height: 100)
    }

}

class ViewController: UIViewController {

    override func viewDidLoad() {
        let newView = CustomView()
        newView.backgroundColor = UIColor.red
        view.addSubview(newView)

        newView.translatesAutoresizingMaskIntoConstraints = false
        let horizontalConstraint = newView.centerXAnchor.constraint(equalTo: view.centerXAnchor)
        let verticalConstraint = newView.centerYAnchor.constraint(equalTo: view.centerYAnchor)
        NSLayoutConstraint.activate([horizontalConstraint, verticalConstraint])
    }

}