GridSplitter with min constraints - wpf

Question

I want a Grid layout with two rows and splitter between them. Rows should have a minimum height of 80 pixels.
This code works great:
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="*" MinHeight="80" />
<RowDefinition Height="5" />
<RowDefinition Height="*" MinHeight="80" />
</Grid.RowDefinitions>
<TextBlock Grid.Row="0" Text="{Binding Path=ActualHeight, RelativeSource={RelativeSource Self}}" />
<GridSplitter Grid.Row="1" VerticalAlignment="Stretch" HorizontalAlignment="Stretch" Background="Red" />
<TextBlock Grid.Row="2" Text="{Binding Path=ActualHeight, RelativeSource={RelativeSource Self}}" />
</Grid>
But I want top row to have an Auto height until user manually change it using the splitter. So I changed the code to this:
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto" MinHeight="80" />
<RowDefinition Height="5" />
<RowDefinition Height="*" MinHeight="80" />
</Grid.RowDefinitions>
<TextBlock Grid.Row="0" Text="{Binding Path=ActualHeight, RelativeSource={RelativeSource Self}}" />
<GridSplitter Grid.Row="1" VerticalAlignment="Stretch" HorizontalAlignment="Stretch" Background="Red" />
<TextBlock Grid.Row="2" Text="{Binding Path=ActualHeight, RelativeSource={RelativeSource Self}}" />
</Grid>
And there is a problem. Splitter still satisfies row constraints, but it begins to increase top row's height infinitely if I drag splitter too low. This results in bottom row to be completely below window's bottom border.
I have done some Reflector on GridSplitter code and see that it uses different logic if rows has Auto or Star height.
Any suggestions how can I "fix" it?

Answer 1

I've run into this problem a few times myself. It appears as though the GridSplitter doesn't play well with Auto. That said, I have found a potential workaround.
You are able to specify the value of a GridLength object using "star coefficients". This acts as a multiplier for the length in question.
In your example, if you take the row you want to remain as star, and set the the star coefficient to a really large number, the row will take up all available space (forcing the other row to become its min-height). While this isn't the same behavior as "auto" (the height of the first row is not determined by its contents height), it might get you closer.
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="*" MinHeight="80" />
<RowDefinition Height="5" />
<RowDefinition Height="10000*" MinHeight="80" />
</Grid.RowDefinitions>
<TextBlock Grid.Row="0" Text="{Binding Path=ActualHeight, RelativeSource={RelativeSource Self}}" />
<GridSplitter Grid.Row="1" VerticalAlignment="Stretch" HorizontalAlignment="Stretch" Background="Red" />
<TextBlock Grid.Row="2" Text="{Binding Path=ActualHeight, RelativeSource={RelativeSource Self}}" />
</Grid>

Answer 2

I have developed a workaround for this problem. Point is to set MaxHeight for the top row while we are dragging splitter. Here the code:
public class FixedGridSplitter : GridSplitter
{
private Grid grid;
private RowDefinition definition1;
private double savedMaxLength;
#region static
static FixedGridSplitter()
{
new GridSplitter();
EventManager.RegisterClassHandler(typeof(FixedGridSplitter), Thumb.DragCompletedEvent, new DragCompletedEventHandler(FixedGridSplitter.OnDragCompleted));
EventManager.RegisterClassHandler(typeof(FixedGridSplitter), Thumb.DragStartedEvent, new DragStartedEventHandler(FixedGridSplitter.OnDragStarted));
}
private static void OnDragStarted(object sender, DragStartedEventArgs e)
{
FixedGridSplitter splitter = (FixedGridSplitter)sender;
splitter.OnDragStarted(e);
}
private static void OnDragCompleted(object sender, DragCompletedEventArgs e)
{
FixedGridSplitter splitter = (FixedGridSplitter)sender;
splitter.OnDragCompleted(e);
}
#endregion
private void OnDragStarted(DragStartedEventArgs sender)
{
grid = Parent as Grid;
if (grid == null)
return;
int splitterIndex = (int)GetValue(Grid.RowProperty);
definition1 = grid.RowDefinitions[splitterIndex - 1];
RowDefinition definition2 = grid.RowDefinitions[splitterIndex + 1];
savedMaxLength = definition1.MaxHeight;
double maxHeight = definition1.ActualHeight + definition2.ActualHeight - definition2.MinHeight;
definition1.MaxHeight = maxHeight;
}
private void OnDragCompleted(DragCompletedEventArgs sender)
{
definition1.MaxHeight = savedMaxLength;
grid = null;
definition1 = null;
}
}
Then just replace GridSplitter with FixedGridSplitter.
Note: this code is not general - it doesn't support columns and assume that bottom row has MinHeight specified.