Xamarin Android WebView access to sessionStorage returns null

Issue

I have this custom WebViewClient where every time a Page finish loading I want to check the sessionStorage for a value using javascript injection to read the storage.

The problem is that callbackobj Value is always (null). What am I doing wrong ?

public class CustomWebViewClient : WebViewClient
{

    public override void OnPageFinished(WebView view, string url)
    {
        string script = "window.sessionStorage.getItem('someKey');";
        var callbackobj = InjectJS(view, script);

        base.OnPageFinished(view, url);
    }

    private JsValue InjectJS(WebView view, string script)
    {
        var valueCallback = new JsValue();
        view.EvaluateJavascript(script, valueCallback);

        return valueCallback;

    }
    public class JsValue : Java.Lang.Object, IValueCallback
    {
        public string Value { get; set; }
        public void OnReceiveValue(Java.Lang.Object value)
        {
            this.Value = value.ToString() ?? throw new ArgumentNullException(nameof(value));
        }
    }
}

I have also a custom renderer implemented from here where I attach the new CustomWebClient.

protected override void OnElementChanged(ElementChangedEventArgs<HybridWebView> e)
    {
        base.OnElementChanged(e);

        if (Control == null)
        {
            var webView = new Android.Webkit.WebView(_context);
            webView.SetWebViewClient(new CustomWebViewClient());

            webView.Settings.JavaScriptEnabled = true;
            webView.Settings.DomStorageEnabled = true;
            SetNativeControl(webView);
        }
        if (e.OldElement != null)
        {
            Control.RemoveJavascriptInterface("jsBridge");
            var hybridWebView = e.OldElement as HybridWebView;
            hybridWebView.Cleanup();
        }
        if (e.NewElement != null)
        {
            Control.AddJavascriptInterface(new JSBridge(this), "jsBridge");
            Control.LoadUrl(Element.Uri);

        }
    }

EDIT: It doesn't even work with simple script as returning document.body, not only storage accessing. The problem must be more general.

Solution

I've played with it and I figured it out.

I don't need jsBridge when using EvaluateJavaScript.

When the script is changed to

string script = "javascript:(function() { return sessionStorage.getItem('someKey'); })()

it works.

Also I changed conversion to be:

public void OnReceiveValue(Java.Lang.Object value)
{
 this.Value = ((Java.Lang.String)value).ToString() ?? throw new ArgumentNullException(nameof(value));
}

EDIT : If there is also a need for awaiting the JS to execute which I needed in this scenario this answer did the work https://forums.xamarin.com/discussion/comment/283373/#Comment_283373

Answered By - gneric