Layout subviews: don't add or remove views here

Why not? LayoutSubviews can run multiple times while the view is presented or about to be presented.

Ideally don't add/remove views within layoutSubviews as can introduce duplicate view bugs.

Especially if a view is regenerated based on another call.

We could have multiple instances of a view hanging around as the object scoped reference gets replaced, but the existing view is kept alive as it's already in the view hierarchy.

Example of regeneration

var titleLabel = UILabel() // this generates the label in the properties

func someMethod() {
    if updateTitle {
        titleLabel = UILabel() // this is REgenerating the label in this method. Not a good idea.
    }
}

What to do instead

If the view is being regenerated, then add/remove the view in that call.

Even better would be you can add the view, update the view with the necessary information, and then hide it if it's not needed.

var titleLabel = UILabel() // this generates the label in the properties

func someMethod() {
    if updateTitle {
        titleLabel.text = "fancy new title" // note there is no = UILabel(), so this updates an existing label instead of regenerating it
    }
}

Why would you ever regenerate?

UILabel is not a great example, because it's to easy to update the label text using titleLabel.text, that you might wonder why any one uses regenerationg.

However imagine you have a custom object.

struct Icecream {
    init(flavour: String, toppings: String?) {}
}

Then you have a store that uses this object. You want to be able to deal with people changing their mind about the flavour.

You may have:

final class IceCreamStore {
    private lazy var icecream = Icecream(flavour: "chocolate")

    init() { }

    func changeFlavour(newFlavour: "peppermint") {
        icecream = Icecream(flavour: "peppermint") // there is no way to change the flavour of the icecream, so you have to regenerate it
    }
}

What would be better would be to update the struct so that the struct has a method that can update the ice cream flavour. Just like UILabel has a method that can update the text.

Then you icecream store could call this:

func changeFlavour(newFlavour: "peppermint") {
        icecream.updateFlavour(to: "peppermint") // no regeneration of the initial struct, Icecream()
    }