No, there is no error, it's the flow I am trying to understand. The code block in-between the last { } get's called multiple times , when the buffer is full I assume, and then the code continues to run everything below it as well.

Experimenting around I found that if I didn't set the child's constraints (see below), then it would always work. Is there a method to remove the constraints of a UIView before deleting it? vc is the Child UIView and pc is its parent func setConstraints (vc: UIViewController, pc: UIView) { vc.view.translatesAutoresizingMaskIntoConstraints = false var constraints = [NSLayoutConstraint]() constraints.append(vc.view.leadingAnchor.constraint(equalTo: pc.safeAreaLayoutGuide.leadingAnchor)) constraints.append(vc.view.trailingAnchor.constraint(equalTo: pc.safeAreaLayoutGuide.trailingAnchor)) constraints.append(vc.view.bottomAnchor.constraint(equalTo: pc.safeAreaLayoutGuide.bottomAnchor)) constraints.append(vc.view.topAnchor.constraint(equalTo: pc.safeAreaLayoutGuide.topAnchor)) NSLayoutConstraint.activate(constraints) }

For anyone coming across this with the same need, I found this to be a great tutorial: https://youtu.be/SZJ8zjMGUcY (no, it's not me)

Doh! Thanks. Am going to blame the laziness of autocomplete. That’s why I like to type everything. Ingrains it better.