路由事件的TargetType

Question

We have TargetType for style, so that we can say please, only buttons :

<Style TargetType="Button">
    ...
</Style>

However, take to example this snippet:

<TabControl >
    <TabControl.Triggers>
        <EventTrigger RoutedEvent="TabControl.SelectionChanged">
            ...
        </EventTrigger>
    </TabControl.Triggers>
    <TabItem>
        <ListBox>
            <ListBoxItem>1</ListBoxItem>
            <ListBoxItem>2</ListBoxItem>
        </ListBox>
    </TabItem>
</TabControl>

We have something (to example, animation) attached to trigger when TabControl (it's even specified in xaml, but doing nothing as a fact) SelectionChanged event occurs. This, however, will happily occurs when ListBox selected item is changed as well.

It's a nature of routed events, because as there is no handler in ListBox , it will be routed to its parent. We could simply use code behind

private void Stop(object sender, SelectionChangedEventArgs e)
{
    e.Handled = true;
}

and attach it to every child , like this:

 <ListBox SelectionChanged="Stop">
 ...
 </ListBox>

However, imagine there are hundreds of children and at some point we decide to animate their parent. Now we have to go through all children and attach them to bloody event handler . Right?

No, it's not right. There should be something missing. An ability to specify a scope or type or anything (make event trigger strict to declaration). Or perhaps some dirty hack. Anyone knows anything like this?

PS: tagged as MVVM, because code-behind is not as good if it is possible to avoid it.

Answer 1

I believe that your problem is caused because both TabControl and ListBox override the Selector.SelectionChanged event . I would however agree with you that if (as we have) we specify that we want to use the overridden TabControl event, this shouldn't be confused with the Listbox.SelectionChanged event as it is. It might even be an idea to leave a bug report on the Microsoft Connect website.

However, there is one fix that I can think of, but it might not suit you... it really depends on what you are doing when the selection of the ListBox es change. What we can do is to add a handler to each ListBox.SelectionChanged event and set the SelectionChangedEventArgs.Handled property to be true , so that it does not interfere with the SelectionChanged event of the TabControl :

<TabControl>
    <TabControl.Triggers>
        <EventTrigger RoutedEvent="TabControl.SelectionChanged">
            <BeginStoryboard>
                <Storyboard>
                    <DoubleAnimation From="0.25" To="1.0" Duration="0:0:1" 
                        Storyboard.TargetProperty="Opacity" />
                </Storyboard>
            </BeginStoryboard>
        </EventTrigger>
    </TabControl.Triggers>
    <TabItem Header="Tab 1">
        <ListBox SelectionChanged="ListBox_SelectionChanged">
            <ListBoxItem>1</ListBoxItem>
            <ListBoxItem>2</ListBoxItem>
        </ListBox>
    </TabItem>
    <TabItem Header="Tab 2">
        <ListBox SelectionChanged="ListBox_SelectionChanged">
            <ListBoxItem>1</ListBoxItem>
            <ListBoxItem>2</ListBoxItem>
        </ListBox>
    </TabItem>
</TabControl>

...

private void ListBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    e.Handled = true;
}

If you want to do something when the selection changes on either ListBox , then I suggest that you use the SelectedItem property to manage that instead and then you won't miss the ListBox.SelectionChanged event.