When are tunneling and bubbling events useful in WPF?

Question

I understand how bubbling and tunneling works. However, i'm confused about using them. Here is why:

I want to handle a mouse click event. To bubble it, there is MouseDown and, to tunnel it, there is PreviewMouseDown. However, MouseDown doesn't necessarily mean the user clicked the control. May be the user pressed the button and moved away from it to cancel the click. I wouldn't want to change anything if the button is not being clicked.

So my question is, how are the Bubbling/Tunneling strategies useful?

Answer 1

If the event is listed RoutedEventArgs, then it's routed event. Routed events support a RoutingStrategy of Bubble, Tunnel, or Direct. Let's take a look at the event handler of Button.Click:

private void Grid_Click(object sender, RoutedEventArgs e)
{
    MessageBox.Show("Button Test clicked!");
}

There specified RoutedEventArgs, so it's routed event. Because the preview were not specified in the name, therefore this Bubble event. This can be demonstrated in the following way:

<Grid ButtonBase.Click="Grid_Click">
    <Button Name="TestButton" Width="100" Height="30" Content="Test" />
</Grid>

When you click on the TestButton, the event is to rise above the Grid, and displays a message:

Button Test clicked!

Usefulness of Bubbling/Tunneling strategies

Tunneling

Many of the standard controls listen to events, such as KeyDown, MouseDown, etc. For example -DataGrid control. I want by pressing the enter key the function was called adding a record. But DataGrid already has KeyDown event, so the event is not raised. So you have to do your logic in the Tunnel event - PreviewKeyDown, it will work before the KeyDown event. The same applies to RichTextBoxControl.

Bubbling

Sometimes, you need a global handler for a specific event, so it worked for all controls in VisualTree. Naturally, the a direct event you can not do it. Hence on the stage comes Bubbling event.

Another reason is the ideology of the WPF. This Button can contain anything: Image, another Button, etc:

The user can click on the TextBlock/Image in the Button. How do we know that the click was in Button? That's right, with the help of Bubbling event.

For more information, please see:

Understanding Routed Events and Commands In WPF

Edit

I changed little bit a Click handler:

private void Grid_Click(object sender, RoutedEventArgs e)
{
    String message = "#" + eventCounter.ToString() + ":\r\n" +
            " Sender: " + sender.ToString() + ":\r\n" +
            " Source: " + e.Source + ":\r\n" +
            " Original Source: " + e.OriginalSource;

    lstEvents.Items.Add(message);
}

Result of click on the Button: