您正在向self添加所有已定义的约束。视图的宽度和高度限制应该添加到你的newView。

另外，据我所知，你想要设置恒定的宽度和高度100:100。 在这种情况下，您应该将代码更改为:

var constW = NSLayoutConstraint(item: newView, 
    attribute: .Width, 
    relatedBy: .Equal, 
    toItem: nil, 
    attribute: .NotAnAttribute, 
    multiplier: 1, 
    constant: 100)
newView.addConstraint(constW)

var constH = NSLayoutConstraint(item: newView, 
    attribute: .Height, 
    relatedBy: .Equal, 
    toItem: nil, 
    attribute: .NotAnAttribute, 
    multiplier: 1, 
    constant: 100)
newView.addConstraint(constH)

操场上多个视图的约束。

斯威夫特 3+

  var yellowView: UIView!
    var redView: UIView!

    override func loadView() {

        // UI

        let view = UIView()
        view.backgroundColor = .white

        yellowView = UIView()
        yellowView.backgroundColor = .yellow
        view.addSubview(yellowView)

        redView = UIView()
        redView.backgroundColor = .red
        view.addSubview(redView)

        // Layout
        redView.translatesAutoresizingMaskIntoConstraints = false
        yellowView.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            yellowView.topAnchor.constraint(equalTo: view.topAnchor, constant: 20),
            yellowView.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 20),
            yellowView.widthAnchor.constraint(equalToConstant: 80),
            yellowView.heightAnchor.constraint(equalToConstant: 80),

            redView.bottomAnchor.constraint(equalTo: view.bottomAnchor, constant: -20),
            redView.trailingAnchor.constraint(equalTo: view.trailingAnchor,constant: -20),
            redView.widthAnchor.constraint(equalToConstant: 80),
            redView.heightAnchor.constraint(equalToConstant: 80)
            ])

        self.view = view
    }

在我看来，xcode游乐场是学习加法最好的地方 通过编程方式约束。

Swift 3更新

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
 }
}

约束的类型

 /*
// 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)
*/

    var xCenterConstraint : NSLayoutConstraint!
    var yCenterConstraint: NSLayoutConstraint!

 xCenterConstraint = NSLayoutConstraint(item: self.view, attribute: .CenterX, relatedBy: .Equal, toItem: (Your view NAme), attribute: .CenterX, multiplier: 1, constant: 0)
            self.view.addConstraint(xCenterConstraint)

 yCenterConstraint = NSLayoutConstraint(item: self.view, attribute: .CenterY, relatedBy: .Equal, toItem: (Your view Name), attribute: .CenterY, multiplier: 1, constant: 0)
            self.view.addConstraint(yCenterConstraint)

这个错误是由autoresizingmask自动创建的约束引起的，它们的创建是因为UIView属性translatesAutoresizingMaskIntoConstraints默认为true。

考虑使用BoxView来摆脱所有手工约束创建样板，并使您的代码简洁易读。用BoxView做布局是非常简单的:

boxView.items = [
   new_view.boxed.centerX().centerY().relativeWidth(1.0).relativeHeight(1.0)
]

下面的代码为我工作在这个场景:一个UIImageView强制景观。

    imagePreview!.isUserInteractionEnabled = true
    imagePreview!.isExclusiveTouch = true
    imagePreview!.contentMode = UIView.ContentMode.scaleAspectFit
    
    // Remove all constraints
    imagePreview!.removeAllConstraints()
    
    // Add the new constraints
    let guide = view.safeAreaLayoutGuide
    imagePreview!.translatesAutoresizingMaskIntoConstraints = false
    imagePreview!.leadingAnchor.constraint(equalTo: guide.leadingAnchor).isActive = true
    imagePreview!.trailingAnchor.constraint(equalTo: guide.trailingAnchor).isActive = true
    imagePreview!.heightAnchor.constraint(equalTo: guide.heightAnchor, multiplier: 1.0).isActive = true

哪里removeAllConstraints是一个扩展

extension UIView {
    
    func removeAllConstraints() {
        var _superview = self.superview
        
        func removeAllConstraintsFromView(view: UIView) { for c in view.constraints { view.removeConstraint(c) } }
        
        while let superview = _superview {
            for constraint in superview.constraints {
                
                if let first = constraint.firstItem as? UIView, first == self {
                    superview.removeConstraint(constraint)
                }
                
                if let second = constraint.secondItem as? UIView, second == self {
                    superview.removeConstraint(constraint)
                }
            }
            
            _superview = superview.superview
        }
        
        self.removeConstraints(self.constraints)
        self.translatesAutoresizingMaskIntoConstraints = true
    }
}