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Dispatcher Object In Silverlight

In this tutorial I will explain what the purpose of Dispatcher object in silverlight is.

Application Execution

Silverlight application runs on the main thread (or main UI thread). Consider a situation in which you have to execute a task with silverlight application that requires considerable amount of time. If you execute the task on the main thread then UI of the application will be hanged until the process is complete.

To avoid this situation you can create a separate thread and executes the process on that thread. This approach is fine but if you want to access the UI element from another thread then you cannot do that.

Elaboration

Consider a silverlight application which contains a textblock UI element.

<Grid x:Name="LayoutRoot" Background="White">
	<TextBlock x:Name="SampleBlock"></TextBlock>
</Grid>

In the constructor of the class I am initiating another thread.

BackgroundWorker _BackgroundWorker = new BackgroundWorker();
_BackgroundWorker.DoWork += new DoWorkEventHandler(_BackgroundWorker_DoWork);
_BackgroundWorker.RunWorkerAsync();

void _BackgroundWorker_DoWork(object sender, DoWorkEventArgs e) {
  //  Doing some time consuming task
}

Here I have created an instance of BackgroundWorker class, add an event handler and finally call the RunWorkerAsync() method to run the task on different thread.

_BackgroundWorker_DoWork() method executes the time consuming task. After the completion of time consuming task I assign the value in the textblock, so the code in _BackgroundWorker_DoWork() method changes to

void _BackgroundWorker_DoWork(object sender, DoWorkEventArgs e) {
  //  Doing some time consuming task
  SampleBlock.Text = "Test Content";
}

When you run this program you will get run time error.

Invalid cross-thread access.

Solution

Here is when Dispatcher object comes into picture. You can access UI element and do manipulations using Dispatcher object asynchronously.

Syntax

this.Dispatcher.BeginInvoke(Delegate d, params Object[] args);

Elaboration

Change the _BackgroundWorker_DoWork() method to

private delegate void AddTextDelegate();
void _BackgroundWorker_DoWork(object sender, DoWorkEventArgs e) {
  //  Doing some time consuming task            
  this.Dispatcher.BeginInvoke(new AddTextDelegate(ChangeText));
}

private void ChangeText() {
  SampleBlock.Text = "Test Content";
}

Here I have created a delegate that points to ChangeText() method. The ChangeText() method will change the text of the TextBlock.

Instead of calling the method directly the method is called asynchronously by dispatcher. When you execute this code, the content of the TextBlock will be changed.

Summary

To change the UI element synchronously if the program is executing on a different thread is not possible. But you can do that asynchronously using Dispatcher object.

Posted By -	Karan Gupta

Posted On -	Tuesday, December 27, 2011

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