WKWebView - evaluate JavaScript without seeing changes

Question

In my SwiftUI App, i used a WKWebView to update some html tags from native side, using SwiftUI TextFields. To get the communication working, i am using the evaluateJavaScript() method to send data from native to webview.

My code looks like this:

struct DemoView: View {
    @State private var headline: String = "Initial"

    var body: some View {
        NavigationView {
            VStack {
                Form {
                    TextField("Your headline", text: $headline)
                }
                WebView(headline: $headline)
            }
        }
    }
}

import WebKit

let bridgeHTML = """
 <!DOCTYPE html>
 <html>
 <head>
 <meta name="viewport" content="width=device-width, height=device-height, initial-scale=1, minimum-scale=1, viewport-fit=cover">
 </head>
 <body>
 <h3 id="headline">Headline</h3>
 <script>
    // to receive messages from native
      webkit.messageHandlers.bridge.onMessage = (msg) => {
        document.getElementById("headline").textContent = msg
      }
    </script>
    </body>
    </html>
"""

struct WebView: UIViewRepresentable {

    @Binding var headline: String

    class Coordinator: NSObject, WKNavigationDelegate, WKScriptMessageHandler {
        func userContentController(_ userContentController: WKUserContentController, didReceive message: WKScriptMessage) {
        }

        private var owner: WebView
        init(owner: WebView) {
            self.owner = owner
        }

        var webView: WKWebView?
        func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
            self.webView = webView
            self.messageToWebview(msg: self.owner.headline)   // initial value loading !!
        }

        func messageToWebview(msg: String) {
            self.webView?.evaluateJavaScript("webkit.messageHandlers.bridge.onMessage('\(msg)')")
        }
    }

    func makeCoordinator() -> Coordinator {
        return Coordinator(owner: self)
    }

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

        let configuration = WKWebViewConfiguration()
        configuration.userContentController = userContentController

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

        guard let path: String = Bundle.main.path(forResource: "index", ofType: "html") else { return _wkwebview }
        let localHTMLUrl = URL(fileURLWithPath: path, isDirectory: false)
        _wkwebview.loadFileURL(localHTMLUrl, allowingReadAccessTo: localHTMLUrl)

//        _wkwebview.loadHTMLString(bridgeHTML, baseURL: nil)    // << used for testing

        return _wkwebview
    }

    func updateUIView(_ webView: WKWebView, context: Context) {
        // this works for update, but for initial it is too early !!
        webView.evaluateJavaScript("webkit.messageHandlers.bridge.onMessage('\(headline)')")
    }
}

It works, inside the didFinish() delegate method i set the initial values to render on the webpage. And when i type something, it automatically updates on the webpage. so far so good, but if I don't do anything for a longer time (about 1 minute) or if I switch the app (not close it) and go back to the app and then change anything in the textfield, the webpage is not updated automatically anymore.

I could of course run webView.reload(), but then I lose my previous state. Instead of Initial I get Headline. Logical because this was set inside the h3 tag, but that's what i want to avoid.

So how can i make sure that whenever evaluteJavascript() is called, the webpage is also updated without losing the previous state, no matter how long i wait or if i switch between apps? Is there any way to solve this?

score 0 · Accepted Answer · answered Jan 10 '22 at 19:34

This looks like Apple's defect, because updateUIView is called, but evaluateJavaScript (internal JavaScript) generates exception, because bridge.onMessage is lost and not a function more.

Here is found work-around - reset WebView on scene activation (app comes into foreground), as it recreated with current states the previous values are preserved. Tested with Xcode 13.2 / iOS 15.2

struct DemoView: View {
    @Environment(\.scenePhase) var scenePhase

    @State private var headline: String = "Initial"
    @State private var reset = false

    var body: some View {
        NavigationView {
            VStack {
                Form {
                    TextField("Your headline", text: $headline)
                }
                WebView(headline: $headline).id(reset)     // << here !!
                    .onChange(of: scenePhase) {
                        if case .active = $0 {
                            self.reset.toggle()            // << here !!
                        }
                    }
            }
        }
    }
}

thank you, but how do I also solve the problem that after inactivity (1min, the app is in foreground) the webpage is also not updated? — Niklas, Jan 10 '22 at 19:46

WKWebView - evaluate JavaScript without seeing changes

1 Answers1