Setting origin of container to another container trouble (Swift 4.2) - ios

Question

I'm trying to set the origin and width/height of one UIView (red) to a second UIView (blue).
I am calling UIView.frame.origin or size and for some reason the y origin doesn't work.
I've also tried with layout constraints (see it commented out below), but this is overriding my blue fully constrained view.
Then I have a button that animates the red view to the side so you can see the blue view underneath, but I can't get them to line up to start with. Below is my code. In interface builder, I have both UIViews set up as containers. Blue is fully constrained with auto layout and red has no constraints.
import UIKit
class ViewController: UIViewController
{
#IBOutlet weak var blueContainer: UIView!
#IBOutlet weak var redContainer: UIView!
#IBOutlet weak var button: UIButton!
override func viewDidLoad() {
super.viewDidLoad()
print(redContainer.frame)
redContainer.frame.origin.x = view.frame.width/2
redContainer.frame.size.width = view.frame.width
//try to line up y with origin and size
redContainer.frame.origin.y = blueContainer.frame.origin.y
redContainer.frame.size.height = blueContainer.frame.size.height
//also tried by using constraints
//redContainer.topAnchor.constraint(equalTo: blueContainer.topAnchor).isActive = true
//redContainer.heightAnchor.constraint(equalTo: blueContainer.heightAnchor).isActive = true
print(redContainer.frame)
}
#IBAction func slideRed(_ sender: Any) {
if redContainer.frame.origin.x == 0 {
UIView.animate(withDuration: 0.5) {
self.redContainer.frame.origin.x = self.view.frame.width/2
}
button.setTitle("Come Back Red!", for: .normal)
} else {
UIView.animate(withDuration: 0.5) {
self.redContainer.frame.origin.x = 0
}
button.setTitle("Go Away Red!", for: .normal)
}
}
}

Answer 1

ViewDidLoad does not guarantee the view has laid out its constraints. So when blueContainer's frame and size is zero, you will not see any effect on redContainer. You should use viewDidLayoutSubviews to get the correct frame and size from blueContainer.
override func viewDidLayoutSubviews() {
super.viewDidLayoutSubviews()
redContainer.frame.origin.x = view.frame.width/2
redContainer.frame.size.width = view.frame.width
//try to line up y with origin and size
redContainer.frame.origin.y = blueContainer.frame.origin.y
redContainer.frame.size.height = blueContainer.frame.size.height
}