DataTrigger that doesn't undo itself when condition no longer met - xaml

Question

Is there a way to do something like the following?
<Style TargetType="{x:Type: TreeViewItem}">
<Style.Triggers>
<DataTrigger Binding="~Complex Binding~" Value="True" DoNotUnset="True">
<Setter Property="IsExpanded" Value="True"/>
</DataTrigger>
</Style.Triggers>
</Style>
What I basically would like is this to be a "No Undo DataTrigger" if you will. When the Value is no longer "True" I don't want it to set "IsExpanded" back to its previous value.
Here is my attempt to do this using enter actions but this also has problems.
<Style TargetType="{x:Type: TreeViewItem}">
<Style.Triggers>
<DataTrigger Binding="~Complex Binding~" Value="True">
<DataTrigger.EnterActions>
<BeginStoryboard>
<BeginStoryboard.Storyboard>
<Storyboard>
<BooleanAnimationUsingKeyFrames Storyboard.TargetProperty="(IsExpanded)" Duration="00:00:01" FillBehavior="Stop">
<BooleanKeyFrameCollection>
<DiscreteBooleanKeyFrame KeyTime="00:00:00" Value="True"/>
</BooleanKeyFrameCollection>
</BooleanAnimationUsingKeyFrames>
</Storyboard>
</BeginStoryboard.Storyboard>
</BeginStoryboard>
</DataTrigger.EnterActions>
</DataTrigger>
</Style.Triggers>
</Style>
First this is insanely verbose but secondly this only works for the duration of the BooleanAnimationUsingKeyFrames. If I change the FillBehavior to "HoldEnd" then it looks correct but now the user can no longer un-expand the treeviewitem by clicks (though oddly enough they still can by using the keyboard).
For those who are wondering: yes I'm trying to expand all items in a tree view by binding because I don't want to build a recursive ItemsGenerator.GetItemContainerFromIndex(i) loop. I'd still like to use a similar "No Undo Datatigger" in other areas of my code.

Answer 1

I would bind the ~Complex Binding~ to a bool in the ViewModel which once set to true is always true.
private bool _onceTrueAlwaysTrue = false;
public bool OnceTrueAlwaysTrue
{
get
{
return _onceTrueAlwaysTrue;
}
set
{
if(value)
{
_onceTrueAlwaysTrue = true;
OnPropertyChanged("OnceTrueAlwaysTrue");
}
}
}
then bind this property to the IsEnabled and you should be fine. If you want to reset it simply make a reset method that sets _onceTrueAlwaysTrue = false;
As for the NoUndo datatrigger, as far as I know there is no such thing. You are gonna have to do some kind of work around every time.
The properties changed by triggers are automatically reset to their previous value when the triggered condition is no longer satisfied. Triggers are optimized for transient states which are expected to change and return to original state, such as IsPressed on Button and IsSelected on ListBoxItem.