在swift 3中以编程方式设置UIImageView AspectRatio约束

Question

I have an UIImageView in storyboard which AspectRatio is 1:1, that I want to change to 2:1 programmatically in ViewController in some cases. I create reference of that constraint in ViewController but unable to set the constraint.

Answer 1

You can change constraint programmatically in swift 3

let aspectRatioConstraint = NSLayoutConstraint(item: self.YourImageObj,attribute: .height,relatedBy: .equal,toItem: self.YourImageObj,attribute: .width,multiplier: (2.0 / 1.0),constant: 0)
self.YourImageObj.addConstraint(aspectRatioConstraint)

Answer 2

As it's stated in Apple's guide , there're three ways to set constraints programmatically:

• You can use layout anchors
• You can use the NSLayoutConstraint class
• You can use the Visual Format Language

The most convenient and fluent way to set constraints is using Layout Anchors.

It's just one line of code to change aspect ratio for your ImageView

imageView.heightAnchor.constraint(equalTo: imageView.widthAnchor, multiplier: 1.0/2.0).isActive = true

To avoid "[LayoutConstraints] Unable to simultaneously satisfy constraints." you should add reference to your ImageView's height constraint then deactivate it:

heightConstraint.isActive = false

Answer 3

根据约束条件将约束的乘数设置为0.5或2，它将变为2：1