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How to position a view using Auto Layout anchors

Swift version: 5.0

Paul Hudson    @twostraws   

Auto Layout anchors make it easy to position your views relative to others. There are lots of anchors to choose from: leading and trailing edges, top and bottom edges, center X and center Y, and more.

To try it out, first create a view something like this:

let child = UIView()
child.translatesAutoresizingMaskIntoConstraints = false
child.backgroundColor = .red
view.addSubview(child)

Now we can position that view by activating various anchors. For example, we could pin it to the top and bottom edges of the screen, make it precisely 128 points wide, then center it horizontally:

child.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor).isActive = true
child.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor).isActive = true
child.widthAnchor.constraint(equalToConstant: 128).isActive = true
child.centerXAnchor.constraint(equalTo: view.safeAreaLayoutGuide.centerXAnchor).isActive = true

To explore anchors further, try typing child.anchor and exploring the code completion options.

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Available from iOS 9.0

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