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How process asynchronous calls in Silverlight, by using custom Dispatcher

In current public alpha of Silverlight 1.1 there is no dispatcher. However there are STA and MTA. So, how to process long operations in background threads in MTA and update UI in STA? Well, let’s write our own dispatcher. But this is not so simple, but it is possible. Let’s start

How, actually works dispatcher? It’s pumps system messages by using timer in Single Threaded Apartment. Let’s write VeryLongMethod. It will be high precise π (PI) calculation. By passing all math stuff, I just point you to how I run the process

public static class BackgroundPiCalculator  { static Thread worker; public static void RunCalculationAsync(int digits, int delayMs) { P = 2 + (digits + 7) / 8; D = delayMs; worker = new Thread(new ThreadStart(calc)); worker.IsBackground = true; worker.Start(); }


As you can see, I open new thread outside of MTA and run this long calculation. I added a couple of wait methods to be able to see the process. The next step is to create callbacks.

public static event EventHandler DoCalculation; public static event EventHandler<RunCalculationCompletedEventArgs> RunCalculationCompleted; public static event EventHandler<ProgressChangedEventArgs> ProgressChanged;


Well. By now, if we’ll subscribe to those events from the right thread (STA), we’ll get them in the thread where we subscribed. Right? Let’s do it.

 public void Page_Loaded(object o, EventArgs e) { // Required to initialize variables  InitializeComponent(); PICalculator.BackgroundPiCalculator.RunCalculationAsync(10000,100); PICalculator.BackgroundPiCalculator.ProgressChanged += new EventHandler<BackgroundPiCalculator.ProgressChangedEventArgs>(BackgroundPiCalculator_ProgressChanged); PICalculator.BackgroundPiCalculator.RunCalculationCompleted += new EventHandler<BackgroundPiCalculator.RunCalculationCompletedEventArgs>(BackgroundPiCalculator_RunCalculationCompleted); }

 void BackgroundPiCalculator_ProgressChanged(object sender, BackgroundPiCalculator.ProgressChangedEventArgs e) { result.Text = e.PreliminaryResult.ToString(); progress.Text = e.ProgressPercentage.ToString() + "%";  }


Well, compile and run to get the error “Invalid cross-thread access”.


Hmmm, it comes from the other thread, but I subscribed to event in my thread, so what’s the problem? Ah, the object I’m trying to use was created in other thread. This why I can not access it from my thread. So what to do?

Windows threats such cases by sending block event and moving the object to executing thread. It just pumps messages. Let’s do it.

We create two queues – one for result text and the other for progress. Then when the data arrived we’ll enqueue the message. Concurrently we’ll create timer in STA to pump and dequeue those messages. The only timer can work in UI thread is HtmlTimer. Please notice, that this class is obsolete due to “This is not a high resolution timer and is not suitable for short-interval animations. A new timer type will be available in a future release.”. Hey, there a new dispatchers will be available as well in a future release? I believe, that they are. So, let’s create a timer, queues and handlers

HtmlTimer t = new HtmlTimer(); t.Interval = 100; t.Tick += new EventHandler(t_Tick); t.Start();

void t_Tick(object sender, EventArgs e) { while (resultMessages.Count > 0) { result.Text = resultMessages.Dequeue(); } while (progressMessages.Count > 0) { progress.Text = progressMessages.Dequeue(); } }


Queue<string> resultMessages = new Queue<string>(); Queue<string> progressMessages = new Queue<string>();


And enqueue messages upon their arrival.

void BackgroundPiCalculator_ProgressChanged(object sender, BackgroundPiCalculator.ProgressChangedEventArgs e) { resultMessages.Enqueue(e.PreliminaryResult.ToString()); progressMessages.Enqueue(e.ProgressPercentage.ToString() + "%"); }


We done. Now the calculations will be performed asynchronously and we’ll get a feedback into our UI.


Well done, so what’s next? Next, we’ll enqueue delegates and invoke them into right thread, but all this in the next post. Have a nice weekend.

Source code for this article.

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6 Responses to “How process asynchronous calls in Silverlight, by using custom Dispatcher”

  1. Tamir Khason Says:

    It is, and this is the problem of WPF as well. The solution is aggresive locking. Seek my blog about multithreading :)

  2. Jack Bond Says:

    Sure tick works, but the issue isn't about getting stuff going again in the ui thread, as you said, "Ah, the object I'm trying to use was created in other thread." When you insert the string into the queue from the background thread, you are accessing the queue that was created in the ui thread. And when you dequeue the inserted string from the UI thread, the string was created in the background thread. What you've done is added the queue in the middle, instead of updating the TextBlock directly from the background thread. There's still plenty of cross thread access going on, it just that string and queue don't cause exceptions, whereas the UI components do. So the real issue is, some objects can be shared accross apartments, and some can't (particularily UI components.) That is the fundamental issue isn't it?

  3. Tamir Khason Says:

    Jack, Tick event works in STA, thus it works. And it still will be relevan in 13 days :)

  4. Jack Bond Says:

    Hopefully this will all be irrelevant in 13 days, but isn't there still cross thread access going on here? When the background thread inserts items into the queue, it is accessing the queue created in the STA UI apartment, and when the Timer ticks, it dequeues the string created in the MTA background worker apartment. Unless I'm missing something, it just seems that the string and Queue classes are a little less tempermental to the cross thread access issues.

  5. Tamir Khason Says:

    It’s just a lot of spaces there :)

  6. shvilam Says:

    the code color is realy unreadable

    you shode do some thing about it

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