DataTrigger on parent property - wpf

Question

How can I set the "IsAtLeastOneUserAvailable" dependency property correctly if at least one of my button is available? The dependency property is set in the code of the xaml. So, we can bind to it like that
"{Binding IsAtLeastOneUserAvailable, ElementName=control}"
I want to hide the label if no control is visible in the ItemsControl.
<UserControl ... Name="control">
<Label Content="Test" Visibility={Binding IsAtLeastOneUserAvailable, ElementName=control">
<ItemsControl ItemsSource="{Binding Users, ElementName=control}" >
<ItemsControl.ItemTemplate>
<DataTemplate>
<Button Visibility="{Binding IsAvailable, Converter={StaticResource BooleanToVisibilityConverter}}">
<Button.Triggers>
<DataTrigger Binding="IsAvailable" Value="True">
<Setter Property="IsAtLeastOneUserAvailable" Value="True" />
</DataTrigger>
</Button.Triggers>
<StackPanel Orientation="Horizontal">
<TextBlock Text="{Binding Name}"/>
</StackPanel>
</Button>
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
</UserControl>

Answer 1

Instead of trying to set the value with a trigger, Why not just set your UserControl.IsAtLeastOneUserAvailable property inside your UserControl based on a linq query?
public bool IsAtLeastOneUserAvailable
{
get
{
return Users.Any(p => p.IsAvailable);
}
}
You can also raise this PropertyChanged event in the User's changed event:
public MyUserControl()
{
Users.CollectionChanged += Users_CollectionChanged;
}
void Users_CollectionChanged(object sender, CollectionChangedEventArgs e)
{
if (e.NewItems != null)
foreach(User user in e.NewItems)
user.PropertyChanged += User_PropertyChanged;
if (e.OldItems != null)
foreach(User user in e.OldItems)
user.PropertyChanged -= User_PropertyChanged;
}
void User_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
if (e.PropertyName == "IsAvailable")
RaisePropertyChanged("IsAtLeastOneUserAvailable");
}