GridSplitter with min constraints

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 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.

Answer 2

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>