Autosizing cells in UICollectionViewLayout (supporting iOS 9) without `invalidateLayout` jump - ios

Question

I have been reading this article about using custom UICollectionViewLayouts and trying to incorporate the idea into a project I'm working on.
We were previously using a UICollectionViewFlowLayout subclass with a huge ugly function to determine the size of the cells by dequeueing a prototype cell and populating it and then asking it for its size.
Needless to say, I wanted to find a better way of doing this.
The problem I'm having is with the initial layout of the cells. A lot of the cells that we use contain a 1 or 2 labels that may contain a lot of text and need to wrap. In the linked article there is a playground which shows multi-line labels that size automatically but it is inserting line breaks into the text to force this...
In the Datasource.swift file...
lazy private var values: [String] = {
return (0...255).map {
_ in
switch arc4random_uniform(3) {
case 0: return "Hello"
case 1: return "Hello\nGoodbye"
default: return "Hello\nGoodbye\nAu revoir"
}
}
}()
However, if I change these to strings of different length that would break naturally it breaks the example. The initial layout only ever has one line in the label. But then, on scrolling the labels display correctly.
I can "fix" this by adding the line layout.invalidateLayout() in the Autosizing file.
Now, I'm in my own project and trying to do the same thing. Using the layout.invalidateLayout() only works in the view controller if I put it in the viewDidAppear function. This means that we get an awkward jump after the transition the layout is incorrect and then "jumps" to the correct layout. However, I feel like the entire approach of using invalidateLayout is broken.
Is there a way I can get this to not break on the initial layout? Or to intelligently invalidate the cells initial layout?
I added a GitHub project that shows the problem I am having. Currently it is laying out correctly because of the code...
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
// This invalidation "fixes" the layout but causes a jump in the UI
collectionView?.collectionViewLayout.invalidateLayout()
}
Ideally I'd like to remove this and have the layout "just work".

Answer 1

The problem here is that on the initial pass, the preferred layout attributes returned by the cell are incorrect - it sizes the label based on a seemingly limitless width, so you end up with it wanting to fit on one line.
Adding this line to the cell setUp() method "fixes" the problem:
label.preferredMaxLayoutWidth = self.bounds.width - (contentView.layoutMargins.left + contentView.layoutMargins.right)
It's a bit dirty, I'm sure there is a better way.
EDIT
OK, here's a better way. This forces your views into the original estimated frame from the layout, giving the AL engine a chance to size the cell properly. Do this in your cell subclass:
private var isInitialLayout = true
public override func preferredLayoutAttributesFitting(_ layoutAttributes: UICollectionViewLayoutAttributes) -> UICollectionViewLayoutAttributes {
if isInitialLayout {
self.contentView.layoutIfNeeded()
isInitialLayout = false
}
return super.preferredLayoutAttributesFitting(layoutAttributes)
}