Nick's .NET Travels

Continually looking for the yellow brick road so I can catch me a wizard....

Limitations of the WebView in Windows 8 Metro Apps

Having worked quite intimately with the WebBrowser control in Windows Phone I was a little flawed by how immature and annoying the WebView for Windows 8 metro app is. As noted by Brent Schooley in his post on Metro WebView Source and HTML workarounds you can work around some of the limitations such as the missing SaveToString method. One of the features of the WebBrowser control that I quite often use is the series of events that are raised around navigating to pages within the control. Unlike the WebBrowser, the WebView is missing events such as Navigating, Navigated and NavigationFailed, and has only the LoadCompleted event. I figured that I should be able to detect when the current page is changing, and thus raise a Navigating event. Turns out that it is possible with a combination of InvokeScript (to invoke some javascript within the page) and window.external.notify (to raise an event when the page in the browser is changing). The following WebViewWrapper class exposes a Navigating event which you can wire an event handler to in order to detect when the browser is being navigated away from the current page (unfortunately there doesn’t seem to be a way to extract the target url, nor to be able to cancel the navigation, unlike the real Navigating event that the WebBrowser control has). I’ve also included a SaveAsString method which wraps the code Brent had in his post.

public class WebViewWrapper
{
    // Maintain a reference to the WebView control so that
    // we can invoke javascript
    public WebView WebView { get; private set; }

    public WebViewWrapper(WebView webView)
    {
        WebView = webView;
    }

    // The Navigating event is a custom event, allowing us to hook/unhook
    // from the ScriptNotify and LoadCompleted events. To invoke this
    // event, we actually invoke the internalNavigating event.
    private event EventHandler<NavigatingEventArgs> internalNavigating;
    public event EventHandler<NavigatingEventArgs> Navigating
    {
        add
        {
            WebView.ScriptNotify+=NavigatingScriptNotify;
            WebView.LoadCompleted+=WireUpNavigating;
            internalNavigating += value;
        }
        remove
        {
            WebView.ScriptNotify -= NavigatingScriptNotify;
            WebView.LoadCompleted-=WireUpNavigating;
            internalNavigating -= value;
        }
    }

    // When each page loads, run a javascript function which wires up
    // an event handler to the onbeforeunload event on the window.  This
    // event is raised when the window is about to unload the current page
    // In the event handler we call window.external.notify in order to raise
    // the ScriptNotify event on the WebView. The javascript function also
    // returns the current document location. This is used to update the
    // AllowedScriptNotifyUris property on the WebView in order to permit
    // the current document to call window.external.notify (remembering
    // that even though we injected the javascript, it’s being invoked in the
    // context of the current document.
    private void WireUpNavigating(object sender, NavigationEventArgs e)
    {
        var unloadFunc = "(function(){" +
                            " function navigating(){" +
                            "  window.external.notify('%%' + location.href);" +
                            "} " +
                            "window.onbeforeunload=navigating;" +
                            "return location.href;" +
                            "})();";
        var host = WebView.InvokeScript("eval", new[] { unloadFunc });
        WebView.AllowedScriptNotifyUris = new[] { new Uri(host) };
    }

   // Check to see if the ScriptNotify was raised by the javascript we
   // injected (ie does it start with %%), and then raise the Navigating
   // event.
   private void NavigatingScriptNotify(object sender, NotifyEventArgs e)
    {
        if (internalNavigating == null) return;
        if(!string.IsNullOrWhiteSpace(e.Value))
        {
            if(e.Value.StartsWith("%%"))
            {
                internalNavigating(this, new NavigatingEventArgs() {LeavingUri = new Uri(e.Value.Trim('%'))});
            }
        }
    }

    public string SaveToString()
    {
        try
        {
            var retrieveHtml = "document.documentElement.outerHTML;";
            var html = WebView.InvokeScript("eval", new[] { retrieveHtml });
            return html;
        }
        catch
        {
            return null;
        }
    }
}

public class NavigatingEventArgs:EventArgs
{
    public Uri LeavingUri { get; set; }
}

Actually making use of the WebViewWrapper class is as simple as creating an instance and then wiring up an event handler.

public BlankPage()
{
    this.InitializeComponent(); 

    var wrapper = new WebViewWrapper(MyWebView);
    wrapper.Navigating += WrapperNavigating;
}

void WrapperNavigating(object sender, NavigatingEventArgs e)
{
    Debug.WriteLine(e.LeavingUri);
    Debug.WriteLine((sender as WebViewWrapper).SaveToString());
}

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