Figure 10-11
Using the BackgroundWorker Control
Because threading is such a common programming task in Windows programming, Microsoft has provided a convenient solution to implementing threading: the BackgroundWorker control for Windows applications. The BackgroundWorker control enables you to run a long background task such as network access, file access, and so forth and receive continual feedback on the progress of the task. It runs on a separate thread.
This section creates a simple Windows application that will show you how the BackgroundWorker component can help make your applications more responsive.
First, start Visual Studio 2008 and create a new Windows application. Populate the default Windows form with the following controls (see Figure 10-12).
Figure 10-12
| Control | Name | Text |
|---|---|---|
Label | Number | |
Label | lblResult | label2 |
Label | Progress | |
TextBox | txtNum | |
Button | btnStart | Start |
Button | btnCancel | Cancel |
ProgressBar | ProgressBar1 |
Drag and drop the BackgroundWorker component from the Toolbox onto the form.
The BackgroundWorker is a nonvisual control, so it appears below the form in the component section (see Figure 10-13).
Figure 10-13
Right-click on the BackgroundWorker component, and select Properties. Set the WorkerReportsProgress and WorkerSupportsCancellation properties to True so that the component can report on the progress of the thread as well as be aborted halfway through the thread (see Figure 10-14).
Figure 10-14
Here is how the application works. The user enters a number in the TextBox control (txtNum) and clicks the Start button. The application then sums all of the numbers from 0 to that number. The progress bar at the bottom of the page displays the progress of the summation. The speed in which the progress bar updates is dependent upon the number entered. For small numbers, the progress bar fills up very quickly. To really see the effect of how summation works in a background thread, try a large number and watch the progress bar update itself. Notice that the window is still responsive while the summation is underway. To abort the summation process, click the Cancel button. Once the summation is done, the result is printed on the Label control (lblResult).
Switch to the code behind of the Windows form to do the coding. When the Start button is clicked, you first initialize some of the controls on the form. You then get the BackgroundWorker component to spin off a separate thread by using the RunWorkAsync() method. You pass the number entered by the user as the parameter for this method:
private void btnStart_Click(object sender, EventArgs e) {
lblResult.Text = string.Empty;
btnCancel.Enabled = true;
btnStart.Enabled = false;
progressBar1.Value = 0;
backgroundWorker1.RunWorkerAsync(txtNum.Text);
}
Now, double-click the BackgroundWorker control in design view to create the event handler for its DoWork event.
private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e) {
BackgroundWorker worker = (BackgroundWorker)sender;
e.Result = SumNumbers(double.Parse(e.Argument.ToString()), worker, e);
}
The DoWork event of the BackgroundWorker component invokes the SumNumbers() function (which you will define next) in a separate thread. This event is fired when
