How to detect when your size class changes

Swift version: 5.4

Paul Hudson @twostraws May 28th 2019

All view controllers have a size class, which is a vague description of how much space is available both horizontally and vertically. These sometimes change when the user rotates their device, but can also change on iPad when they split the screen between two apps in various ways.

If your app needs to respond to size class changes, it’s important you implement the traitCollectionDidChange method. There you can check the size class of the current trait collection and respond appropriately. For example:

override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
    if traitCollection.horizontalSizeClass == .compact {
        // load slim view
    } else {
        // load wide view
    }
}