`UIScrollView` `bounds` are different in `viewDidAppear` than last `viewDidLayoutSubviews` - ios

Question

In attempt to improve the UI for smaller devices, when a UIScrollView contentSize.height is greater than it's bounds.height (I'm calling this an overflow) I want to tweak the UI.
My issue is: I'm finding the layout of the views is different after appearing on screen (in viewDidAppear(:)) than it is in the last viewDidLayoutSubviews() (could be called multiple times).
In order to determine if a scrollView is overflowing, we need to know both the scrollView.bounds and scrollView.contentSize. Which I thought would be determined in the last call of viewDidLayoutSubviews().
Below examples my scenario. It's running an interface built in xib with the scrollView set up there too. Note there are no hiding/showing of views to interfere with the layout values, simply runs through.
override func viewDidLayoutSubviews() {
super.viewDidLayoutSubviews()
printOverflow(type: #function)
}
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
printOverflow(type: #function)
}
private func printOverflow(type: String) {
let isOverflowY = scrollView.contentSize.height > scrollView.bounds.height
debugPrint("[Overflow] \(type) \(isOverflowY ? "Overflow" : "Fits")")
}
// After running prints:
// "[Overflow] viewDidLayoutSubviews() Fits"
// "[Overflow] viewDidAppear(_:) Overflow"
My question is: why can I not get the final layout of the subviews in the last viewDidLayoutSubviews()?
I also tried adding a view.layoutIfNeeded() in viewWillAppear(:), indeed this fires another viewDidLayoutSubviews() as you'd expect but the bounds properties are still different to that in viewDidAppear(:) (despite actually now printing "Overflow")

Answer 1

From Apple's docs (italics are mine):
func viewDidLayoutSubviews()
When the bounds change for a view controller'€™s view, the view adjusts the positions of its subviews and then the system calls this method. However, this method being called does not indicate that the individual layouts of the view'€™s subviews have been adjusted. Each subview is responsible for adjusting its own layout.
So, viewDidLayoutSubviews() can (and will) be called before subviews have been completely configured by auto-layout. That's also why we see viewDidLayoutSubviews() being called multiple times during loading / displaying.
If you subclass UIScrollView like this:
class MyScrollView: UIScrollView {
override func layoutSubviews() {
super.layoutSubviews()
printOverflow(type: #function)
}
private func printOverflow(type: String) {
let isOverflowY = contentSize.height > bounds.height
debugPrint("MyScrollView {Content Size: \(contentSize.height) Bounds Height: \(bounds.height)} [Overflow] \(type) \(isOverflowY ? "Overflow" : "Fits")")
}
}
You'll see that you get the correct heights.
What exactly are trying to do to "improve the UI for smaller devices"? There may be other (better?) options...
Edit
Here's a very simple example showing the sizing at each stage. The custom view changes its height anchor (if necessary) in its layoutSubviews() function:
import UIKit
class MySizingView: UIView {
override func layoutSubviews() {
super.layoutSubviews()
if frame.height != 200.0 {
heightAnchor.constraint(equalToConstant: 200.0).isActive = true
}
print(#function, "self height:", frame.height)
}
}
class SizingTestViewController: UIViewController {
let myView: MySizingView = {
let v = MySizingView()
v.translatesAutoresizingMaskIntoConstraints = false
v.backgroundColor = .cyan
return v
}()
override func viewDidLoad() {
super.viewDidLoad()
view.addSubview(myView)
NSLayoutConstraint.activate([
// constrain width and center X & Y
// its height will be set by its own height constraint
myView.widthAnchor.constraint(equalToConstant: 300.0),
myView.centerXAnchor.constraint(equalTo: view.centerXAnchor),
myView.centerYAnchor.constraint(equalTo: view.centerYAnchor),
])
}
override func viewDidLayoutSubviews() {
super.viewDidLayoutSubviews()
print(#function, "myView height:", myView.frame.height)
}
override func viewWillAppear(_ animated: Bool) {
super.viewWillAppear(animated)
print(#function, "myView height:", myView.frame.height)
}
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
print(#function, "myView height:", myView.frame.height)
}
}
Debug console output should be:
viewWillAppear(_:) myView height: 0.0
viewDidLayoutSubviews() myView height: 0.0
layoutSubviews() self height: 0.0
viewDidLayoutSubviews() myView height: 200.0
layoutSubviews() self height: 200.0
viewDidAppear(_:) myView height: 200.0