Set a trigger's property to a child - wpf

Question

How can I set the trigger to be depend on a children's property?
Like I want to change the header of my Expander depending whether the Expander's ListView
does have children or not.
But I always get an Comilor error, that HasItems can not be resolved...
<Expander Header="Expand to add new ports">
<Expander.Resources>
<Style TargetType="{x:Type Expander}">
<Style.Triggers>
<Trigger Property="Content.HasItems" Value="False">
<Setter Property="Header" Value="No children" />
</Trigger>
</Style.Triggers>
</Style>
</Expander.Resources>
<ListView ItemsSource="{Binding Path=SomeItems}">
</ListView>

Answer 1

You can use a DataTrigger bound to the ListView using ElementName:
<Expander>
<Expander.Resources>
<Style TargetType="{x:Type Expander}">
<Setter Property="Header" Value="Expand to add new ports" />
<Style.Triggers>
<DataTrigger Binding="{Binding ElementName=ListView, Path=HasItems}" Value="False">
<Setter Property="Header" Value="No children" />
</DataTrigger>
</Style.Triggers>
</Style>
</Expander.Resources>
<ListView x:Name="ListView" ItemsSource="{Binding Path=SomeItems}">
</ListView>
</Expander>
Also note that if you set a property along with the declaration of the control the Setter in the trigger won't have any effect.
Use this:
<Setter Property="Header" Value="Expand to add new ports" />
Instead of this:
<Expander Header="Expand to add new ports">

Answer 2

There are multiple issues in your code -
First, you should set the property in style setter instead of declaring it locally.
Second, use DataTrigger in place of Trigger.
<Expander>
<Expander.Style>
<Style TargetType="{x:Type Expander}">
<Setter Property="Header" Value="Expand to add new ports"/>
<Style.Triggers>
<DataTrigger Binding="{Binding Path=SomeItems.Count}" Value="0">
<Setter Property="Header" Value="No children" />
</DataTrigger>
</Style.Triggers>
</Style>
</Expander.Style>
<ListView ItemsSource="{Binding Path=SomeItems}">
</ListView>
</Expander>