Modify expander header children when expanded - wpf

Question

I can't figure out how to do it in XAML. I came up with workarounds that use Expanded/Collapsed events, but they just don't feel right.
I have a DataGrid, with groups, that are templated as expanders. I have a button inside expander, that's hidden by default, and only needs to be shown when an expander is expanded.
<GroupStyle.ContainerStyle>
<Style TargetType="{x:Type GroupItem}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type GroupItem}">
<Expander IsExpanded="True">
<Expander.Header>
<StackPanel Orientation="Horizontal">
<TextBlock Text="Some Text"/>
<Button Name="MyButton" Visibility="Collapsed" Content="Add All"/>
</StackPanel>
</Expander.Header>
<Expander.Content>
<ItemsPresenter />
</Expander.Content>
</Expander>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</GroupStyle.ContainerStyle>
So basically the code in question is right in the middle:
<Expander IsExpanded="True">
<Expander.Header>
<StackPanel Orientation="Horizontal">
<TextBlock Text="Some Text"/>
<Button Name="MyButton" Visibility="Collapsed" Content="Add All"/>
</StackPanel>
</Expander.Header>
<Expander.Content>
<ItemsPresenter />
</Expander.Content>
</Expander>
I'm trying to set Visibility of the Button to false, when the expander is expanded. I tried using a trigger like the following:
<Expander.Style>
<Style TargetType="Expander">
<Style.Triggers>
<DataTrigger Binding="{Binding IsExpanded, RelativeSource={RelativeSource Self}}" Value="False">
<Setter TargetProperty="MyButton" Property="Visibility" Value="Visible" />
</DataTrigger>
</Style.Triggers>
</Style>
</Expander.Style>
, but the compiler doesn't accept it, because it can't find MyButton (I'm guessing because it's inside a header). Any ideas on how I can get this to work?

Answer 1

You have to move the DataTrigger in a ControlTemplate like this:
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type GroupItem}">
<Expander Name="Expander" IsExpanded="True">
<Expander.Header>
<StackPanel Orientation="Horizontal">
<TextBlock Text="Some Text" />
<Button Name="MyButton"
Visibility="Collapsed"
Content="Add All" />
</StackPanel>
</Expander.Header>
<Expander.Content>
<ItemsPresenter />
</Expander.Content>
</Expander>
<ControlTemplate.Triggers>
<DataTrigger Binding="{Binding Path=IsExpanded, ElementName=Expander}" Value="False">
<Setter TargetName="MyButton" Property="Visibility" Value="Visible" />
</DataTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
Notes
I explicitly set the name for the Expander, because the construction RelativeSource={RelativeSource Self} points to control himself, and there in GroupItem is no property IsExpanded.
In Setter does not have a TargetProperty property, but there are TargetName.