Question

While reading about DataTrigger on MSDN, it says

Represents a trigger that applies property values or performs actions when the bound data meets a specified condition.

It means

When a trigger is true it changes the value to the desired value.

But, can this be inferred?

When it is no longer true it returns the value to the previous value.

---

How I came to this conclusion

I did this

<Style x:Key="fd" TargetType="SomeControl">
 <Setter Property="Control.Template">
  <Setter.Value>
   <ControlTemplate>
    <Button Content="Foo" x:Name="mybutton">
    <ControlTemplate.Triggers>
     <DataTrigger Binding="{Binding RelativeSource={RelativeSource Self}, Path=FooProperty}" 
                  Value="Collapsed">
      <Setter Property="IsEnabled" Value="False" TargetName="mybutton"/>
      <Setter Property="Opacity" Value="0.5" TargetName="mybutton"/>
     </DataTrigger>
    </ControlTemplate.Triggers>
   </ControlTemplate>
  </Setter.Value>
 </Setter>
</Style>

When FooProperty becomes collapsed it changes mybutton’s IsEnabled to False and Opacity to 0.5

Now when FooProperty becomes visible it changes mybutton’s IsEnabled to True and Opacity to 1 even though I have not written any trigger for reverting back to previous values.

Is this an inbuilt feature of DataTrigger to revert back to previous value When it is no longer true?

If yes, is there any Microsoft/MSDN doc to prove this?

Answer 1

What you are telling, is correct.

As for actual doc to prove this,
search from here:

http://msdn.microsoft.com/en-us/library/ms745683.aspx

http://msdn.microsoft.com/en-us/library/system.windows.trigger.aspx

“Remarks section”

WPF defines properties that correspond to end-user actions, such as
the IsMouseOver property that is set to true when the user hovers the
cursor over a UIElement or the corresponding IsMouseOver property of a
ContentElement. Representing end-user actions in property values,
along with the Trigger element, allows WPF styles to change property
values based on those end-user actions, all from within markup.

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. The Property of interest must be a
dependency property.

Note that you must specify both the Property and Value properties on a
Trigger for the trigger to be meaningful. If one or both of the
properties are not set, an exception is thrown.

good luck 🙂