Objective-C/Swift (iOS) When are the auto-constraints applied in the View/ViewController work flow?

Question

I'm having some issues of figuring out when the auto-contraints setup on a XIB are applied in the view setup process.

For more explanation:

• I've setup a XIB for a view
• I set the "Simulated Metrics" Size to iPhone 3.5-Inch
• I've added auto-constraints to the subviews inside this view
• In the View Controller I perform certain operations dependent on the subview (IBOutlet) frames/bounds in the viewDidLoad method
• In the View I perform certain operations dependent on the subview (IBOutlet) frames/bounds in the awakeFromNib method

In those 2 methods (ViewController::viewDidLoad and View::awakeFromNib) the IBOutlet views HAVE been loaded but the constraints have no yet been applied. The actual frame is still set to the iPhone 3.5" size (width 320) when using a larger simulator (such as the iPhone 6 simulator).

When are these auto-constraints applied and when should any necessary operations that would need the ACTUAL frame/bounds of the subviews take place?

I'm using XCode 6.3, Swift (1.2)

Answer 1

The constraints are applied in the layoutSubviews method. So if you want to do something after they are applied in a UIView subclass, override the method:

   override func layoutSubviews() {
    super.layoutSubviews()
    //the frames have now their final values, after applying constraints
  }

In a UIViewController subclass, use viewDidLayoutSubviews for the same purpose:

  override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()
    //the frames have now their final values, after applying constraints
  }

Please note that you shouldn't set frame / bounds for a view if you added auto layout constraints to this view.