Performing UpdateLayout on SizeChanged event of ScrollViewer

Question

UPDATED I have an obtuse requirement I'm trying to fulfill working with existing elements and I'm at a point I could really use another pair of eyes.

For quicker explanation, some pseudo;

<Grid>
  <Grid.RowDefinitions>
     <RowDefinition Height="Auto"/>
     <RowDefinition Height="Auto"/>
     <RowDefinition Height="*"/>
     <RowDefinition Height="Auto"/>
   </Grid.RowDefinitions>

   <Grid/>
   <Grid Grid.Row="1"/>
   <Grid Grid.Row="2"/>

   <Grid Grid.Row="3" Visibility="Collapsed">
      <Grid.RowDefinitions>
         <RowDefinition Height="20"/>
         <RowDefinition Height="*"/>
      </Grid.RowDefinitions>

        <ScrollViewer Grid.Row="1">
          <ItemsControl/>
        </ScrollViewer/>

    </Grid>

</Grid>

So what we have here is a parent Grid with a child Grid whose visibility gets toggled. Currently the parent's row has an Auto height set so when the child disappears, the row collapses.

Originally, there was just a MaxHeight set on the child which invoked the ScrollViewer scrollbar just fine when it's made visible. Except I need to ditch the MaxHeight and let it take up all the space it has available/needs and at the same time invoke the scrollbar on the ScrollViewer when the size changes. I should also mention I'm trying to accomplish this entire in XAML with no code behind if I can.

I tried rigging an EventTrigger to SizeChanged and also tried Loaded and let it do a CallMethodAction to perform UpdateLayout hoping it would re-draw/measure the ScrollViewer to invoke the scrollbar if it's required. However no joy...

Something like this attached to the ScrollViewer;

<i:Interaction.Triggers>
   <i:EventTrigger EventName="Loaded">
       <ei:CallMethodAction MethodName="UpdateLayout"/>
   </i:EventTrigger>
</i:Interaction.Triggers>

I'm hoping someone who's got more sleep than I have can offer some insight to my folly or what I'm forgetting? Seems like something like this should work.

All I need it to do, is still respect the Visibility change and allow the parent's row to collapse, but still invoke a scrollbar when it's Visible and it's required. Any tricks?

PS - I'm also trying InvalidateArrange & InvalidateMeasure with no luck, am I perhaps using CallMethodAction incorrectly in this scenario?

Answer 1

Setting VerticalAlignment="Top" and no Height (or Height="*") on the second RowDefinition in the outer Grid should work:

<Grid VerticalAlignment="Top">
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto"/>
        <RowDefinition/>
    </Grid.RowDefinitions>

    <Grid Grid.Row="1" Visibility="Collapsed">
        <Grid.RowDefinitions>
            <RowDefinition Height="20"/>
            <RowDefinition/>
        </Grid.RowDefinitions>

        <ScrollViewer Grid.Row="1">
            <ItemsControl/>
        </ScrollViewer>
    </Grid>
</Grid>