ios - Why is viewDidLayoutSubviews called multiple times?

Question

I'm developing iOS Universal App with Swift, using auto layout and support only portrait.

I found UIViewController.viewDidLayoutSubviews called multiple times. Instead, viewDidLoad is only called once on starting up MyApp's UIViewController.

Why is viewDidLayoutSubviews called multiple times? Will constraints on each UIView (UIButtons,UITextFields, etc..) be performed in order?

Any information will be appreciated.

Answer 1

LoadView is only called once: when the view needs to be loaded.

LayoutSubviews, however, is called once per run loop on any view that has had setNeedsLayout or setNeedsDisplayInRect called on it - this includes whenever a subview has been added to the view, scrolling, resizing, etc.