MouseDoubleClick events don't bubble

问题:

My scenario, simplified: I have a ListView containing rows of Employees, and in each Employee row, there are buttons "Increase" and "Decrease" adjusting his salary.

Pretend that in my program, double-clicking an Employee row means "fire this person".

The problem is that while I'm clicking "Increase" rapidly, this triggers a double click event on the ListViewItem. Naturally, I don't want to fire people when I'm just increasing their salary.

According to how all other events work, I expect to be able to solve this by setting Handled=true on the event. This, however, doesn't work. It appears to me that WPF generates two separate, completely unlinked, double click events.

The following is a minimal example to reproduce my issue. The visible components:

<ListView>
    <ListViewItem MouseDoubleClick="ListViewItem_MouseDoubleClick">
            <Button MouseDoubleClick="Button_MouseDoubleClick"/>
    </ListViewItem>
</ListView>

And the handler code:

private void Button_MouseDoubleClick(object s, MouseButtonEventArgs e) {
    if (!e.Handled) MessageBox.Show("Button got unhandled doubleclick.");
    e.Handled = true;
}

private void ListViewItem_MouseDoubleClick(object s, MouseButtonEventArgs e) {
    if (!e.Handled) MessageBox.Show("ListViewItem got unhandled doubleclick.");
    e.Handled = true;
}

After firing up this program and double-clicking the listed button, both messageboxes show up in sequence. (Also, the button is stuck in the down position after this.)

As a "fix" I can, on the ListViewItem handler, inspect the visual tree attached to the event and check that "there is a button there somewhere" and thus discard the event, but this is a last resort. I want to at least understand the issue before coding such a kludge.

Does anyone know why WPF does this, and an elegant idiomatic way to avoid the problem?

回答1:

I think you'll find that the MouseDoubleClick event is an abstraction on top of the MouseDown event. That is, if two MouseDown events occur in quick enough succession, the MouseDoubleClick event will also be raised. Both the Button and ListViewItem appear to have this logic, so that explains why you're seeing two distinct MouseDoubleClick events.

As per MSDN:

Although this routed event seems to follow a bubbling route through an element tree, it actually is a direct routed event that is raised along the element tree by each UIElement. If you set the Handled property to true in a MouseDoubleClick event handler, subsequent MouseDoubleClick events along the route will occur with Handled set to false.

You could try handling MouseDown on the Button and setting that to handled so that it doesn't propagate to the ListViewItem.

Wish I could verify this myself but I'm .NET-less at the moment.

回答2:

The MSDN documentation for the MouseDoubleClick does give a suggestion on how to keep the MouseDoubleClick event from bubbling up:

Control authors who want to handle mouse double clicks should use the MouseLeftButtonDown event when ClickCount is equal to two. This will cause the state of Handled to propagate appropriately in the case where another element in the element tree handles the event.

So you could hanlde the MouseLeftButtonDown event and set hanged to true if ClickCount is two. But this fails on Buttons because they already handle the MouseLeftButtonDown and don't raise that event.

But there is still the PreviewMouseLeftButtonDown event. Use that on your buttons to set handled to true when ClickCount equals two as below:

 private void Button_PreviewMouseLeftButtonDown(object sender, MouseButtonEventArgs e)   {
     if (e.ClickCount == 2)
         e.Handled = true;
 }

回答3:

Since there have been no definite answers to this question, this is the workaround I ended up using:

protected override void ListViewItem_MouseDoubleClick(MouseButtonEventArgs e) {
    var originalSource = e.OriginalSource as System.Windows.Media.Visual;
    if (originalSource.IsDescendantOf(this)) {
        // Test for IsDescendantOf because other event handlers can have changed
        // the visual tree such that the actually clicked original source
        // component is no longer in the tree.
        // You may want to handle the "not" case differently, but for my
        // application's UI, this makes sense.
        for (System.Windows.DependencyObject depObj = originalSource;
             depObj != this;
             depObj = System.Windows.Media.VisualTreeHelper.GetParent(depObj))
        {
            if (depObj is System.Windows.Controls.Primitives.ButtonBase) return;
        }
    }

    MessageBox.Show("ListViewItem doubleclicked.");
}

Class names are here unnecessarily typed with full namespaces for documentation purposes.

回答4:

Well it may not be elegant or idiomatic, but you might like it better than your current workaround:

    int handledTimestamp = 0;

    private void ListViewItem_MouseDoubleClick(object sender, MouseButtonEventArgs e)
    {
        if (e.Timestamp != handledTimestamp)
        {
            System.Diagnostics.Debug.WriteLine("ListView at " + e.Timestamp);
            handledTimestamp = e.Timestamp;
        }
        e.Handled = true;
    }

    private void Button_MouseDoubleClick(object sender, MouseButtonEventArgs e)
    {
        if (e.Timestamp != handledTimestamp)
        {
            System.Diagnostics.Debug.WriteLine("Button at " + e.Timestamp);
            handledTimestamp = e.Timestamp;
        }
        e.Handled = true;
    }

The weird thing is this doesn't work if you don't set e.Handled = true. If you don't set e.Handled and put a breakpoint or a Sleep into the button's handler, you will see the delay in the ListView's handler. (Even without an explicit delay there will still be some small delay, enough to break it.) But once you set e.Handled it doesn't matter how long of a delay there is, they will have the same timestamp. I'm not sure why this is, and I'm not sure if this is documented behavior that you can rely on.

回答5:

Control.MouseDoubleClick is not a bubble event but a direct event.

Since checking this question with Snoop, which is a tool for browsing visual trees and routed events, I see that Control.MouseDoubleClick events of 'ListView' and 'ListBoxItem' are fired at one time. You could check with this Snoop tool.

---

First, to find an answer, it is needed to check that both event arguments of the MouseDoublClick are same objects. You would expect they are same objects. If it is true, it is very strange as your question, but they are not same instances. We can check it with following codes.

RoutedEventArgs _eventArg;
private void Button_MouseDoubleClick(object s, RoutedEventArgs e)
{
    if (!e.Handled) Debug.WriteLine("Button got unhandled doubleclick.");
    //e.Handled = true;
    _eventArg = e;
}

private void ListViewItem_MouseDoubleClick(object s, RoutedEventArgs e)
{
    if (!e.Handled) Debug.WriteLine("ListViewItem got unhandled doubleclick.");
    e.Handled = true;
    if (_eventArg != null)
    {
        var result = _eventArg.Equals(e);
        Debug.WriteLine(result);
    }
}

It means that the event argument of the MouseDoublClick is created newly at somewhere, but I don't understand deeply why it is.

To be clearer, let's check for the event argument of the BottonBase.Click. It will be return the true about checking same instances.

    <ListView>
        <ListViewItem ButtonBase.Click="ListViewItem_MouseDoubleClick">
            <Button Click="Button_MouseDoubleClick" Content="click"/>
        </ListViewItem>
    </ListView>

If you only focus on the execution as you mentioned there'll be lots of solutions. As above, I think that using the flag(_eventArg) is also good choice.

回答6:

I've just had this same problem. There is a simple but non-obvious solution.

Here is how double click is raised by Control ....

private static void HandleDoubleClick(object sender, MouseButtonEventArgs e)
{
    if (e.ClickCount == 2)
    {
        Control control = (Control)sender;
        MouseButtonEventArgs mouseButtonEventArgs = new MouseButtonEventArgs(e.MouseDevice, e.Timestamp, e.ChangedButton, e.StylusDevice);
        if (e.RoutedEvent == UIElement.PreviewMouseLeftButtonDownEvent || e.RoutedEvent == UIElement.PreviewMouseRightButtonDownEvent)
        {
            mouseButtonEventArgs.RoutedEvent = Control.PreviewMouseDoubleClickEvent;
            mouseButtonEventArgs.Source = e.OriginalSource;
            mouseButtonEventArgs.OverrideSource(e.Source);
            control.OnPreviewMouseDoubleClick(mouseButtonEventArgs);
        }
        else
        {
            mouseButtonEventArgs.RoutedEvent = Control.MouseDoubleClickEvent;
            mouseButtonEventArgs.Source = e.OriginalSource;
            mouseButtonEventArgs.OverrideSource(e.Source);
            control.OnMouseDoubleClick(mouseButtonEventArgs);
        }
        if (mouseButtonEventArgs.Handled)
        {
            e.Handled = true;
        }
    }
}

So if you handle PreviewMouseDoubleClick setting e.Handled = true on the child control MouseDoubleClick won't fire on the parent control.

回答7:

1. You cannot easily change the way double clicking events get fired because they are dependent on user settings and that delay is customized in control panel.
2. You should checkout RepeatButton that allows you to press's button and while it is pressed it generates multiple click events in regular sequence.
3. In case if you want to customize event bubbling then you should search for Preview events that allows you to block propogation of events. What are WPF Preview Events?