CALayerInvalidGeometry: CALayer position contains NaN when trying to size a small, remote svg with variable size and aspect ratio - svg

Question

What I want to achieve:
Display small, variable sized svgs in my app with auto layout.
The svgs contain pre-rendered latex formulas. The svgs are loaded asynchronously so I don't know upfront which size or aspect ratio they are.
The hardest pre-conditions I could imagine.
I tried several svg displaying libraries, and no one turned out to work with our svgs, so I decided to use a UIWebView.
The problem here is, that the svgs should not be scrollable and the UIWebView of minimal size.
The svgs should span all available width and have a height based on the aspect ratio.
What I tried:
class FormulaView: UIView, UIWebViewDelegate, XMLParserDelegate {
private var view: UIView!
#IBOutlet weak var webView: UIWebView!
#IBOutlet weak var topMargin: NSLayoutConstraint!
#IBOutlet weak var bottomMargin: NSLayoutConstraint!
private var width = -1.0
private var height = -1.0
// MARK: Initialisation
override init(frame: CGRect) {
super.init(frame: frame)
setUpView()
}
required init?(coder: NSCoder) {
super.init(coder: coder)
setUpView()
}
// MARK: Set Up
// Performs the initial setup.
// Loads a XIB file into a view and returns this view.
func loadSVG(fromURL url: URL) {
let request = NSURLRequest(url: url)
webView.delegate = self
webView.loadRequest(request as URLRequest)
}
func loadSVG(fromString string: String) {
let url = URL(string: string)
if url == nil {
self.isHidden = true
return
}
self.loadSVG(fromURL: url!)
}
var tableView: UITableView?
private func viewFromNibForClass() -> UIView {
let bundle = Bundle(for: type(of: self))
let nib = UINib(nibName: String(describing: type(of: self)), bundle: bundle)
let view = nib.instantiate(withOwner: self, options: nil).first as! UIView
return view
}
private func setUpView() {
view = viewFromNibForClass()
view.frame = bounds
// Auto-layout stuff.
view.autoresizingMask = [
UIViewAutoresizing.flexibleWidth,
UIViewAutoresizing.flexibleHeight
]
addSubview(view)
// Formula should take no space if no formula is loaded
topMargin.constant = 0
bottomMargin.constant = 0
self.translatesAutoresizingMaskIntoConstraints = false
self.addConstraints([
NSLayoutConstraint(item: self, attribute: .top, relatedBy: .equal,
toItem: view, attribute: .top, multiplier: 1.0, constant: 0),
NSLayoutConstraint(item: self, attribute: .right, relatedBy: .equal,
toItem: view, attribute: .right, multiplier: 1.0, constant: 0),
NSLayoutConstraint(item: self, attribute: .bottom, relatedBy: .equal,
toItem: view, attribute: .bottom, multiplier: 1.0, constant: 0),
NSLayoutConstraint(item: self, attribute: .left, relatedBy: .equal,
toItem: view, attribute: .left, multiplier: 1.0, constant: 0)
])
}
// MARK: - UIWebView delegate
func webViewDidFinishLoad(_ webView: UIWebView) {
// Get xml of svg file
let xml = webView.stringByEvaluatingJavaScript(from: "document.documentElement.outerHTML")
let parser = XMLParser(data: (xml?.data(using: .utf8))!)
parser.delegate = self
parser.parse()
}
// MARK: - XMLParser delegate
func parser(_ parser: XMLParser, didStartElement elementName: String, namespaceURI: String?, qualifiedName qName: String?, attributes attributeDict: [String : String] = [:]) {
// Get width and height of svg
if elementName == "svg" {
self.height = Double(attributeDict["height"]?
.replacingOccurrences(of: "pt", with: "") ?? "")
?? -1.0
self.width = Double(attributeDict["width"]?
.replacingOccurrences(of: "pt", with: "") ?? "")
?? -1.0
let aspectRatio = self.width / self.height
tableView?.beginUpdates()
// Calculate height the webview should have based on the width and the normal aspect ratio of the svg
let webviewHeight = self.webView.bounds.size.width / CGFloat(aspectRatio)
if let constraint = (webView.constraints.filter {
$0.firstAttribute == .height && $0.secondAttribute == .notAnAttribute
}.first) {
constraint.constant = webviewHeight
} else {
webView.heightAnchor.constraint(equalToConstant: webviewHeight).isActive = true
}
// Apply a 24 margin as a svg is now loaded
topMargin.constant = 24
bottomMargin.constant = 24
tableView?.endUpdates()
self.layoutIfNeeded()
self.invalidateIntrinsicContentSize()
// Zoom webview appropriately that the svg is completely visible and no scrolling is neccessary or even possible
let scrollView = webView.scrollView
let zoom = webView.bounds.size.width / scrollView.contentSize.width
scrollView.maximumZoomScale = zoom
scrollView.setZoomScale(zoom, animated: true) // Error occurs here
// prevent scrolling
scrollView.isScrollEnabled = false
scrollView.isPagingEnabled = false
scrollView.isUserInteractionEnabled = false
scrollView.isDirectionalLockEnabled = true
}
}
}
This version seems to work fine as I tested it on the simulator,
but in some cases (EDIT: I only see these crashes in our Bug reporting tool), I get an error (CALayerInvalidGeometry: CALayer position contains NaN: [nan nan]) at this line scrollView.setZoomScale(zoom, animated: true).
It seems the calculating the zoom results in CGFloat.nan,
(1) but when does this occur,
(2) how can I prevent this and
(3) if it occurs, how can I calculate the zoom in these cases?
EDIT: Or in other words, why are webView.bounds.size.width and/or scrollView.contentSize.width non positiv values?
If I could reproduce it, I could say more, but unluckily I can't...
Thanks in advance,
Dennis