wpf - How do I refresh visual control properties (TextBlock.text) set inside a loop?

Question

With each loop iteration, I want to visually update the text of a textblock. My problem is that the WPF window or control does not visually refresh until the loop is complete.

for (int i = 0; i < 50; i++)
{
    System.Threading.Thread.Sleep(100);
    myTextBlock.Text = i.ToString();                
}

In VB6, I would call DoEvents() or control.Refresh. At the moment I'd just like a quick and dirty solution similar to DoEvents(), but I'd also like to know about alternatives or the "right" way to do this. Is there a simple binding statement I could add? What is the syntax? Thanks in advance.

Answer 1

If you really want the quick and dirty implementation and don't care about maintaining the product in the future or about the user experience, you can just add a reference to System.Windows.Forms and call System.Windows.Forms.Application.DoEvents():

for (int i = 0; i < 50; i++)
{
    System.Threading.Thread.Sleep(100);
    MyTextBlock.Text = i.ToString();
    System.Windows.Forms.Application.DoEvents();
}

The downside is that it's really really bad. You're going to lock up the UI during the Thread.Sleep(), which annoys the user, and you could end up with unpredictable results depending on the complexity of the program (I have seen one application where two methods were running on the UI thread, each one calling DoEvents() repeatedly...).

This is how it should be done:

1. Any time your application has to wait for something to happen (ie a disk read, a web service call, or a Sleep()), it should be on a separate thread.
2. You should not set TextBlock.Text manually - bind it to a property and implement INotifyPropertyChanged.

Here is an example showing the functionality you've asked for. It only takes a few seconds longer to write and it's so much easier to work with - and it doesn't lock up the UI.

Xaml:

<StackPanel>
    <TextBlock Name="MyTextBlock" Text="{Binding Path=MyValue}"></TextBlock>
    <Button Click="Button_Click">OK</Button>
</StackPanel>

CodeBehind:

public partial class MainWindow : Window, INotifyPropertyChanged
{
    public MainWindow()
    {
        InitializeComponent();
        this.DataContext = this;
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        Task.Factory.StartNew(() =>
        {
            for (int i = 0; i < 50; i++)
            {
                System.Threading.Thread.Sleep(100);
                MyValue = i.ToString();
            }
        });
    }

    private string myValue;
    public string MyValue
    {
        get { return myValue; }
        set
        {
            myValue = value;
            RaisePropertyChanged("MyValue");
        }
    }

    private void RaisePropertyChanged(string propName)
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(propName));
    }
    public event PropertyChangedEventHandler PropertyChanged;
}

The code might seem a bit complicated, but it's a cornerstone of WPF, and it comes together with a bit of practice - it's well worth learning.