想在Imanou Petit的回答中添加一些理论概念，这样人们就可以了解自动布局是如何工作的。

要理解自动布局，请将您的视图视为橡胶的对象，最初是收缩的。

要在屏幕上放置一个对象，我们需要4件强制性的事情:

物体的X坐标(水平位置)。 物体的Y坐标(垂直位置) 对象的宽度 物体的高度。

X坐标:有多种方式给视图X坐标。

如前向约束，后向约束，水平中心 等。

2 Y坐标:给视图Y坐标有多种方式:

如顶部约束，底部约束，垂直中心等。

有两种方法来限制一个视图的宽度:

a.添加固定宽度约束(将此约束视为固定宽度的铁棒，并与橡胶对象水平挂钩，使橡胶对象不会收缩或膨胀)

b.不要添加任何宽度约束，而是在视图的前端和前端添加x坐标约束，这两个约束将通过从前端和前端拉动橡胶对象来扩展/收缩。

对象的高度:类似于宽度，有两种方法来限制视图的高度:

a.添加固定高度约束(将此约束视为固定高度的铁棒，并与橡胶物体垂直挂钩，使橡胶物体不会收缩或膨胀)

b.不要添加任何高度约束，但在视图的顶部和底部两端添加x坐标约束，这两个约束将从两端、顶部和底部拉伸/推动橡胶对象。

它帮助我直观地学习，所以这是一个补充答案。

样板代码

override func viewDidLoad() {
    super.viewDidLoad()

    let myView = UIView()
    myView.backgroundColor = UIColor.blue
    myView.translatesAutoresizingMaskIntoConstraints = false
    view.addSubview(myView)

    // Add constraints code here
    // ...
}

下面的每个例子都是相互独立的。

引脚左边缘

myView。leading = leadingMargin + 20

方法1:锚式

let margins = view.layoutMarginsGuide
myView.leadingAnchor.constraint(equalTo: margins.leadingAnchor, constant: 20).isActive = true

除了leadingAnchor，还有trailingAnchor, topAnchor和bottomAnchor。

方法2:NSLayoutConstraint样式

NSLayoutConstraint(item: myView, attribute: NSLayoutAttribute.leading, relatedBy: NSLayoutRelation.equal, toItem: view, attribute: NSLayoutAttribute.leadingMargin, multiplier: 1.0, constant: 20.0).isActive = true

除了。leading，还有。tailed，。top，和。bottom。 除了。leadingmargin，还有。trailingmargin， . topmargin和.bottom margin。

设置宽度和高度

宽度= 200 高度= 100

方法1:锚式

myView.widthAnchor.constraint(equalToConstant: 200).isActive = true
myView.heightAnchor.constraint(equalToConstant: 100).isActive = true

方法2:NSLayoutConstraint样式

NSLayoutConstraint(item: myView, attribute: NSLayoutAttribute.width, relatedBy: NSLayoutRelation.equal, toItem: nil, attribute: NSLayoutAttribute.notAnAttribute, multiplier: 1, constant: 200).isActive = true
NSLayoutConstraint(item: myView, attribute: NSLayoutAttribute.height, relatedBy: NSLayoutRelation.equal, toItem: nil, attribute: NSLayoutAttribute.notAnAttribute, multiplier: 1, constant: 100).isActive = true

集装箱中心

myView。centerX = centerX myView。世纪=世纪

方法1:锚式

myView.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
myView.centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = true

方法2:NSLayoutConstraint样式

NSLayoutConstraint(item: myView, attribute: NSLayoutAttribute.centerX, relatedBy: NSLayoutRelation.equal, toItem: view, attribute: NSLayoutAttribute.centerX, multiplier: 1, constant: 0).isActive = true
NSLayoutConstraint(item: myView, attribute: NSLayoutAttribute.centerY, relatedBy: NSLayoutRelation.equal, toItem: view, attribute: NSLayoutAttribute.centerY, multiplier: 1, constant: 0).isActive = true

笔记

锚的风格是首选的方法超过NSLayoutConstraint风格，但它只适用于iOS 9，所以如果你是支持iOS 8，那么你仍然应该使用NSLayoutConstraint风格。 上面的例子只显示了我们所关注的一两个约束条件。然而，为了在我的测试项目中正确地放置myView，我需要有四个约束。

进一步的阅读

以编程方式创建约束文档

在xcode 7.3.1中略有不同。这是我想到的

   // creating the view
        let newView = UIView()
        newView.backgroundColor = UIColor.redColor()
        newView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(newView)

        // creating the constraint 

        // attribute and relation cannot be set directyl you need to create a cariable of them
        let layout11 = NSLayoutAttribute.CenterX
        let layout21 = NSLayoutRelation.Equal
        let layout31 = NSLayoutAttribute.CenterY
        let layout41 = NSLayoutAttribute.Width
        let layout51 = NSLayoutAttribute.Height
        let layout61 = NSLayoutAttribute.NotAnAttribute

        // defining all the constraint
        let horizontalConstraint = NSLayoutConstraint(item: newView, attribute: layout11, relatedBy: layout21, toItem: view, attribute: layout11, multiplier: 1, constant: 0)
        let verticalConstraint = NSLayoutConstraint(item: newView, attribute: layout31, relatedBy: layout21, toItem: view, attribute: layout31, multiplier: 1, constant: 0)
        let widthConstraint = NSLayoutConstraint(item: newView, attribute: layout41, relatedBy: layout21, toItem: nil, attribute: layout61, multiplier: 1, constant: 100)
        let heightConstraint = NSLayoutConstraint(item: newView, attribute: layout51, relatedBy: layout21, toItem: nil, attribute: layout61, multiplier: 1, constant: 100)

        // adding all the constraint
        NSLayoutConstraint.activateConstraints([horizontalConstraint,verticalConstraint,widthConstraint,heightConstraint])

通过对图像施加约束来实现自动布局。使用NSLayoutConstraint。我们可以在所有设备上实现理想而漂亮的设计。请尝试下面的代码。

import UIKit

class ViewController: UIViewController {

override func viewDidLoad() {
super.viewDidLoad()

let myImageView:UIImageView = UIImageView()
myImageView.backgroundColor = UIColor.red
myImageView.image = UIImage(named:"sample_dog")!
myImageView.translatesAutoresizingMaskIntoConstraints = false
myImageView.layer.borderColor = UIColor.red.cgColor
myImageView.layer.borderWidth = 10
self.view.addSubview(myImageView)
        
view.removeConstraints(view.constraints)


view.addConstraint(NSLayoutConstraint(
item: myImageView,
attribute: .top,
relatedBy: .equal,
toItem: view,
attribute: .top,
multiplier: 1,
constant:100)
    
)

view.addConstraint(NSLayoutConstraint(
item: myImageView,
attribute: .centerX,
relatedBy: .equal,
toItem: view,
attribute: .centerX,
multiplier: 1,
constant:0)
)
    
view.addConstraint(NSLayoutConstraint(
item: myImageView,
attribute: .height,
relatedBy: .equal,
toItem: view,
attribute: .width,
multiplier: 0.5,
constant:40))
    
view.addConstraint(NSLayoutConstraint(
item: myImageView,
attribute: .width,
relatedBy: .equal,
toItem: view,
attribute: .width,
multiplier: 0.5,
constant:40))
    
}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
}
}

这是一种以编程方式添加约束的方法

override func viewDidLoad() {
            super.viewDidLoad()


let myLabel = UILabel()
        myLabel.labelFrameUpdate(label: myLabel, text: "Welcome User", font: UIFont(name: "times new roman", size: 40)!, textColor: UIColor.red, textAlignment: .center, numberOfLines: 0, borderWidth: 2.0, BorderColor: UIColor.red.cgColor)
        self.view.addSubview(myLabel)


         let myLabelhorizontalConstraint = NSLayoutConstraint(item: myLabel, attribute: NSLayoutAttribute.centerX, relatedBy: NSLayoutRelation.equal, toItem: self.view, attribute: NSLayoutAttribute.centerX, multiplier: 1, constant: 0)
        let myLabelverticalConstraint = NSLayoutConstraint(item: myLabel, attribute: NSLayoutAttribute.centerY, relatedBy: NSLayoutRelation.equal, toItem: self.view, attribute: NSLayoutAttribute.centerY, multiplier: 1, constant: 0)
        let mylabelLeading = NSLayoutConstraint(item: myLabel, attribute: NSLayoutAttribute.leading, relatedBy: NSLayoutRelation.equal, toItem: self.view, attribute: NSLayoutAttribute.leading, multiplier: 1, constant: 10)
        let mylabelTrailing = NSLayoutConstraint(item: myLabel, attribute: NSLayoutAttribute.trailing, relatedBy: NSLayoutRelation.equal, toItem: self.view, attribute: NSLayoutAttribute.trailing, multiplier: 1, constant: -10)
        let myLabelheightConstraint = NSLayoutConstraint(item: myLabel, attribute: NSLayoutAttribute.height, relatedBy: NSLayoutRelation.equal, toItem: nil, attribute: NSLayoutAttribute.notAnAttribute, multiplier: 1, constant: 50)
        NSLayoutConstraint.activate(\[myLabelhorizontalConstraint, myLabelverticalConstraint, myLabelheightConstraint,mylabelLeading,mylabelTrailing\])
}

extension UILabel
{
    func labelFrameUpdate(label:UILabel,text:String = "This is sample Label",font:UIFont = UIFont(name: "times new roman", size: 20)!,textColor:UIColor = UIColor.red,textAlignment:NSTextAlignment = .center,numberOfLines:Int = 0,borderWidth:CGFloat = 2.0,BorderColor:CGColor = UIColor.red.cgColor){
        label.translatesAutoresizingMaskIntoConstraints = false
        label.text = text
        label.font = font
        label.textColor = textColor
        label.textAlignment = textAlignment
        label.numberOfLines = numberOfLines
        label.layer.borderWidth = borderWidth
        label.layer.borderColor = UIColor.red.cgColor
    }
}

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
    }
}