DataTriggerBehavior Binding to Property on ViewModel - mvvm

Question

I'm trying to bind a DataTriggerBehavior to a Property on my ViewModel, but it doesn't ever fire.
I've used DataTriggerBehaviors bound to various Properties of Controls with no trouble but can't get the VM binding to work.
DataContext is set to the VM.
I can see the binding value in debug but nothing triggers.
I've tested the InvokeCommandAction by changing the DataTriggerBehavior to an EventTriggerBehavior so that works fine.
<AppBarButton Icon="Library">
<i:Interaction.Behaviors>
<core:DataTriggerBehavior Binding="{Binding HelpPhase}" ComparisonCondition="Equal" Value="Add" >
<core:InvokeCommandAction Command="{Binding DataContext.StoreRateCommand, ElementName=LayoutRoot}"/>
</core:DataTriggerBehavior>
</i:Interaction.Behaviors>
</AppBarButton>
In VM (inherits VMBase that implements IPCN)
Private mHelpPhase As String
Public Property HelpPhase() As String
Get
Return Settings.HelpPhase
End Get
Set(value As String)
SetProperty(Settings.HelpPhase, value)
End Set
End Property

Answer 1

The EventTriggerBehavior listens for a specific event on its source and executes an action when the event is fired. It is different from the DataTriggerBehavior.
The DataTriggerBehavior performs an action when the data the behaviors is bound to meets a specified condition. In your question, when the bound data of the HelpPhase's value change to "Add", the behavior triggers an action to fire the command.
You should be able check if you have bind the HelpPhase to the DataTriggerBehavior and set the "Add" to the HelpPhase. You can bind the HelpPhase to Text property of TextBlock if the TextBlock show "Add".
There is an official DataTriggerBehavior sample, please refer the XamlBehaviors sample.