On Android, the web view is represented as WebView.

Java

Copy
@Override
public boolean shouldShowMessage(FullscreenMessage fullscreenMessage) {
  // access to the whole message from the parent
  Message message = (Message) fullscreenMessage.getParent();
      
  WebView webView = message.getWebView();
  
  ...
}
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@Override
public boolean shouldShowMessage(FullscreenMessage fullscreenMessage) {
  // access to the whole message from the parent
  Message message = (Message) fullscreenMessage.getParent();
      
  WebView webView = message.getWebView();
  
  ...
}

On iOS, the web view is represented as WKWebView.

Swift

Copy
func shouldShowMessage(message: Showable) -> Bool {
    // access to the whole message from the parent
    let fullscreenMessage = message as? FullscreenMessage
    let message = fullscreenMessage?.parent

    let messageWebView = message?.view as? WKWebView

    ...
}
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func shouldShowMessage(message: Showable) -> Bool {
    // access to the whole message from the parent
    let fullscreenMessage = message as? FullscreenMessage
    let message = fullscreenMessage?.parent

    let messageWebView = message?.view as? WKWebView

    ...
}

With a reference to the WebView, the instance method public void evaluateJavascript(@NonNull String script, @Nullable ValueCallback<String> resultCallback) can now be leveraged to call a JavaScript method.

Further details of this API are explained in the Android documentation - the example below is provided for the purpose of demonstration:

Java

Copy
@Override
public boolean shouldShowMessage(FullscreenMessage fullscreenMessage) {
  // access to the whole message from the parent
  Message message = (Message) fullscreenMessage.getParent();
      
  WebView webView = message.getWebView();
  // webview operations must be run on the ui thread
  webView.post(new Runnable() {
    @Override
    public void run() {
      webView.evaluateJavascript("startTimer()", new ValueCallback<String>() {
        @Override
        public void onReceiveValue(String s) {
          // do something with the content
        }
      });
    }
  });
  
  ...
}
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@Override
public boolean shouldShowMessage(FullscreenMessage fullscreenMessage) {
  // access to the whole message from the parent
  Message message = (Message) fullscreenMessage.getParent();
      
  WebView webView = message.getWebView();
  // webview operations must be run on the ui thread
  webView.post(new Runnable() {
    @Override
    public void run() {
      webView.evaluateJavascript("startTimer()", new ValueCallback<String>() {
        @Override
        public void onReceiveValue(String s) {
          // do something with the content
        }
      });
    }
  });
  
  ...
}

With a reference to the WKWebView, the instance method evaluateJavaScript(_:completionHandler:) can now be leveraged to call a JavaScript method.

Further details of this API are explained in the Apple documentation - the example below is provided for the purpose of demonstration:

Swift

Copy
func shouldShowMessage(message: Showable) -> Bool {
    // access to the whole message from the parent
    let fullscreenMessage = message as? FullscreenMessage
    let message = fullscreenMessage?.parent

    // the `shouldShowMessage` delegate method is called on a background thread.
    // need to dispatch code that uses the webview back to the main thread.
    DispatchQueue.main.async {
        let messageWebView = message?.view as? WKWebView

        messageWebView?.evaluateJavaScript("startTimer();") { result, error in
            if error != nil {
                // handle error
                return
            }

            if result != nil {
                // do something with the result
            }
        }                
    }

    ...
}
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func shouldShowMessage(message: Showable) -> Bool {
    // access to the whole message from the parent
    let fullscreenMessage = message as? FullscreenMessage
    let message = fullscreenMessage?.parent

    // the `shouldShowMessage` delegate method is called on a background thread.
    // need to dispatch code that uses the webview back to the main thread.
    DispatchQueue.main.async {
        let messageWebView = message?.view as? WKWebView

        messageWebView?.evaluateJavaScript("startTimer();") { result, error in
            if error != nil {
                // handle error
                return
            }

            if result != nil {
                // do something with the result
            }
        }                
    }

    ...
}