How to use IsKeyboardFocusWithin and IsSelected to

问题:

I have a style defined for my ListBoxItems with a trigger to set a background color when IsSelected is True:

    <Style x:Key="StepItemStyle" TargetType="{x:Type ListBoxItem}">
        <Setter Property="SnapsToDevicePixels" Value="true"/>
        <Setter Property="OverridesDefaultStyle" Value="true"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="ListBoxItem">
                    <Border Name="Border" Padding="0" SnapsToDevicePixels="true">
                        <ContentPresenter />
                    </Border>
                    <ControlTemplate.Triggers>
                        <Trigger Property="IsSelected" Value="True">
                            <Setter TargetName="Border" Property="Background" Value="#40a0f5ff"/>
                        </Trigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

This style maintains the selected item even when the ListBox and ListBoxItem loses focus, which in my case is an absolute must. The problem is that I also want the ListBoxItem to be selected when one of its TextBox's child gets focused. To achieve this I add a trigger that sets IsSelected to true when IsKeyboardFocusWithin is true:

<Trigger Property="IsKeyboardFocusWithin" Value="True">
    <Setter Property="IsSelected" Value="True" />
</Trigger>

When I add this trigger the Item is selected when the focus is on a child TextBox, but the first behaviour disappears. Now when I click outside the ListBox, the item is de-selected.

How can I keep both behaviours?

回答1:

When your listbox looses focus, it will set selected item to null because of your trigger. You can select on focus using some code behind that will not unselect when you loose focus.

XAML:

<Window x:Class="SelectedTest.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Height="300" Width="300">

    <StackPanel>
        <TextBox Text="Loose focus here" />
        <ListBox Name="_listBox" ItemsSource="{Binding Path=Items}">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <StackPanel Orientation="Horizontal" GotFocus="OnChildGotFocus">
                        <TextBox Text="{Binding .}" Margin="10" />
                        <TextBox Text="{Binding .}" Margin="10" />
                    </StackPanel>
                </DataTemplate>
            </ListBox.ItemTemplate>
            <ListBox.ItemContainerStyle>
                <Style TargetType="{x:Type ListBoxItem}">
                    <Setter Property="SnapsToDevicePixels" Value="true"/>
                    <Setter Property="OverridesDefaultStyle" Value="true"/>
                    <Setter Property="Template">
                        <Setter.Value>
                            <ControlTemplate TargetType="ListBoxItem">
                                <Border Name="Border" SnapsToDevicePixels="true" Background="Transparent">
                                    <ContentPresenter />
                                </Border>
                                <ControlTemplate.Triggers>
                                    <Trigger Property="IsSelected" Value="True">
                                        <Setter TargetName="Border" Property="Background" Value="Red"/>
                                    </Trigger>                                   
                                </ControlTemplate.Triggers>
                            </ControlTemplate>
                        </Setter.Value>
                    </Setter>
                </Style>
            </ListBox.ItemContainerStyle>
        </ListBox>
    </StackPanel>
</Window>

Code behind:

private void OnChildGotFocus(object sender, RoutedEventArgs e) 
{   
   _listBox.SelectedItem = (sender as StackPanel).DataContext; 
}

回答2:

"When I add this trigger the Item is selected when the focus is on a child TextBox, but the first behaviour disappears. Now when I click outside the ListBox, the item is de-selected."

Actually, I don't think it has lost that original behavior. What I suspect is happening is you're clicking directly in the textbox from somewhere else so the underlying ListBoxItem never actually became selected. If it did however, you'd see the selection would still remain after you left as you want.

You can test this by forcing the ListBoxItem to be selected by clicking directly on it (side-note: you should always give it a background, even if just 'transparent' so it can receive mouse clicks, which it won't if it's null) or even just hitting 'Shift-Tab' to set the focus there, back from the textbox.

However, that doesn't solve your issue, which is that the TextBox gets the focus but doesn't let the underlying ListBoxItem know about it.

The two approaches you can use for that are an event trigger or an attached behavior.

The first is an event trigger on the IsKeyboardFocusWithinChanged event where you set 'IsSelected' to true if the keyboard focus changed to true. (Note: Sheridan's answer does a faux-change-notification but it should not be used in cases where you can multi-select in the list because everything becomes selected.) But even an event trigger causes issues because you lose the multi-select behaviors such as toggling or range-clicking, etc.

The other (and my preferred approach) is to write an attached behavior which you set on the ListBoxItem, either directly, or via a style if you prefer.

Here's the attached behavior. Note: You again would need to handle the multi-select stuff if you want to implement that. Also note that although I'm attaching the behavior to a ListBoxItem, inside I cast to UIElement. This way you can also use it in ComboBoxItem, TreeViewItem, etc. Basically any ContainerItem in a Selector-based control.

public class AutoSelectWhenAnyChildGetsFocus
{
    public static readonly DependencyProperty EnabledProperty = DependencyProperty.RegisterAttached(
        "Enabled",
        typeof(bool),
        typeof(AutoSelectWhenAnyChildGetsFocus),
        new UIPropertyMetadata(false, Enabled_Changed));

    public static bool GetEnabled(DependencyObject obj){ return (bool)obj.GetValue(EnabledProperty); }
    public static void SetEnabled(DependencyObject obj, bool value){ obj.SetValue(EnabledProperty, value); }

    private static void Enabled_Changed(DependencyObject sender, DependencyPropertyChangedEventArgs e)
    {
        var attachEvents = (bool)e.NewValue;
        var targetUiElement = (UIElement)sender;

        if(attachEvents)
            targetUiElement.IsKeyboardFocusWithinChanged += TargetUiElement_IsKeyboardFocusWithinChanged;
        else
            targetUiElement.IsKeyboardFocusWithinChanged -= TargetUiElement_IsKeyboardFocusWithinChanged;
    }

    static void TargetUiElement_IsKeyboardFocusWithinChanged(object sender, DependencyPropertyChangedEventArgs e)
    {
        var targetUiElement = (UIElement)sender;

        if(targetUiElement.IsKeyboardFocusWithin)
            Selector.SetIsSelected(targetUiElement, true);
    }

}

...and you simply add this as a property setter in your ListBoxItem's style

<Setter Property="behaviors:AutoSelectWhenAnyChildGetsFocus.Enabled" Value="True" />

This of course assumes you've imported an XML namespace called 'behaviors' that points to the namespace where the class is contained. You can put the class itself in a shared 'Helper' library, which is what we do. That way, everywhere we want it, its a simple property set in the XAML and the behavior takes care of everything else.

回答3:

I figured out that IsKeyboardFocusWithin is not the best solution.

What I did in this case was to set the style on all of the controls used as DataTemplate to send the GotFocus-event to be handled in code behind. Then, in code behind, I searched up the visual tree (using VisualTreeHelper) to find the ListViewItem and set IsSelected to true. This way it does not "touch" the DataContext and works just with the View elements.

<Style TargetType="{x:Type Control}" x:Key="GridCellControlStyle">
...
<EventSetter Event="GotFocus" Handler="SelectListViewItemOnControlGotFocus"/>
...

private void SelectListViewItemOnControlGotFocus(object sender, RoutedEventArgs e)
{
var control = (Control)sender;
FocusParentListViewItem(control);
}

private void FocusParentListViewItem(Control control)
{
var listViewItem = FindVisualParent<ListViewItem>(control);
if (listViewItem != null)
    listViewItem.IsSelected = true;
}

public static T FindVisualParent<T>(UIElement element) where T : UIElement
{
UIElement parent = element; 

while (parent != null)
{
    var correctlyTyped = parent as T; 

    if (correctlyTyped != null)
    {
        return correctlyTyped;
    }

    parent = VisualTreeHelper.GetParent(parent) as UIElement;
} 

return null;
}