UIScrollView contentLayoutGuide and zooming centered

Question

The problem to be solved here is how to zoom in a UIScrollView while staying centered. If you don't take some sort of precautions, the default is that as we zoom out, the zoomed view slides up to the top left corner of the scroll view, like this:

So how to prevent this, and keep the zoomed view in the center as we zoom? As you probably know, there are traditional ways of handling this by messing with the scroll view's layout, as described by Josh and Eliza in the brilliant classic WWDC video 104 from 2010. This can be done by using a delegate or by subclassing UIScrollView, and gives the desired result:

Now comes WWDC 2017 video 201 (https://developer.apple.com/videos/play/wwdc2017/201/?time=1496), and there's Eliza making a claim that the new (iOS 11) contentLayoutGuide solves the problem of zooming while staying centered in a new way: she says to center the content view at the center of the content layout guide.

But she doesn't demonstrate. And when I try it for myself, I find it isn't solving the problem. I'm zooming in just fine, but when zooming out, so that the zoom scale is smaller than 1, the content view moves up to the top left, just as it always has.

Has anyone figured out what this claim in the video actually means? How does iOS 11 make it easier to zoom centered than in the past?

EDIT I actually received a sample project from Apple in response to my bug report, which they claimed illustrated how to solve this, and it didn't! So I conclude that even Apple doesn't know what they're talking about here.

Answer 1

The view goes to the top left because the contentSize of the scroll view is not defined. When using the new Auto Layout guides in iOS 11, it's still necessary to define the contentSize.

Add the following constraints:

scrollView.contentLayoutGuide.widthAnchor.constraint(equalTo: scrollView.frameLayoutGuide.widthAnchor),
scrollView.contentLayoutGuide.heightAnchor.constraint(equalTo: scrollView.frameLayoutGuide.heightAnchor)

---

This worked for me, when I had a contentView with a fixed width/height and the following additional constraints:

//  give the centerView explicit height and width constraints
centerView.widthAnchor.constraint(equalToConstant: 500),
centerView.heightAnchor.constraint(equalToConstant: 500),

//  pin the center of the centerView to the center of the scrollView's contentLayoutGuide
centerView.centerXAnchor.constraint(equalTo: scrollView.contentLayoutGuide.centerXAnchor),
centerView.centerYAnchor.constraint(equalTo: scrollView.contentLayoutGuide.centerYAnchor)

Answer 2

This is the solution you are / everybody is looking for. In my case I want to center a view inside a table view scroll view. So if the table view scrolls the custom view will always be in the center of the scroll view content.

// create a view
let v:UIView = UIView(frame:CGRect.zero)    // use zero if using constraints
    ibTableView.addSubview(v)
    ibTableView.bringSubview(toFront:v)
v.translatesAutoresizingMaskIntoConstraints = no
v.backgroundColor = .yellow
v.widthAnchor.constraint(equalToConstant:100).isActive = yes
v.heightAnchor.constraint(equalToConstant:100).isActive = yes

// set scrollview guides
ibTableView.contentLayoutGuide.widthAnchor.constraint(equalTo:ibTableView.frameLayoutGuide.widthAnchor).isActive = yes
ibTableView.contentLayoutGuide.heightAnchor.constraint(equalTo:ibTableView.frameLayoutGuide.heightAnchor).isActive = yes

// anchor view
v.centerXAnchor.constraint(equalTo:ibTableView.contentLayoutGuide.centerXAnchor).isActive = yes
v.centerYAnchor.constraint(equalTo:ibTableView.contentLayoutGuide.centerYAnchor).isActive = yes