Update GUI using BackgroundWorker

Question

I've been searching and found that a good way to perform background work and update the GUI is using background workers. However, doing this (stupid) little task (counting from 1 to 10000) it doesn't update the label content but prints to the debug! (This is just a spike solution for another project of course...)

Here's the code:

public partial class MainWindow : Window
{
    BackgroundWorker bw = new BackgroundWorker();

    public MainWindow()
    {
        InitializeComponent();
    }

    private void button1_Click(object sender, RoutedEventArgs e)
    {

        bw.DoWork += new DoWorkEventHandler(bw_DoWork);
        bw.ProgressChanged += new ProgressChangedEventHandler(bw_ProgressChanged);
        bw.WorkerReportsProgress = true;
        bw.RunWorkerCompleted += new RunWorkerCompletedEventHandler(bw_RunWorkerCompleted);
        bw.RunWorkerAsync();

    }

    void bw_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
    {
        MessageBox.Show("DONE");

    }

    void bw_ProgressChanged(object sender, ProgressChangedEventArgs e)
    {
        label1.Content = "going here: "+e.ProgressPercentage;
        Debug.WriteLine(e.ProgressPercentage);
    }

    void bw_DoWork(object sender, DoWorkEventArgs e)
    {
        for (int i=0; i < 10000; i++)
        {
            bw.ReportProgress((i*100)/10000);
        }
    }

}

score 12 · Accepted Answer · answered Mar 04 '11 at 10:50

The ProgressChanged event is raised on the UI thread, not the worker thread. In your code, the worker thread is doing almost nothing (just loop from 0 to 10000 and call ReportProgress), most of the work is done on the UI thread. Basically, you're sending too many progress notifications. Because of this, the UI thread is almost always busy and has no time to render the new content of the label.

Rendering in WPF is not performed immediately when you change a property of a control, it is done on a separate dispatcher frame, which is processed when the dispatcher has nothing more urgent to do, based on the priority of the task. The priority used for rendering has a value of 7 (DispatcherPriority.Render); the ProgressChanged event is marshalled to the UI thread with a priority of 9 (DispatcherPriority.Normal), as specified on MSDN. So the ProgressChanged notifications always have a higher priority than rendering, and since they keep coming, the dispatcher never has time to process the rendering tasks.

If you just decrease the frequency of the notifications, your app should work fine (currently you're sending 100 notifications for each percentage value, which is useless):

    void bw_DoWork(object sender, DoWorkEventArgs e)
    {
        for (int i = 0; i < 10000; i++)
        {
            if (i % 100 == 0)
                bw.ReportProgress(i / 100);
        }
    }

Beliving or not, I was actually thinking it before seeing your comment! It makes complete sence that the UI gets frozen with so much calls... — Miguel Ribeiro, Mar 04 '11 at 11:58
Of course it's easier to bind to a property and update that from the worker thread. — EightyOne Unite, Mar 04 '11 at 12:10
@Stimul8d, absolutely, and this should be the preferred way of doing things in WPF — Thomas Levesque, Mar 04 '11 at 12:43

score 3 · Answer 2 · answered Mar 04 '11 at 10:51

3

this.Dispatcher.BeginInvoke( (Action) delegate(){
   label1.Content = "going here: "+e.ProgressPercentage;
});

answered Mar 04 '11 at 10:51

Akash Kava

39,066
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score 0 · Answer 3 · edited Jan 12 '19 at 18:55

0

Try to change the label using womething like this:

string Text = "going here: " + e.ProgressPercentage;
this.Invoke((MethodInvoker)delegate {
    label1.Content = newText;
});

Note that i'm not sure it will work. I can not test it now. If it does not work, let me know and I will delete the answer.

If you need the a canonical way to do exactly what you want, look at the Hath answer in this post: How do I update the GUI from another thread?

edited Jan 12 '19 at 18:55

keepAlive

6,369
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answered Mar 04 '11 at 10:07

Jonathan

11,809
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Well... Your answer could work if I was using windows forms... in WPF I don't have the Invoke property from controls... Only the control's dispatcher as it. I've been also trying to "mess" with it but no success :( – Miguel Ribeiro Mar 04 '11 at 10:16
1

You can do the same in WPF with Dispatcher.Invoke. But this is irrelevant anyway: the ProgressChanged event is raised on the UI thread, so Invoke is no required. – Thomas Levesque Mar 04 '11 at 10:34

Update GUI using BackgroundWorker

3 Answers3

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