Is it Possible to Interleave WebView and CALayerViews in Swift

I have a view that acts as a container for multiple CAShapeLayers. These contain bezier that form a composition on the screen that the user can manage in different ways to do with line weight, colour, shape fill, opacity etc.

I want to introduce text via a WebView that will occupy the same container. The trick is that ideally I want to be able to control the zPosition in the container of each element, relative to the others.

Can a WebView be in a container in a way that it shares zPosition range with CAShapeLayers in that container?

Solution

Couple options...

Use html Canvas and draw shapes in the page content itself.
Create a clear web view and draw shapes in views under / on-top-of the web view.
Add sublayers to the web view's layer (will still need to implement clear html to put layers under the html content).

Here's a quick example of Option 3:

class ViewController: UIViewController {
    
    var webView: WKWebView!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        webView = WKWebView()
        webView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(webView)
        
        let g = view.safeAreaLayoutGuide
        NSLayoutConstraint.activate([
            webView.topAnchor.constraint(equalTo: g.topAnchor, constant: 20.0),
            webView.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 20.0),
            webView.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -20.0),
            webView.bottomAnchor.constraint(equalTo: g.bottomAnchor, constant: -20.0),
        ])
        
        // this will allow us to see the shape layers
        //  if we put them *behind* the html content
        webView.isOpaque = false
        webView.backgroundColor = .clear
        webView.scrollView.backgroundColor = .clear
        
        // webView will be clear, so let's give it a border
        // so we can see the frame
        webView.layer.borderWidth = 1
        webView.layer.borderColor = UIColor.black.cgColor
        
        webView.loadHTMLString(html, baseURL: nil)
    }
    
    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        
        // we'll create 4 CAShapeLayers
        //  with oval paths
        //  each 1/4 the height of the webView
        //  overlapping a little
        var y: CGFloat = 12.0
        let yInc: CGFloat = (webView.bounds.height - 32.0) * 0.25
        var pathRect: CGRect = webView.bounds.insetBy(dx: 20.0, dy: 0.0)
        pathRect.size.height *= 0.25
        
        let colors: [UIColor] = [
            .systemRed, .systemGreen, .systemBlue, .cyan
        ]
        
        for (i, c) in colors.enumerated() {
            
            // oval CAShapeLayer with
            //  solid color thick stroke
            //  50% transparent color fill
            let cLayer = CAShapeLayer()
            cLayer.lineWidth = 8
            cLayer.strokeColor = c.cgColor
            cLayer.fillColor = c.withAlphaComponent(0.5).cgColor
            cLayer.path = UIBezierPath(ovalIn: pathRect.offsetBy(dx: 0.0, dy: y)).cgPath
            
            // add shape layers to the webView's layer
            //  shape layers will NOT scroll
            var targetLayer = webView.layer
            
            // or, add shape layers to the webView's scrollView's layer
            //  shape layers WILL scroll
            //targetLayer = webView.scrollView.layer
            
            if i < 2 {
                // insert the first two layers *behind* the web page content
                targetLayer.insertSublayer(cLayer, at: UInt32(i))
            } else {
                // add the second two layer *on top of* the web page content
                targetLayer.addSublayer(cLayer)
            }
            
            y += yInc
        }
        
    }
    
    let html: String =
"""
<html>
<head>
<style type=\"text/css\">
body{
 font-size: 100px;
 font-family:verdana;
 text-align: center;
 color: #ffdd00;
}
</style>
</head>
<body>
<p>This is some web page body text in a WKWebView.</p>
<p>Red and Green shape layers are <i><b>behind</b></i> the html content.</p>
<p>Blue and Cyan shape layers are <i><b>on top of</b></i> the html content.</p>
<p>Let's add enough text to exceed the height of the web view so we can see what happens when we scroll.</p>
</body>
</html>
"""
    
}

Looks like this: