Friday, September 21, 2012

Windows 8: Windows Store apps - JavaScript passing data to the code containing the WebView control

Over the past few weeks, in my spare time, I've been working on building a Windows Store app that will wrap an HTML web app that I've built.

To use the web app, you enter a token in a textbox on the welcome page and then click the Verify button.

If the token is valid, a URI is given to the user which can then be used for the tracking of one's time using the start/stop timer view in the web app.

The issue that we're going to tackle in this blog post is the following:

When the user clicks the Verify button on the welcome page and a URI is generated, I would like the JavaScript to call the code that holds the WebView control to have the URI saved.

This will save the user the trouble of having to copy the URI, open up the settings flyout, and paste in the new URI.

I know that the JavaScript, of a page loaded in a WebBrowser control, can talk to the C# code when the ObjectForScripting option is turned on but do we have access to a similar feature in a Windows Store app when using a WebView control?

ScriptNotify and AllowedScriptNotifyUris

As it turns out, it is possible for the JavaScript of a page loaded into a WebView control to call into the C# code of the page if you set up a ScriptNotify event.

According to the Microsoft documentation on the ScriptNotify event (, depending on which method you use to display the page in the WebView control, you may or may not have to specify URIs that are allowed to call the event handler.

If you use the WebView's Navigate method then you must specify the list of URIs that are allowed to call the ScriptNotify event handler.

If you use the WebView's NavigateToString method then the list of URIs is not necessary.

I've been simply setting the WebView's 'Source' property with the desired URI so the question is: Do I need to specify the list of URIs or not?

As it turns out, if I don't specify the list of URIs when using the WebView's Source property, the ScriptNotify event does not trigger.

You specify a list of allowed URIs in the following manner:
List<Uri> lstAllowedUris = new List<Uri>();
lstAllowedUris.Add(new Uri(""));
wvBrowser.AllowedScriptNotifyUris = lstAllowedUris;

Note: The property AllowedScriptNotifyUris almost suggests that you can say one page can make ScriptNotify event handler calls but none of the other pages on the site can.

In reality, based on my testing, even if you specify a URI to a particular page, any page within that domain can call the event handler.

The AllowedScriptNotifyUris property is really a list of allowed domains who's pages are allowed to call the ScriptNotify event handler.

The following is an example of how to wire up a ScriptNotify event handler:
public MainPage()
// Only allow the following domain's pages to call our ScriptNotify
// event handler
List<Uri> lstAllowedUris = new List<Uri>();
lstAllowedUris.Add(new Uri(""));
wvBrowser.AllowedScriptNotifyUris = lstAllowedUris;

// Attach our ScriptNotify event handler
wvBrowser.ScriptNotify += wvBrowser_ScriptNotify;

The following is an example of the event handler itself:
void wvBrowser_ScriptNotify(object sender, NotifyEventArgs e)
// The string received from the JavaScript code can be found
// in e.Value

On the JavaScript side of things, the code is similar to what you would write when calling the C# code of a WebBrowser control with the exception that the function you call is 'notify' where with a WebBrowser control you could define your own callback function name.

The following is an example of the JavaScript code that calls the C# code:
// We have a couple of checks to make sure the 'external'
// object and 'notify' method exist before we try using them
// because our project is a web application and might not be
// running in a WebView control (the checks prevent
// JavaScript errors if the method doesn't exist)
if ((typeof (window.external) !== "undefined") &&
(typeof (window.external.notify) !== "undefined"))

In Conclusion

Even though the JavaScript is only capable of passing a string to the ScriptNotify event handler, it is highly recommended that you verify the string is in the expected format before trying to use it even if the string came from a source you included in the list of allowed URIs.

Additional Resources

If you are interested in details on how to implement a Settings flyout, especially if you are using a WebView control, I have a previous article that might be of interest: Windows 8: Windows Store apps and the WebView control (we discussed how to display a Settings flyout and focused mostly around making the WebView control play nice with the flyout).

If you are interested in RoamingSettings and the DataChanged event, the following article might also be of interest: Windows 8: Windows Store apps - RoamingSettings and the DataChanged event

A download of the project (C# and built using Visual Studio Express 2012 for Windows 8) can be found in the following location:

Monday, September 17, 2012

Windows 8: Windows Store apps - RoamingSettings and the DataChanged event

In a previous article (Windows 8: Windows Store apps and the WebView control) we discussed how to display a Settings flyout and focused mostly around making the WebView control play nice with the flyout.

We created a Settings flyout because we want to store the URI that is used by the WebView control.

Now that we have the Settings flyout working, how do we store settings in a Windows Store app?

ApplicationData and RoamingSettings

It turns out that Windows Store apps have access to a class called ApplicationData which allows the storage of session state, user preferences, and other settings.

You have access to the following types of storage with the ApplicationData class:
  • local - persistent data but only on the current device
  • roaming - data that is available to all devices the user has your app installed on
  • temporary - data that can be removed by the system at any point after your app is closed

Local would do the trick but roaming caught my attention.

Being able to set a setting on one device and have all devices, that your app is installed on, automatically know about the new setting sounds pretty neat and would make things a lot easier for the user.

Setting a RoamingSettings key/value pair can be done as follows:
using Windows.Storage;

ApplicationData.Current.RoamingSettings.Values["URI"] = sURI;

Pulling a RoamingSettings value can be done as follows:
string sURI = ApplicationData.Current.RoamingSettings.Values["URI"] as string;
if (!string.IsNullOrWhiteSpace(sURI))
// do something with the setting

The DataChanged event

Another neat feature of the roaming settings is that you can subscribe to a DataChanged event so that your app can know when a setting was changed on another device.

One thing to be aware of here is that the DataChanged event might not fire at the instant that you set a RoamingSettings value. The synchronization of the roaming app data is controlled by Windows itself.

In our case passing the roaming settings data to other devices instantly is not a concern but what would be nice is if the Settings flyout changes would trigger the DataChanged event in the app where it was changed. As it turns out this is possible...

You can simulate a DataChanged event by calling the SignalDataChanged method after you apply the new settings and it will send a DataChanged event to all registered event handlers:

Handling the DataChanged event

Initially, I let Visual Studio wire up my DataChanged event handler (I typed in the += characters and pressed Tab) which gave me the following:
// Don't use this
ApplicationData.Current.DataChanged += DataChangedHandler;

When I ran the application, I discovered an issue where the event is being triggered, because my breakpoint was being hit, but the WebView control was not being updated with the new URI.

After some digging, I discovered that the MSDN documentation for implementing a DataChanged event handler uses the following method:
// Use this
ApplicationData.Current.DataChanged += new TypeEventHandler(DataChangedHandler);

Changing how the DataChanged event handler is registered didn't solve my issue of the WebView control not updating when the URI changes.

Background Threads and the UI Thread

As I was thinking about the issue a thought occurred to me...

Updating the WebView control would be the UI thread's responsibility.

What if the DataChanged event is being received on a background thread?

I've seen the code to call a UI thread in a developer training session at some point in the past (I think it was in regards to Silverlight but I'm not sure).

Either way, I thought I knew what the issue was but I couldn't remember how to get around it so I started searching the internet and I ran across the following MSDN sample code that had just the information I needed:

The following is the DataChanged event handler you need if you wish to update (directly or indirectly) the UI of your app since the DataChanged event might be triggered on a background thread which has no access to the UI:
async void Current_DataChanged(ApplicationData sender, object args)
// DataChangeHandler may be invoked on a background thread, so
// use the Dispatcher to invoke the UI-related code on the UI
// thread.
await this.Dispatcher.RunAsync(CoreDispatcherPriority.Normal, () =>
// Cause the web browser to reload in its content

In Conclusion

Overall, RoamingSettings in Windows Store apps are pretty straightforward.

One thing to remember with the DataChanged event, if you need to update the UI (directly or indirectly), is that the event might arrive on a background thread and you will need to dispatch the call to the UI thread.

Additional Resources

The following are some additional resources with regards to ApplicationData and RoamingSettings:
A download of the project (C# and built using Visual Studio Express 2012 for Windows 8) can be found in the following location:

Wednesday, September 12, 2012

Windows 8: Windows Store apps and the WebView control

I recently started looking into creating a simple Windows 8, Windows Store app, that would contain a browser window and a setting somewhere to allow the URI to be modified.

The app itself is quite simple in concept so I figured it would be a breeze to implement for Windows 8.

To display a web page I needed to use a WebView control and I simply set the Source property to the desired Uri.

The Settings Flyout

After successfully building my solution, and seeing the web page displayed, I decided it was time to move on to building the option to allow a user to modify the URI.

To try and keep things consistent with other Windows Store apps, I decided that it would be best to try and integrate the settings window into the Settings Charm of Windows.

I did some research and integrating into the Settings Charm seemed pretty straightforward in that you simply add an event handler for the CommandsRequested event of the Settings Charm and when the event is called, create your SettingsCommand callback.

Showing a Settings flyout seemed straightforward but I couldn't seem to get it to display.

While searching the internet, looking for articles or forum posts that might shed some light on my issue, I ran across the Callisto open-source toolkit, created by Tim Heuer, that offers boilerplate code for Windows Store apps.

You can find Tim's Callisto blog here:

The GitHub repository for the Callisto toolkit can be found here:

To install the Callisto package into your project via NuGet (, you can use the following command:

PM> Install-Package Callisto

Unfortunately, even with the Callisto toolkit, my settings flyout still wasn't being displayed.

When I was trying to create the settings flyout myself, before I discovered Callisto, I needed to set the height of the flyout so I started to wonder if perhaps the WebView control's VerticalAlignment setting of Stretch was the issue since I wasn't setting an explicit height value.

I set the height of the WebView control to 700 pixels and, when I ran my app, I discovered what the issue really was.

It turns out that the settings flyout was being displayed all along but the WebView control was hiding it because I can now see it being displayed behind the WebView control.


After a bit more research, I came to discover that you need a workaround with the WebView control when displaying a flyout (unfortunately, this reminds me of IE 6 and how Select objects would show through a floating div).

The following is the suggested workaround for displaying a flyout over a WebView control:
  • Add a Rectangle object to your view's XMAL with the same dimensions as your WebView control
  • Just before you show the flyout, create a WebViewBrush with the current contents of the WebView control (basically a screen shot)
  • Apply the brush to the Rectangle control
  • Hide the WebView control
  • Display the flyout
  • When the flyout closes, redisplay the WebView control
  • Clear the brush from the Rectangle control

I modified my view to use the workaround and my settings flyout now displays which I'm very happy about.

The main issue that I see now is that there is a noticeable flicker when switching to the WebViewBrush.

My initial thought, since the Redraw method of the brush is asynchronous, was that the WebView control was being hidden a fraction of a second before the brush had time to finish loading resulting in the background of the view being briefly visible causing the flicker.

I tried everything I could think of to get rid of the flicker including making the WebViewBrush a member variable and constructing it during the view's constructor.

I even tried placing the Redraw call before the settings flyout code creation to try and give the brush a few more milliseconds to load but the flicker remained.

async Task.Delay

Something I was curious about was if causing the app to sleep, in between the Redraw call of the brush and the hiding of the WebView, would help.

As it turns out there is no Thread.Sleep method available in a Windows Store app but I did find a workaround using an async Task.Delay(5000) call (I used 5000 milliseconds just so that it was obvious that the sleep call worked)

Much to my pleasant surprise, the flicker disappeared!

When I looked closer at the code, a thought crossed my mind...

I had both the WebViewBrush’s Redraw method as well as the Rectangle's Fill property being set before the delay.

What if the flicker wasn't because of the Redraw method of the brush after all?

What if the issue is with the Fill property of the Rectangle object?

I moved the rectangle's Fill call to after the delay and the flicker returned which indicates to me that the issue is not the brush but rather the Rectangle’s Fill property.

As I started thinking about what the issue might be, a thought occurred to me...

What if this is behaving similar to how the UI thread behaves in a web browser?

In a web browser there is only the one UI thread for a window so when you want to update the UI while processing, your UI request gets added to the end of the list of things that the window plans to do once the currently executing code completes.

Typically, if your code might take a few seconds to complete, you would want to display a processing indicator of some sort while your code executes (so that the user doesn't think that the page froze).

If you simply tell the UI processing control to display and then start processing, the UI control won't display until your code completes since the UI request is queued up for execution next by the browser window and your code is what it's currently working on.

To get around this UI update issue, in a web browser, you tell the UI control to display which adds that item to the end of the window's queue and then you set a timeout to call the function that will do the actual processing which puts the processing function on the queue just after the UI update.

When the original function exits, the UI is updated and then the processing function is called.

Based on the behavior I'm seeing, I believe the Windows Store app is using a similar technique to the UI thread of a web browser window

If I set the Rectangle's Fill property with the WebViewBrush and then add an async delay of 1 millisecond before proceeding to display the settings flyout, there is no flicker!

I still saw the flicker once in a while when closing the Settings flyout via the back button and then redisplaying the flyout really quick. Increasing the delay to about 100 milliseconds seems to improve that scenario.

Example Code

The following is an example of the XAML needed:
<WebView x:Name="wvBrowser" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" />

<Rectangle x:Name="rectWebViewBrush" HorizontalAlignment="Stretch" VerticalAlignment="Stretch"></Rectangle>

The following is an example of the source code needed to show the Settings flyout when dealing with a WebView control:
protected override void OnNavigatedTo(NavigationEventArgs e)
SettingsPane.GetForCurrentView().CommandsRequested += MainPage_CommandsRequested;

protected override void OnNavigatingFrom(NavigatingCancelEventArgs e)
SettingsPane.GetForCurrentView().CommandsRequested -= MainPage_CommandsRequested;


// Called by the Settings charm to find out what Settings links
// to display and the code to call when clicked.
void MainPage_CommandsRequested(SettingsPane sender, SettingsPaneCommandsRequestedEventArgs args)
// Set up the Link for the Settings charm
SettingsCommand cmdSettingsOptions = new SettingsCommand("cmdSettingsOptionsLabel", "Options", (x) =>
// Get a brush from the WebView's content (basically,
// a screen shot)
WebViewBrush wvbBrush = new WebViewBrush();
wvbBrush.SourceName = "wvBrowser";

// Fill the Rectangle object with the brush
rectWebViewBrush.Fill = wvbBrush;

// Show the settings flyout

// Add our Setting link to our applications list of settings

// Creates and displays the Settings flyout:
async void ShowSettingsFlyout()
// Give the Rectangle a chance to refresh so that we
// don't have flicker
await Task.Delay(100);

// Create the Settings flyout and display it (this is using
// the Callisto open-source toolkit). Additional attributes
// can be set like the background color and header brush
// to better represent your app's look and feel
SettingsFlyout settings = new SettingsFlyout();
settings.FlyoutWidth = Callisto.Controls.SettingsFlyout.SettingsFlyoutWidth.Narrow;
settings.HeaderText = "Options";

// Intercept the setting's closed event so that we can
// switch back to the WebView control
settings.Closed += settings_Closed;

// Our UserControl derived class that holds the controls
// for our settings
settings.Content = new SettingsOptionsContent();

// Switch to the WebViewBrush and then show the
// settings flyout
settings.IsOpen = true;

void SwitchToWebViewScreenShot(bool bSwitchToScreenShot)
// If we're to show the screen shot then...
if (bSwitchToScreenShot)
// Hide the WebView control (MainPage_CommandsRequested
// has already set the rectangle's fill with a screen shot of the
// contents of the WebView control)
wvBrowser.Visibility = Windows.UI.Xaml.Visibility.Collapsed;
else // We're to show the WebView again...
// Show the WebView control and remove the WebViewBrush
// from the Rectangle
wvBrowser.Visibility = Windows.UI.Xaml.Visibility.Visible;
rectWebViewBrush.Fill = new SolidColorBrush(Colors.Transparent);

// Called when the Settings flyout closes
void settings_Closed(object sender, object e)
// Switch back to the WebView control rather than the screen shot

In Summary

WebView controls don't allow a flyout to appear over it and require the use of a WebViewBrush to basically display a screen shot while the flyout is displayed.

The filling of a Rectangle object with a WebViewBrush, and immediately hiding the WebView control, introduces a flicker that can be circumvented by interrupting the UI's processing flow by adding an async delay before hiding the WebView control.

Windows Store apps don't support Thread.Sleep but you can accomplish a similar effect by using 'async Task.Delay'

When I was researching the WebViewBrush flicker issue, several forum posts indicated that there was a bug filed for it so there is potential that the flicker will not be an issue forever.

In the mean time, however, I hope this helps.

Download the Source Code

A download of the project (C# and built using Visual Studio Express 2012 for Windows 8) can be found in the following location: