如何在 iOS 中以编程方式设置纵横比约束？

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[self.yourview setTranslatesAutoresizingMaskIntoConstraints:NO];
[self.yourview addConstraint:[NSLayoutConstraint
constraintWithItem:self.yourview
attribute:NSLayoutAttributeHeight
relatedBy:NSLayoutRelationEqual
toItem:self.yourview
attribute:NSLayoutAttributeWidth
multiplier:(self.yourview.frame.size.height / self.yourview.frame.size.width)
constant:0]];

or in the place of (self.yourview.frame.size.height / self.yourview.frame.size.width) you can use any float value. Thanks.

Swift 3.0 -

self.yourview!.translatesAutoresizingMaskIntoConstraints = false
self.yourview!.addConstraint(NSLayoutConstraint(item: self.yourview!,
attribute: NSLayoutAttribute.height,
relatedBy: NSLayoutRelation.equal,
toItem: self.yourview!,
attribute: NSLayoutAttribute.width,
multiplier: self.yourview.frame.size.height / self.yourview.frame.size.width,
constant: 0))

You can set "Height constraint" at Design-time in Interface Builder. Just check '"Remove at build time", and it removes when App will running.

After that you can add "Aspect Ratio" constraint, for example, in viewDidLoad method.

In Xamain.iOS for "16:9" it's looks like this:

    this.ImageOfTheDay.AddConstraint(NSLayoutConstraint.Create(
this.ImageOfTheDay,
NSLayoutAttribute.Height,
NSLayoutRelation.Equal,
this.ImageOfTheDay,
NSLayoutAttribute.Width,
9.0f / 16.0f,
0));

Or, as Mahesh Agrawal said:

    [self.ImageOfTheDay addConstraint:[NSLayoutConstraint
constraintWithItem:self.ImageOfTheDay
attribute:NSLayoutAttributeHeight
relatedBy:NSLayoutRelationEqual
toItem:self.ImageOfTheDay
attribute:NSLayoutAttributeWidth
multiplier:(9.0f / 16.0f)
constant:0]];

Swift 3:

yourView.addConstraint(NSLayoutConstraint(item: yourView,
attribute: .height,
relatedBy: .equal,
toItem: yourView,
attribute: .width,
multiplier: 9.0 / 16.0,
constant: 0))

I have try all answers above but didn't work, and this is my solution with swift 3:

let newConstraint = NSLayoutConstraint(item: yourView, attribute: .width, relatedBy: .equal, toItem: yourView, attribute: .height, multiplier: 560.0/315.0, constant: 0)
yourView.addConstraint(newConstraint)
NSLayoutConstraint.activate([newConstraint])
NSLayoutConstraint.deactivate(yourView.constraints)
yourView.layoutIfNeeded()

Layout Anchors is the most convenient way to set constraints programmatically.

Say you want to set 5:1 aspect ratio for your button then you should use:

button.heightAnchor.constraint(equalTo: button.widthAnchor, multiplier: 1.0/5.0).isActive = true

Here's the full code:

class ViewController: UIViewController {


override func viewDidLoad() {
super.viewDidLoad()


let button = UIButton(type: .custom)
button.setTitle("Login", for: .normal)
button.backgroundColor = UIColor.darkGray


self.view.addSubview(button)


button.translatesAutoresizingMaskIntoConstraints = false


let margins = view.layoutMarginsGuide


button.leadingAnchor.constraint(equalTo: margins.leadingAnchor, constant: 20.0).isActive = true
button.trailingAnchor.constraint(equalTo: margins.trailingAnchor, constant: -20.0).isActive = true
button.bottomAnchor.constraint(equalTo: margins.bottomAnchor, constant: -20.0).isActive = true
button.heightAnchor.constraint(equalTo: button.widthAnchor, multiplier: 1.0/5.0).isActive = true
}


}

Here're results achieved with code written above. You can see that button keeps its 5:1 aspect ratio across various devices:

Helper Extension on UIView

extension UIView {
    

func aspectRatio(_ ratio: CGFloat) -> NSLayoutConstraint {
        

return NSLayoutConstraint(item: self, attribute: .height, relatedBy: .equal, toItem: self, attribute: .width, multiplier: ratio, constant: 0)
}
}

And usage is:

view.aspectRatio(1.0/1.0).isActive = true

Swift 5 solution, Xcode 12.4. Requires ios 9.0+

Simply anchor your height constraint to a certain ratio:

let aspectRatio: CGFloat = 9 / 16 // Example of ratio you want to apply
yourView.heightAnchor.constraint(equalTo: imageView.widthAnchor,
multiplier: aspectRatio).activate()