Silverlight ToggleButton with images

I am trying to create a Silverlight Toggle button style that would flip between a plus and a minus sign. Unfortunately it is always showing a minus sign. Can anyone tell me what's the problem with this style:

<Style x:Key="PlusMinusToggleButtonStyle" TargetType="ToggleButton">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="ToggleButton">
                <Grid>
                    <VisualStateManager.VisualStateGroups>
                        <VisualStateGroup x:Name="CheckStates">
                            <VisualState x:Name="Unchecked">
                                <Storyboard>
                                    <DoubleAnimation Storyboard.TargetName="UncheckedVisual" Storyboard.TargetProperty="Opacity" To="0" Duration="1" />
                                    <DoubleAnimation Storyboard.TargetName="CheckedVisual" Storyboard.TargetProperty="Opacity" To="1" Duration="0" />
                                </Storyboard>
                            </VisualState>
                            <VisualState x:Name="Checked">
                                <Storyboard>
                                    <DoubleAnimation Storyboard.TargetName="UncheckedVisual" Storyboard.TargetProperty="Opacity" To="1" Duration="0" />
                                    <DoubleAnimation Storyboard.TargetName="CheckedVisual" Storyboard.TargetProperty="Opacity" To="0" Duration="1" />开发者_高级运维
                                </Storyboard>
                            </VisualState>
                        </VisualStateGroup>
                    </VisualStateManager.VisualStateGroups>
                    <Grid>
                        <Image x:Name="UncheckedVisual" Source="plus.png" Stretch="None" />
                        <Image x:Name="CheckedVisual" Source="minus.png" Stretch="None" />
                    </Grid>
                </Grid>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

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You need to expand on the DoubleAnimation, put in KeyFrame Animation in it. Also set the base opacity to 0 and then fade in the respective one you need. For example:

  <VisualState x:Name="Checked">
      <Storyboard>
           <DoubleAnimationUsingKeyFrames Storyboard.TargetName="CheckedVisual" Storyboard.TargetProperty="Opacity">
                <EasingDoubleKeyFrame KeyTime="0" Value="0"/>
                <EasingDoubleKeyFrame KeyTime="0:0:1" Value="1"/>
            </DoubleAnimationUsingKeyFrames>
       </Storyboard>
   </VisualState>

Everytime a state is changed, it goes back to the base state then to the state that was called. Because a togglebutton is always in an on or off state, it will not be in an invisible state.