wpf BackgroundWorker - Regarding updating UI

Question

I use a browse for files dialog to allow a user to select multiple images. If a lot of images are selected, as expected it takes a bit. Below is an example of what I do with the selected images. I loop through the filepaths to images and create an instance of a user control, the user control has an Image control and a few other controls. I create the instance of this control then add it to a existing stackPanel created in the associating window xaml file. The example just below works fine, but I'm trying to understand BackGroundWorker better, I get the basics of how to set it up, with it's events, and pass back a value that could update a progress bar, but because my loop that takes up time below adds the usercontrol instance to an existing stackPanel, It won't work, being in a different thread. Is BackGroundWorker something that would work for an example like this? If so, what's the best way to update the ui (my stackpanel) that is outside the thread. I'm fairly new to wpf and have never used the BackGroundWorker besides testing having it just update progress with a int value, so I hope this question makes sense, if I'm way off target just let me know. Thanks for any thoughts.

Example of how I'm doing it now, which does work fine.

protected void myMethod(string[] fileNames) {  
    MyUserControl uc;  

    foreach (String imagePath in fileNames) {  
        uc = new MyUserControl();  
        uc.setImage(imagePath);  
        stackPanel.Children.Add(uc);  
        progressBar.Value = ++counter;  
        progressBar.Refresh();  
    }  
}

below this class i have this so I can have the progressBar refresh:

public static class extensionRefresh {  
    private static Action EmptyDelegate = delegate() { };  

    public static void Refresh(this UIElement uiElement) {  
        uiElement.Dispatcher.Invoke(DispatcherPriority.Background, EmptyDelegate);  
    }  
}

kenwarner kenwarner · Accepted Answer · 2011-03-30T15:40:07

Check out this article on Building more responsive apps with the Dispatcher

Now that you have a sense of how the Dispatcher works, you might be surprised to know that you will not find use for it in most cases. In Windows Forms 2.0, Microsoft introduced a class for non-UI thread handling to simplify the development model for user interface developers. This class is called the BackgroundWorker

In WPF, this model is extended with a DispatcherSynchronizationContext class. By using BackgroundWorker, the Dispatcher is being employed automatically to invoke cross-thread method calls. The good news is that since you are probably already familiar with this common pattern, you can continue using BackgroundWorker in your new WPF projects

Basically the approach is

BackgroundWorker _backgroundWorker = new BackgroundWorker();

// Set up the Background Worker Events
_backgroundWorker.DoWork += _backgroundWorker_DoWork;
_backgroundWorker.RunWorkerCompleted += _backgroundWorker_RunWorkerCompleted;

// Run the Background Worker
_backgroundWorker.RunWorkerAsync(5000);

// Worker Method
void _backgroundWorker_DoWork(object sender, DoWorkEventArgs e)
{
    // Do something
}

// Completed Method
void _backgroundWorker_RunWorkerCompleted(object sender,  RunWorkerCompletedEventArgs e)
{
    // Doing UI stuff
    if (e.Cancelled)
    {
        statusText.Text = "Cancelled";
    }
    else if (e.Error != null) 
    {
        statusText.Text = "Exception Thrown";
    }
    else 
    {
        statusText.Text = "Completed";
    }
}

wpf BackgroundWorker - Regarding updating UI

2 Answers