WebView Performance

WebTimingApp.swift

import SwiftUI

@main
struct WebTimingApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }
}

struct ContentView: View {
    var body: some View {
        WebView(url: URL(string: "https://google.com")!)
            .edgesIgnoringSafeArea(.all)
    }
}

WebViewWrapper.swift

import SwiftUI
import WebKit

struct WebView: UIViewRepresentable {
    let url: URL

    func makeCoordinator() -> Coordinator {
        Coordinator()
    }

    func makeUIView(context: Context) -> WKWebView {
        let contentController = WKUserContentController()
        contentController.add(context.coordinator, name: "onLoadTime")

        // Inject JS to record window.onload timestamp
        let js = """
        window.addEventListener('load', () => {
            const onLoadTime = performance.now();
            window.webkit.messageHandlers.onLoadTime.postMessage(onLoadTime);
        });
        """
        let userScript = WKUserScript(source: js, injectionTime: .atDocumentEnd, forMainFrameOnly: true)
        contentController.addUserScript(userScript)

        let config = WKWebViewConfiguration()
        config.userContentController = contentController

        let webView = WKWebView(frame: .zero, configuration: config)
        webView.navigationDelegate = context.coordinator

        let request = URLRequest(url: url)
        webView.load(request)

        return webView
    }

    func updateUIView(_ uiView: WKWebView, context: Context) {}

    class Coordinator: NSObject, WKNavigationDelegate, WKScriptMessageHandler {
        private var onLoadTime: Double?

        func userContentController(_ userContentController: WKUserContentController, didReceive message: WKScriptMessage) {
            if message.name == "onLoadTime", let time = message.body as? Double {
                self.onLoadTime = time
                print("onload event fired at \(time) ms")
            }
        }

        func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
            let finishTime = Date().timeIntervalSince1970 * 1000 // in ms

            if let onLoadTime = self.onLoadTime {
                let loadDuration = finishTime - onLoadTime
                print("Time from window.onload to didFinish: \(loadDuration) ms")
            } else {
                print("onload time not yet captured")
            }
        }
    }
}