How to Javascript notify() with ms-appdata on Windows 10 UWP

Due to file-writing security issues we have tried to switch our app from using ms-appx-web: to using ms-appdata:. But it immediately fails because we rely on window.external.notify() which works fine with ms-appx-web: but seems to behave as a no-op with ms-appdata:. As a test we loaded the following html into a WebView object:

<html>
<head>
    <script>
        function demofunc( str ) {
            document.getElementById("demo").innerHTML += str;
            window.external.notify( str );
        }
    </script>
</head>
<body onLoad="demofunc('demofunc()');">
    demo &lt;body&gt;
    <div id="demo"></div>
</body>
</html>

which produces this result as it should:

demo <body> 
demofunc()

BUT, does NOT produce a popup message of any kind. Why? Clearly the demofunc() method is being called to add the second line of output in the demo div, but window.external.notify() is not producing a popup message. Are there special rules regarding notify() along with ms-appdata:?

Update - the question Can't run javascript alerts in universal app's webview at payment gateway is similar and works for ms-appx-web: but not for ms-appdata:. That question catches ScriptNotify() which then uses Windows.UI.Popups.MessageDialog to popup a dialog. With ms-appx-web: the ScriptNotify() is called but with ms-appdata: the ScriptNotify() is not called. That is our problem, there is no popup occurring.

Solution

But it immediately fails because we rely on window.external.notify() which works fine with ms-appx-web: but seems to behave as a no-op with ms-appdata:.

For your scenario please use the scheme ms-local-stream:///, rather than ms-appdata:///. And I have tested ms-local-stream:/// scheme, it is working pretty well.

To use the NavigateToLocalStreamUri method, you must pass in an IUriToStreamResolver implementation that translates a URI pattern into a content stream. Please reference the following code.

StreamUriWinRTResolver

public sealed class StreamUriWinRTResolver : IUriToStreamResolver
    {
        /// <summary>
        /// The entry point for resolving a Uri to a stream.
        /// </summary>
        /// <param name="uri"></param>
        /// <returns></returns>
        public IAsyncOperation<IInputStream> UriToStreamAsync(Uri uri)
        {
            if (uri == null)
            {
                throw new Exception();
            }
            string path = uri.AbsolutePath;
            // Because of the signature of this method, it can't use await, so we
            // call into a separate helper method that can use the C# await pattern.
            return getContent(path).AsAsyncOperation();
        }

        /// <summary>
        /// Helper that maps the path to package content and resolves the Uri
        /// Uses the C# await pattern to coordinate async operations
        /// </summary>
        private async Task<IInputStream> getContent(string path)
        {
            // We use a package folder as the source, but the same principle should apply
            // when supplying content from other locations
            try
            {
                Uri localUri = new Uri("ms-appdata:///local" + path);
                StorageFile f = await StorageFile.GetFileFromApplicationUriAsync(localUri);
                IRandomAccessStream stream = await f.OpenAsync(FileAccessMode.Read);
                return stream.GetInputStreamAt(0);
            }
            catch (Exception) { throw new Exception("Invalid path"); }
        }
    }

MainPage

public MainPage()
 {
     this.InitializeComponent();
     MyWebView.ScriptNotify += MyWebView_ScriptNotify;
     Uri url = MyWebView.BuildLocalStreamUri("MyTag", "/Test/HomePage.html");
     StreamUriWinRTResolver myResolver = new StreamUriWinRTResolver();
     MyWebView.NavigateToLocalStreamUri(url, myResolver);
 }

I have uploaded the code sample to github. Please check.