IsPressed触发器不生效

[OneOnce]

自定义ImageButton，实现如下Trigger时IsPressed无效果
1. IsMouseOver
2. IsPressed无效果
3. IsEnabled

Generic.xaml：

<Style TargetType="{x:Type local:ImageButton}" BasedOn="{x:Null}">
    <Setter Property="SnapsToDevicePixels" Value="true"/>
    <Setter Property="FocusVisualStyle" Value="{StaticResource ButtonFocusVisual}"/>
    <Setter Property="VerticalContentAlignment" Value="Center"/>
    <Setter Property="HorizontalContentAlignment" Value="Center"/>
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type local:ImageButton}">
                <Button >
                    <StackPanel Orientation="Horizontal">
                        <Image x:Name="BG_PART" Margin="2 0"
                                Source="{Binding NormalImage, RelativeSource={RelativeSource TemplatedParent}}"
                                VerticalAlignment="Center"/>

                        <ContentPresenter HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" 
                                            VerticalAlignment="{TemplateBinding VerticalContentAlignment}" 
                                            RecognizesAccessKey="True" Margin="{TemplateBinding Padding}"/>
                    </StackPanel>
                </Button>

                <ControlTemplate.Triggers>
                    <Trigger Property="IsMouseOver" Value="true">
                        <Setter TargetName="BG_PART" Property="Source" Value="{Binding HoverImage, RelativeSource={RelativeSource TemplatedParent}}"/>
                    </Trigger>

                    <Trigger Property="Button.IsPressed" Value="true">
                        <Setter TargetName="BG_PART" Property="Source" Value="{Binding PressedImage, RelativeSource={RelativeSource TemplatedParent}}"/>
                    </Trigger>

                    <Trigger Property="IsEnabled" Value="false">
                        <Setter TargetName="BG_PART" Property="Source" Value="{Binding DisabledImage, RelativeSource={RelativeSource TemplatedParent}}" />
                    </Trigger>
                </ControlTemplate.Triggers>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

ImageButton.cs：

public class ImageButton : Button
{
    static ImageButton()
    {
        DefaultStyleKeyProperty.OverrideMetadata(typeof(ImageButton), new FrameworkPropertyMetadata(typeof(ImageButton)));
    }

    public ImageButton()
    {
    }

    #region Properties
 
    public static readonly DependencyProperty NormalImageProperty =
                        DependencyProperty.Register("NormalImage", typeof(ImageSource), typeof(ImageButton), new PropertyMetadata(null));
    public ImageSource NormalImage
    {
        get { return (ImageSource)GetValue(NormalImageProperty); }
        set { SetValue(NormalImageProperty, value); }
    }

    public static readonly DependencyProperty HoverImageProperty =
                        DependencyProperty.Register("HoverImage", typeof(ImageSource), typeof(ImageButton), new PropertyMetadata(null));
    public ImageSource HoverImage
    {
        get { return (ImageSource)GetValue(HoverImageProperty); }
        set { SetValue(HoverImageProperty, value); }
    }

    public static readonly DependencyProperty PressedImageProperty =
                        DependencyProperty.Register("PressedImage", typeof(ImageSource), typeof(ImageButton), new PropertyMetadata(null));
    public ImageSource PressedImage
    {
        get { return (ImageSource)GetValue(PressedImageProperty); }
        set { SetValue(PressedImageProperty, value); }
    }

    public static readonly DependencyProperty DisabledImageProperty =
                        DependencyProperty.Register("DisabledImage", typeof(ImageSource), typeof(ImageButton), new PropertyMetadata(null));
    public ImageSource DisabledImage
    {
        get { return (ImageSource)GetValue(DisabledImageProperty); }
        set { SetValue(DisabledImageProperty, value); }
    }
    #endregion

}