Auto-Layout fit to parent via code in Swift

Question

I have a view i'm creating via code and adding to another view as subview. The new superview can change it's frame over time and I want the newly created subview to change it's frame accordingly. How can I do that using Auto-Layout via code in Swift?

Answer 1

Here is an example:

let view = UIView() // existing view

let subview = UIView()
subview.setTranslatesAutoresizingMaskIntoConstraints(false)

view.addSubview(subview)
view.addConstraint(NSLayoutConstraint(item: subview, attribute: .Top, relatedBy: .Equal, toItem: view, attribute: .Top, multiplier: 1.0, constant: 0.0))
view.addConstraint(NSLayoutConstraint(item: subview, attribute: .Leading, relatedBy: .Equal, toItem: view, attribute: .Leading, multiplier: 1.0, constant: 0.0))
view.addConstraint(NSLayoutConstraint(item: view, attribute: .Bottom, relatedBy: .Equal, toItem: subview, attribute: .Bottom, multiplier: 1.0, constant: 0.0))
view.addConstraint(NSLayoutConstraint(item: view, attribute: .Trailing, relatedBy: .Equal, toItem: subview, attribute: .Trailing, multiplier: 1.0, constant: 0.0))

Answer 2

iOS 13, swift 5

First, you add this code

    subview.translatesAutoresizingMaskIntoConstraints = false
    view.addSubview(subview)

Then, there are two ways of doing this in newer versions of iOS.

1. With NSLayoutConstraint class

   NSLayoutConstraint(item: subview, attribute: .top, relatedBy: .equal, toItem: view, attribute: .top, multiplier: 1, constant: 0).isActive = true
   NSLayoutConstraint(item: subview, attribute: .bottom, relatedBy: .equal, toItem: view, attribute: .bottom, multiplier: 1, constant: 0).isActive = true
   NSLayoutConstraint(item: subview, attribute: .left, relatedBy: .equal, toItem: view, attribute: .left, multiplier: 1, constant: 0).isActive = true
   NSLayoutConstraint(item: subview, attribute: .right, relatedBy: .equal, toItem: view, attribute: .right, multiplier: 1, constant: 0).isActive = true
   

2. With NSLayoutAnchor class (less verbose)

   subview.topAnchor.constraint(equalTo: view.topAnchor).isActive = true
   subview.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
   subview.leftAnchor.constraint(equalTo: view.leftAnchor).isActive = true
   subview.rightAnchor.constraint(equalTo: view.rightAnchor).isActive = true
   

Either way, on iOS 8 and later Apple recommends using isActive() instead of adding constraints directly to a view.

Additionally, I believe the purpose of the NSLayoutAnchor method is to be more concise and readable compared to NSLayoutConstraint.