What’s the difference between leading, trailing, left, and right anchors?

Swift version: 5.1

Paul Hudson @twostraws October 26th 2019

If you wanted to pin a button to the left edge of its parent, you might write code like this:

button.leftAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leftAnchor).isActive = true

However, that has a problem: in right-to-left languages, the user interface ought to be flipped horizontally for the most part, but your button won’t move – you’ve specifically asked it to be pinned to the left edge regardless of the user’s device settings.

If that’s what you want, you don’t have a problem. However, if you meant “left edge for left-to-right languages and right edge for right-to-left languages,” then you should use leadingAnchor instead of leftAnchor, like this:

button.leadingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor).isActive = true

For the same effect on the opposite edge, you should use trailingAnchor rather than rightAnchor. Again, if you specifically want something to appear on the right regardless of language then this does not apply.

Available from iOS 8.0