c# - XAML 弹出模板中的 opaqueRect 有何用途

Question

我试图理解 XAML 弹出模板中控件的每个部分的含义。原代码如下:

    <Popup x:Name="PART_Popup"
        AllowsTransparency="true"
        Grid.ColumnSpan="2"
        IsOpen="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"
        PopupAnimation="{DynamicResource {x:Static SystemParameters.ComboBoxPopupAnimationKey}}"
        Placement="Bottom">
    <Themes:SystemDropShadowChrome x:Name="shadow"
                                    Color="Transparent"
                                    MaxHeight="{TemplateBinding MaxDropDownHeight}"
                                    MinWidth="{Binding ActualWidth, ElementName=templateRoot}">
        <Border x:Name="dropDownBorder"
                BorderBrush="#FFEBEBEB"
                BorderThickness="1"
                CornerRadius="8"
                Background="White">
                <ScrollViewer x:Name="DropDownScrollViewer"
                                Template="{StaticResource UniversalScrollViewerTemplate}">
                    <Grid x:Name="grid"
                            RenderOptions.ClearTypeHint="Enabled">
                        <Grid.RowDefinitions>
                            <RowDefinition />
                            <RowDefinition />
                        </Grid.RowDefinitions>
                        <Canvas x:Name="canvas"
                                HorizontalAlignment="Left"
                                Height="0"
                                VerticalAlignment="Top"
                                Width="0">
                            <Rectangle x:Name="opaqueRect"
                                        Fill="{Binding Background, ElementName=dropDownBorder}"
                                        Height="{Binding ActualHeight, ElementName=dropDownBorder}"
                                        Width="{Binding ActualWidth, ElementName=dropDownBorder}" />
                        </Canvas>
                        <ItemsPresenter x:Name="ItemsPresenter"
                                        Grid.Row="1"
                                        KeyboardNavigation.DirectionalNavigation="Contained"
                                        SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" />
                    </Grid>
                </ScrollViewer>
        </Border>
    </Themes:SystemDropShadowChrome>
</Popup>

我想知道 Canvas Control canvas 在 DropDownScrollViewer -> grid 中的作用是什么，因为它的高度和宽度都是 0 .

Answer 1

因为 Grid 允许 ClearType 文本呈现。必须将 ClearType 文本呈现为不透明背景。

Set the ClearTypeHint property to Enabled to indicate that a subtree is safe for ClearType text rendering. Do this only when you can be certain that the text is rendering to a fully opaque background.

https://msdn.microsoft.com/en-us/library/system.windows.media.renderoptions.cleartypehint(v=vs.110).aspx