webview-scrolldelegate.swift

import UIKit
import WebKit

// Disableing `WKWebView` user zooming by returning `nil` in `UIScrollViewDelegate`'s
// `viewForZooming` delegate method.
// On iOS 12, the delegate method only called when set the web view itself as the
// scroll view delegate.

class WebView: WKWebView {}

extension WebView: UIScrollViewDelegate {
    func viewForZooming(in scrollView: UIScrollView) -> UIView? {
        return nil
    }
}

class ViewController: UIViewController {

    var webView: WebView!

    override func viewDidLoad() {
        super.viewDidLoad()

        view.backgroundColor = .red

        webView = WebView(frame: .zero)

        // This does not work on iOS 12.
        // `viewForZooming` in "extension ViewController" not called at all.
        // But on iOS 11 and earlier, it works fine.

        // webView.scrollView.delegate = self

        // Only this works on iOS 12!
        webView.scrollView.delegate = webView

        webView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(webView)

        let safeAreaLayoutGuide = view.safeAreaLayoutGuide
        [
            webView.topAnchor.constraint(equalTo: safeAreaLayoutGuide.topAnchor),
            webView.leadingAnchor.constraint(equalTo: safeAreaLayoutGuide.leadingAnchor),
            webView.bottomAnchor.constraint(equalTo: safeAreaLayoutGuide.bottomAnchor),
            webView.trailingAnchor.constraint(equalTo: safeAreaLayoutGuide.trailingAnchor)
        ]
            .forEach { $0.isActive = true }

        webView.load(URLRequest(url: URL(string: "https://google.com")!))
    }
}

extension ViewController: UIScrollViewDelegate {
    func viewForZooming(in scrollView: UIScrollView) -> UIView? {
        return nil
    }
}

Note: this appears to work, even if you don't set the webView as the scrollView's delegate. This tells us it already is the scrollView's delegate.

So this is a bit of magic, but it works :/