空集合时，组合框的样式不同

Question

我给自己设计了一个包含正常Popup的ComboBox。现在，我想在集合为空（Count = 0）的情况下向用户显示Popup中的消息，而不是在绑定属性Null时。该消息应该只是一些带有一些文本的TextBlock。
我试图用一个不起作用的Trigger来改变它。Popup如下所示：

<Popup Name="PART_Popup" IsOpen="{Binding IsDropDownOpen, RelativeSource={RelativeSource TemplatedParent}}" Grid.ColumnSpan="2" Placement="Bottom">
<Border x:Name="DropDownBorder" Height="Auto" MaxHeight="100" Width="100" BorderBrush="Blue" BorderThickness="1">
    <ScrollViewer x:Name="DropDownScrollViewer" Background="White">
        <ContentControl x:Name="PopupContent">
            <Grid RenderOptions.ClearTypeHint="Enabled">
                <Canvas HorizontalAlignment="Left" Height="0" VerticalAlignment="Top" Width="0">
                    <Rectangle x:Name="OpaqueRect" Height="{Binding ActualHeight, ElementName=DropDownBorder}" Width="{Binding ActualWidth, ElementName=DropDownBorder}"/>
                </Canvas>
                <ItemsPresenter x:Name="ItemsPresenter" KeyboardNavigation.DirectionalNavigation="Contained" />
            </Grid>
        </ContentControl>
    </ScrollViewer>
</Border></Popup>

Answer 1

您可以使用 ComboBox 的 HasItems 属性。在这里，我在Popup内部包含一个TextBlock，并根据HasItems控制了它的Visibility。

     <Popup
                x:Name="PART_Popup"
                Grid.ColumnSpan="2"
                Margin="1"
                AllowsTransparency="true"
                IsOpen="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"
                Placement="Bottom"
                PopupAnimation="{DynamicResource {x:Static SystemParameters.ComboBoxPopupAnimationKey}}">
                <Popup.Resources>
                    <BooleanToVisibilityConverter x:Key="BooleanToVisibilityConverter" />
                    <local:InverseBooleanToVisiblityConverter x:Key="InverseBooleanToVisiblityConverter" />
                </Popup.Resources>
                <Grid>
                    <TextBlock Text="No Items" Visibility="{Binding HasItems, Mode=OneWay, RelativeSource={RelativeSource TemplatedParent}, Converter={StaticResource InverseBooleanToVisiblityConverter}}" />
                    <Border
                        x:Name="dropDownBorder"
                        Background="{DynamicResource {x:Static SystemColors.WindowBrushKey}}"
                        BorderBrush="{DynamicResource {x:Static SystemColors.WindowFrameBrushKey}}"
                        BorderThickness="1"
                        Visibility="{Binding HasItems, Mode=OneWay, RelativeSource={RelativeSource TemplatedParent}, Converter={StaticResource BooleanToVisibilityConverter}}">
                        <ScrollViewer x:Name="DropDownScrollViewer">
                            <Grid x:Name="grid" RenderOptions.ClearTypeHint="Enabled">
                                <Canvas
                                    x:Name="canvas"
                                    Width="0"
                                    Height="0"
                                    HorizontalAlignment="Left"
                                    VerticalAlignment="Top">
                                    <Rectangle
                                        x:Name="opaqueRect"
                                        Width="{Binding ActualWidth, ElementName=dropDownBorder}"
                                        Height="{Binding ActualHeight, ElementName=dropDownBorder}"
                                        Fill="{Binding Background, ElementName=dropDownBorder}" />
                                </Canvas>
                                <ItemsPresenter
                                    x:Name="ItemsPresenter"
                                    KeyboardNavigation.DirectionalNavigation="Contained"
                                    SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" />
                            </Grid>
                        </ScrollViewer>
                    </Border>
                </Grid>
            </Popup>

而转换器，

  public class InverseBooleanToVisiblityConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return (bool)value  ? Visibility.Collapsed : Visibility.Visible;
    }
    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

Answer 2

使用组合框的 itemsource 并初始化集合。如果您的集合不包含任何项目，请使弹出窗口可见，依此类推...

集合.计数