Why is the TreeViewItem's MouseDoubleClick event being raised multiple times per double click?

Question

XAML

<TreeView Name="GroupView" ItemsSource="{Binding Documents}">
            <TreeView.ItemContainerStyle>
                <Style TargetType="{x:Type TreeViewItem}">
                    <EventSetter Event="MouseDoubleClick" Handler="OnTreeNodeDoubleClick"/>
                </Style>
            </TreeView.ItemContainerStyle>
            ....
</TreeView>

Code-Behind

private void OnTreeNodeDoubleClick(object sender, MouseButtonEventArgs mouseEvtArgs)
       {
           Console.WriteLine("{3} MouseDoubleClick Clicks={0} ChangedButton={1} Source={2} Handled={4} ButtonState={5}",
               mouseEvtArgs.ClickCount, mouseEvtArgs.ChangedButton, mouseEvtArgs.OriginalSource,
               mouseEvtArgs.Timestamp, mouseEvtArgs.Handled, mouseEvtArgs.ButtonState);
       }

I find that for one double click, the event handler is called multiple times. I'm trying to open up a document in tab on a double-click on the corresponding tree node; so I'd need to filter out the extra calls.

23479156 MouseDoubleClick Clicks=1 ChangedButton=Left Source=System.Windows.Controls.TextBlock Handled=False ButtonState=Pressed
23479156 MouseDoubleClick Clicks=1 ChangedButton=Left Source=System.Windows.Controls.TextBlock Handled=False ButtonState=Pressed

In my slightly-complicated app, it is being raised 4 times per double-click. On a simple repro-app, it is being raised 2 times per double click. Also all the event argument parameters are the same too, so I can't distinguish the last one of a set.

Any ideas why this is the way it is?

Vernalize

Are you using the treeview inside UpdatePanel? – Angilaangina

@Kangkan: No. This is not a web-app; a simple desktop app. – Vernalize

I had the same issue once, never figured it out. I installed the doubleclick event handler on the treeview (instead of on the treeviewitems) and just used the selecteditem property... – Skyscape

Answer 1

I know this is an old question, but as I came across it in my searches for the solution, here are my findings for any future visitors!

TreeViewItems are recursively contained within each other. TreeViewItem is a HeaderedContentControl (see msdn), with the child nodes as the Content. So, each TreeViewItem's bounds include all of its child items. This can be verified using the excellent WPF Inspector by selecting a TreeViewItem in the visual tree, which will highlights the bounds of the TreeViewItem.

In the OP's example, the MouseDoubleClick event is registered on each TreeViewItem using the style. Therefore, the event will be raised for the TreeViewItems that you double-clicked on - and each of its parent items - separately. This can be verified in your debugger by putting a breakpoint in your double-click event handler and putting a watch on the event args' Source property - you will notice that it changes each time the event handler is called. Incidentally, as can be expected, the OriginalSource of the event stays the same.

To counter this unexpected behaviour, checking whether the source TreeViewItem is selected, as suggested by Pablo in his answer, has worked the best for me.

Attributive

Answer 2

When a TreeViewItem is double clicked, that item is selected as part of the control behavior. Depending on the particular scenario it could be possible to say:

...
TreeViewItem tviSender = sender as TreeViewItem;

if (tviSender.IsSelected)
    DoAction();
...

Spruce

Answer 3

I've done some debugging and it appears to be a bug in WPF. Most answers already given are correct, and the workaround is to check if the tree view item is selected.

@ristogod's answer is the closest to the root problem - it mentions that setting e.Handled = true the first time handler is invoked doesn't have the desired effect and the event continues to bubble up, calling parent TreeViewItems' handlers (where e.Handled is false again).

The bug seems to be in this code in WPF: http://referencesource.microsoft.com/#PresentationFramework/src/Framework/System/Windows/Controls/Control.cs,5ed30e0aec6a58b2

It receives the MouseLeftButtonDown event (which is handled by the child control already), but it fails to check if e.Handled is already set to true. Then it proceeds to create a new MouseDoubleClick event args (with e.Handled == false) and invokes that always.

The question also remains why after having set it to handled the first time the event continues to bubble? Because in this line, when we register the handler Control.HandleDoubleClick: http://referencesource.microsoft.com/#PresentationFramework/src/Framework/System/Windows/Controls/Control.cs,40

we pass true as the last argument to RegisterClassHandler: http://referencesource.microsoft.com/#PresentationCore/Core/CSharp/System/Windows/EventManager.cs,161 which is handledEventsToo.

So the unfortunate behavior is a confluence of two factors:

1. Control.HandleDoubleClick is called always (for handled events too), and
2. Control.HandleDoubleClick fails to check if the event had already been handled

I will notify the WPF team but I'm not sure this bug is worth fixing because it might break existing apps (who rely on the current behavior of event handlers being called even if Handled was set to true by a previous handler).

Elspet

Answer 4

private void TreeView_OnItemMouseDoubleClick(object sender, MouseButtonEventArgs e)
{
    if (e.Source is TreeViewItem
        && (e.Source as TreeViewItem).IsSelected)
    {
        // your code
        e.Handled = true;
   }
}

Presumably

Answer 5

This is not actually a bubbling issue. I've seen this before. Even when you tell the event that you handled it, it continues to keep bubbling up. Except that I don't think that it's actually bubbling up, but rather firing the node above's own double click event. I could be totally wrong on that. But in either case, it's important to know that saying:

e.handled = true;

Does nothing to stop this from happening.

One way to prevent this behavior is to note that when you are double clicking, you are first single clicking and that the selected event should fire first. So while you can't stop the Double Click events from occurring, you should be able to check inside the handler to see whether the event logic should run. This example leverages that:

TreeViewItem selectedNode;

private void MouseDoubleClickEventHandler(object sender, MouseButtonEventArgs e)
{
    if(selectedNode = e.Source)
    {
        //do event logic
    }
}

private void TreeViewSelectedEventHandler(object sender, RoutedEventArgs e)
{
    selectedNode = (TreeViewItem)e.Source;
}

Sometimes however you have situations where the nodes are being selected by other beans than through the TreeView SelectedItemChanged event. In that case you can do something like this. If you happen to have a TreeView with a single declared top node, you can give that node a specific name and then do something like this:

bool TreeViewItemDoubleClickhandled;

private void MouseDoubleClickEventHandler(object sender, MouseButtonEventArgs e)
{
    if (!TreeViewItemDoubleClickhandled)
    {
        //do logic here

        TreeViewItemDoubleClickhandled = true;
    }

    if (e.Source == tviLoadTreeTop)
    {
        TreeViewItemDoubleClickhandled = false;
    }
    e.Handled = true;
}

Regardless of the method you use, the important thing is to note that for whatever reason with TreeViewItem double clicking that you can't stop the events from firing up the tree. At least I haven't found a way.

Poniard

Spot on! The fact that setting e.Handled to true has no expected effect is a bug in WPF. I'll add my own answer with more details. – Elspet

Answer 6

I have a little bit more elegant solution than checking for selection or creating flags:

A helper method:

public static object GetParent(this DependencyObject obj, Type expectedType) {

    var parent = VisualTreeHelper.GetParent(obj);
    while (parent != null && parent.GetType() != expectedType)
        parent = VisualTreeHelper.GetParent(parent);

    return parent;
}

And then your handler:

public void HandleDoubleClick(object sender, MouseButtonEventArgs e)
{
    if (e.OriginalSource is DependencyObject)
        if (sender == (e.OriginalSource as DependencyObject).GetParent(typeof(TreeViewItem))) 
    {
        // sender is the node, which was directly doubleclicked
    }
}

Clinometer

You should specify that OriginalSource is the original reporting source as determined by pure hit testing, so if the event sender is the same as the parent TreeViewItem, we are on the node directly clicked. – Blackshear

Answer 7

This is the wonderful world of event bubbling. The event is bubbling up the node hierarchy of your TreeView and your handler is called once for every node in the hierarchy path.

Just use something like

        // ...
        if (sender != this)
        {
            return;
        }
        // Your handler code goes here ...
        args.Handled = true;
        // ...

in your handler code.

Crankle

Bubbling would cause different handlers up the UI element hierarchy to be invoked for an event. It should not cause the same handler to be called multiple times per event. – Vernalize

Another thing to your identical handler parameters. It doesn´t matter to which event you subscribe, the sender is always the TreeView. The real sender is hidden in the RoutedEvent.OriginalSource parameter. Depending on the event sometimes it´s the TreeViewItem but for mouse events it´s the control type defined in your TreeViewItem´s (Hierarchical)DataTemplate on which the user has clicked. Probably MS has´nt overwritten the FrameworkElement implementation. – Crankle

Answer 8

There are some pretty major problems with this solution, but it could work in case someone needs to solve this problem in multiple places and I did find a scenario where the accepted solution doesn't work (double clicking on a toggle button that opens up a popup, where the toggle button is inside another element that handles double click.)

public class DoubleClickEventHandlingTool

{ private const string DoubleClickEventHandled = "DoubleClickEventHandled";

public static void HandleDoubleClickEvent()
{
    Application.Current.Properties[DoubleClickEventHandled] = DateTime.Now.AddSeconds(1);
}

public static bool IsDoubleClickEventHandled()
{
    var doubleClickWasHandled = Application.Current.Properties[DoubleClickEventHandled] as DateTime?;

    return doubleClickWasHandled.HasValue && !IsDateTimeExpired(doubleClickWasHandled.Value);
}

private static bool IsDateTimeExpired(DateTime value)
{
    return value < DateTime.Now;
}

public static void EnableDoubleClickHandling()
{
    Application.Current.Properties[DoubleClickEventHandled] = null;
}

public static bool IsDoubleClickEventHandledAndEnableHandling()
{
    var handled = IsDoubleClickEventHandled();
    EnableDoubleClickHandling();

    return handled;
}

}

Use DoubleClickEventHandlingTool.HandleDoubleClickEvent() inside the inner/low level element eg:

    private void OnPreviewMouseDown(object sender, MouseButtonEventArgs e)
{if (e.ClickCount == 2) DoubleClickEventHandlingTool.HandleDoubleClickEvent();}

High level double click event then only performs it's action when:

if (!DoubleClickEventHandlingTool.IsDoubleClickEventHandledAndEnableHandling())

Each

Answer 9

The most likely reason is that the doubleclick handler is installed multiple times, so each instance of the handler is being called once for each click.

Whitsuntide

@John - did a find in all files, the only place where OnTreeNodeDoubleClick in mentioned is in the Style definition – Vernalize