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Using BLOB storage as a media server (part 2) - A WPF-based adaptive-streaming video player

3/16/2011 6:23:42 PM
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2. A WPF-based adaptive-streaming video player

Now we’re going to tell you how to build a WPF-based media player that can adaptively stream your video. Your media player will look exactly like the WPF media player displayed in figure 1.

Adaptive streaming is a technique that most content delivery networks (CDNs) use to deliver high-performance video. This technique is also used in IIS adaptive streaming, although BLOB storage won’t deliver in multiple bitrates.

Warning

The following code is intended as an example to show you how you can use adaptive streaming. This isn’t production-quality code (not by a long shot).


Playing the Video

The following XAML is used to play the video in WPF:

<MediaElement x:Name="myVideo" Source="videopodcast01.wmv"
LoadedBehavior="Manual"/>

MediaElement is a built-in control that allows you to play movies in WPF applications. In this example, we’re downloading the movie (mymovie.wmv) to the same folder that the WPF movie player application resides in. The source attribute states where the MediaElement should look for the movie.

By default, the MediaElement automatically starts playing the movie on startup. Because the movie hasn’t been downloaded yet, you need to prevent the movie from automatically playing by setting the attribute LoadedBehavior to Manual.

The Chunking Methodology for WPF

You’re going to download the movie in chunks of 100 Kb. Only one chunk will be downloaded at a time and each newly downloaded chunk will be appended to the previously downloaded chunk. Listing 1 shows that as soon as the movie player is loaded, it starts downloading the chunks. After 10 seconds, the movie starts playing.

Listing 1. Chunking movies with the WPF movie player

Before we look more carefully at the code used to progressively download the video, let’s talk a bit about the Range header.

Using the Range Header

When a GET request is made using either the storage client or via an HTTP request, the entire file is downloaded by default. The code shown in listing 2 will download videopodcast01.wmv from the public container, podcasts.

Listing 2. Using the storage client to download a video file

The code in listing 2 will download the entire movie; in this example, we want to split the movie up into manageable chunks. Currently, the storage client sample code doesn’t provide the ability to download a specified portion of the file, even though the underlying REST API does support this. The code shown in listing 3 will download the entire podcast using HttpWebRequest.

Listing 3. Using HttpWebRequest to download a video file

At , you generate the HttpWebRequest with the standard required headers . At , you use the Download-File method to invoke the request and download the file to disk. The implementation of the DownloadFile method is available in the online samples.

If you don’t want to download the entire video file, you can use the Range header to specify the range of bytes that you want to download. The following code would restrict the download to the first 100,000 bytes of the file, and can be used in listing 3 at :

hwr.AddRange(0, 100000);

Not only can you restrict the number of bytes using the Range header, you can also use it to progressively download the file in chunks.

Downloading Chunks of Data

When the WPF movie player was loaded, we made a call to GetNextChunk() in listing 1. In listing 4, we show you how GetNextChunk is implemented, and how to use the Range header to progressively download the movie.

Listing 4. Downloading the data in chunks

The GetNextChunk method will download 100 Kb of data from the BLOB storage service asynchronously. This method will be called every time you need to download a new chunk of video; it’s called for the first time when the application is loaded in the fourth line in listing 1.

In listing 4, you create a standard HTTP web request at ; this is the same method that you used to create requests earlier. The video (videopodcast01.wmv) that you’re downloading resides in a public container called podcasts that’s held in your storage account. After you’ve created the request, you add the Range header to restrict the download to the next 100 Kb chunk.

At you’re making an asynchronous HTTP web request to the BLOB storage service, with a callback to . On the request callback , the downloaded data is retrieved, and you call the SaveChunk method , which will append the downloaded chunk to the videopodcast01.wmv file on the local disk. Finally, you call GetNextChunk again to get the next chunk of data.

Time-based buffering

In our simple example, we used time-based buffering to delay the playback of the movie. Although time-based buffering is suitable for our example, in a production scenario you should start playing the movie after a portion of the movie has been downloaded. We also don’t make any provision for variable download speeds; you might want to extend the sample to handle situations in which the playback speed is faster than the download speed.


Now you have a working version of a WPF progressive-downloading media player, which shows that you can use BLOB storage as a storage service for desktop clients. With some tweaks you can build media players that perform similarly to a streaming service.

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