wpf - 为什么 WPF 触发器必须声明为一种样式(甚至内联)？

Question

我不明白为什么 WPF 允许我同时写

<Grid>
 <Grid.Triggers>
  <DataTrigger Binding="{Binding HasNeverBeenSeen}" Value="true">
   <Setter Property="Background" Value="Red"/>
  </DataTrigger>
 </Grid.Triggers>
</Grid>

和

<Grid>
 <Grid.Style>
  <Style TargetType="{x:Type Grid}">
   <Style.Triggers>
    <DataTrigger Binding="{Binding HasNeverBeenSeen}" Value="true">
     <Setter Property="Background" Value="Red"/>
    </DataTrigger>
   </Style.Triggers>
  </Style>
 </Grid.Style>
</Grid>

但只有第二个似乎有效。如果我们必须使用样式，为什么 Grid 元素有一个 Triggers 标签？

谢谢

Answer 1

对您问题的简短回答是因为 WPF 团队就是这样设计的。

FrameworkElement.Triggers 只能有 EventTriggers 虽然属性是 TriggerBase 的集合。 MSDN page上也有明确说明:

Note that the collection of triggers established on an element only supports EventTrigger, not property triggers (Trigger). If you require property triggers, you must place these within a style or template and then assign that style or template to the element either directly through the Style property, or indirectly through an implicit style reference.