Windows Phone 7 Advanced Programming Model : Advanced Data Binding (part 3) - Showing Progress & Lazy Load Images

9/20/2011 3:44:04 PM

2. Showing Progress

In this section, I demonstrate how to have your ViewModel drive UI, such as when to show progress bars or not, via data binding. It is a quick example based on the previous example that downloads the AppHub feed but the concept is very important to help you understand how to put more code into your ViewModels and less code in your View.

The SyndicatedServicesViewModel.cs code file is copied and renamed ShowProgressViewModel as well as the constructor. The new ShowProgressViewModelViewModelLocator just as before, and a new View named ShowingProgress.xaml is created and data bound to the ShowProgressViewModel. Run the project and it should work just as the previous sample. property is added to the

A new Boolean property named ShowProgressBar is added to the ShowProgressViewModel class. The ShowProgressBar property is set to true just before the HttpWebRequest.CreateHttp method call in the ShowProgressViewModel .DownloadAppHubFeed method. The property must be modified on the UI thread so that data binding propagates to the UI and an error doesn't happen from cross-thread access:

public void DownloadAppHubFeed()
Deployment.Current.Dispatcher.BeginInvoke(() =>
ShowProgressBar = true;

At the end of the ShowProgressViewModel.ReceiveFeedData method set ShowProgressBar to false. This completes the changes in the ViewModel. The progress bar should be enabled right before making the web call and then disabled once the new data is data binded to the UI. Again the property must be modified on the UI thread:

Deployment.Current.Dispatcher.BeginInvoke(() =>
FeedItems = items.ToList<FeedItem>();
//Artificial delay to test progress bar
ShowProgressBar = false;

In the ShowProgress.xaml page, add a progress bar that applies to the width of the screen in the middle of the page. The progress bar should look like the native WP7 progress bar, otherwise known as "the flying dots." Luckily the Silverlight for Windows Phone Toolkit has been updated to include two new controls, TiltEffectPerformanceProgressBar. Previously you had to create a customized version of the built-in progress bar in order to have good UI thread performance. and

Once the latest Silverlight toolkit for WP7 is installed, add PerformanceProgressBar to the Toolbox window in Visual Studio and drag the control on to the ShowingProgress.xaml View. Switch over to Expression Blend and set the HorizontalAlignment to stretch and reset Margin to just a top Margin of 300. Next set the IndeterminateProperty to True and you can see the flying dots at design-time in Expression Blend. We only want to see the flying dots when loading data so let's data bind the LoadingDataPerfProgressBar's Indeterminate property to the ShowProgressViewModel.ShowProgressBar property. Here is the XAML for the PerformanceProgressBar from ShowingProgress.xaml:

<toolkit:PerformanceProgressBar x:Name="LoadingDataPerfProgressBar" Margin="0,300,0,0"
VerticalAlignment="Top" IsIndeterminate="{Binding ShowProgressBar}" />

By default, the data is obtained by ShowProgressViewModel when the application loads and the ShowProgressViewModel is created. We modify the constructor to not automatically call the DownloadAppHubFeed() method. The ShowingProgressView is modified to have an Application Bar button that manually downloads the feed. Clicking the Application Bar button will enable the progress bar, download the feed, and then disable the progress bar by calling the DownlaodAppHubFeed()Figure 6-4 shows the flying dots in the emulator. method.

Figure 4. PerformanceProgressBar in action

This example demonstrates the power of Silverlight and its data binding infrastructure to allow the ViewModel non-visual class to drive the UI based on the applications current state. This technique is critical to effective MVVM development and can be extended to other data-driven UI to indicate state and display additional UI when needed.

3. Lazy Load Images

When developing for a mobile device with constrained networking and hardware capabilities relative to a desktop computer, you sometimes have to take additional steps beyond the "default" programming model. As an example, loading a ListBox with data and images from a remote server can peg the UI thread resulting in a poor user experience for scrolling and animations. Anytime work can be taken off of the UI thread is a win for performance.

A Silverlight for Windows Phone team member David Anson blogged here about a way to offload image loading to a background thread that results in much better UI performance in this article.

In the blog post, Dave introduces the LowProfileImageLoader class to address the very specific issue of loading lots of images from the web simultaneously. The LowProfileImageLoader slows the loading of images but the UI remains responsive as images are loaded because the UI thread is not slammed waiting for images to load and then data bind.

To test out the LowProfileImageLoader, let's work with the Netflix API. I picked this API because it has an interesting set of images (movie art) in a well-documented API. The API is available at


You must register an account to use the Netflix APIs at

We will use the OData Client Library for Windows Phone 7 to access the Netflix API. As before we add a reference to System.Data.Services.Client.dll. We use the DataSvcUtil.exe to generate a proxy class:

DataSvcUti.exe /uri: /out:Netfli xOdataAPI.cs
/Version:2.0 /DataServiceCollection

Adding the DataServiceCollection option has the tool implement the INotifyPropertyChanged interface, which is important for change notification purposes. The OData access is wired into the LazyLoadViewModel.cs class, which is a copy of the ShowProgressViewModel:

public void DownloadNetflixTopTitles()
Deployment.Current.Dispatcher.BeginInvoke(() =>
ShowProgressBar = true;
topMovieTitles = new DataServiceCollection<Title>(ODataContext);
topMovieTitles.LoadCompleted +=
new EventHandler<LoadCompletedEventArgs>(topMovieTitles_LoadCompleted);
topMovieTitles.LoadAsync(new Uri("/Titles()",UriKind.Relative));


void topMovieTitles_LoadCompleted(object sender, LoadCompletedEventArgs e)
Deployment.Current.Dispatcher.BeginInvoke(() =>
ShowProgressBar = false;
TopMovieTitles = topMovieTitles;

The LazyLoad.xaml View page is modified to include a ListBox with a simple DataTemplate that shows the movie title and image. Figure 5 shows the UI.

Figure 5. Netflix API top titles via OData

Not every movie title has box art, but many do. Let's now modify this application to use the LowProfileImageLoader class to asynchronously display the images. Download the source code at the blog post link above and grab the PhonePerformance.dll. Add a reference to it in the AdvancedDatabinding project and then add a namespace reference to LazyLoadImage.xaml:


Next update the NetflixTopTitleDataTemplate so that the image is not loaded from the built-in Source property but instead is loaded using this XAML:

<Image delay:LowProfileImageLoader.UriSource="{Binding BoxArt.SmallUrl}"
HorizontalAlignment="Left" Stretch="UniformToFill" Width="150"/>

The performance difference is much more obvious on a physical device over 3G, but as you can see it is pretty trivial to add support for the LowProfileImageLoader to see if it can help with any performance issues related to image loading in your applications.

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