What does viewDidLayoutSubviews() do?

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TL/DR

Simply put, viewDidLayoutSubviews() allows you to make customizations to views after they’ve been positioned by AutoLayout, but before they are visible to the user.

Whenever the bounds change for a UIViewController’s view (i.e. device rotation), it’s likely that the position and size of all the subviews will need to be updated as well. So, the system will calllayoutSubviews() to perform this change.

Then, once your UIViewController has finished laying out all of its subviews (all of your subviews are in their correct location and their frames honor whatever AutoLayout constraints you’ve specified), the system will call viewDidLayoutSubviews().

From here, if you need to further customize or override any changes prior to the view being visible on screen, viewDidLayoutSubviews() gives youan opportunity to do so.

If you’re wondering why we can’t make these changes in viewDidLoad() or viewWillAppear(), it’s because the frames of the subviews aren’t finalized by the time those functions are called. They’re only considered finalized once layoutSubviews() finishes running, so our only option to make customizations is in viewDidLayoutSubviews().

The order of the UIViewController lifecycle is as follows:

1. loadView()
2. viewDidLoad()
3. viewWillAppear()
4. viewWillLayoutSubviews()
5. viewDidLayoutSubviews()
6. viewDidAppear()
7. viewWillDisappear()
8. viewDidDisappear()