GroupItem Update Expander Header when Property on Items Collection changes
<p>I show a summary of a int Property (Math.Max) of Items in a CollectionListView which is grouped. This can be done by ValueConverter which works fine, but whenever this Proptery of an underlaying item gets changed, the Expander.Header is not updated. If i manually subscribe each PropertyChanged event of each single item and fire the CollectionChangedEvent the whole view is updated and the expander states get lost. How can i achive what i want without code behind just with XAML?</p>

<pre><code>&lt;Style x:Key="GroupHeaderStyle" TargetType="{x:Type GroupItem}"&gt;
        &lt;Setter Property="Template"&gt;
            &lt;Setter.Value&gt;
                &lt;ControlTemplate TargetType="{x:Type GroupItem}"&gt;
                    &lt;Expander IsExpanded="True"&gt;
                        &lt;Expander.Header&gt;
                            &lt;StackPanel Orientation="Horizontal"&gt;
                                &lt;TextBlock Text="Assign To: "/&gt;
                                &lt;TextBlock Text="{Binding Items, Converter{StaticResource PropertyCollectionConverter}}"/&gt;
                            &lt;/StackPanel&gt;       
                        &lt;/Expander.Header&gt;
                        &lt;ItemsPresenter/&gt;
                    &lt;/Expander&gt;
                &lt;/ControlTemplate&gt;
            &lt;/Setter.Value&gt;
        &lt;/Setter&gt;
    &lt;/Style&gt;
</code></pre>

<p>What i want is something like this (pseudo code):</p>

<pre><code>StackPanel Orientation="Horizontal"&gt;
    &lt;TextBlock Text="Assign To: "/&gt;
    &lt;TextBlock Text="{Binding Items.Max(t =&gt; t.MyIntProperty)}"/&gt;
&lt;/StackPanel&gt; 
</code></pre>

<p>Any tips how can i achive this?</p>