WPF - Border with a OpacityMask/VisualBrush: Memory Leaks

A brief explanation about my app:

the application in which I'm working on is such a greeting cards designer. Imagine something in which there is a background image, and an indefinite number of "layers" (in particular, pictures) that stay over the background and can be moved, resized, moved front and back, etc...

It is also possibile to apply particular shapes to these layers, like a star, an ellipse, .. and after the card is made, it's possibile to save is to jpeg file.

The problem

Everything works correctly, but I detected that when a shape is applied to a layer, a memory leak is generated.

Here is the code of the UserControl of each layer:

<UserControl>
.....
    <Grid x:Name="_myGrid"  >
        <Border x:Name="im_the_problem" BorderThickness="0" OpacityMask="{Binding Path=MyMask.Data, Converter={StaticResource MaskConverter}}">
        <!-- My Image... -->
        </Border>
    </Grid>
</UserControl>

where MaskConverter code is the following:

public class MaskConverter : IValueConverter
{

    public object Convert(object value, Type targetType, object parameter,
       System.Globalization.CultureInfo culture)
    {
        String maskData = value as String;
        if (maskData == null) 
            return null;
        if (maskData == "")
            return null;
        VisualBrush vb = new VisualBrush();
        vb.Visual = XamlReader.Parse(maskData) as Visual;
        return vb;
    }

    public object ConvertBack(object value, Type targetType, object parameter,
        System.Globalization.CultureInfo culture)
    {
        throw new NotSupportedException();
    }
}

The Parameter "MyMask.Data" is a XAML Path (that is the shape that I'm applying) that I dinamically load from a textfile that contains different shapes.

So, the principle is that if I have the bord开发者_运维技巧er named *im_the_problem*, the memory is NOT released. If I comment *im_the_problem* (so I'll just have rectangular layers/pictures without shapes) everything work like a charm, without memory leaks.

The problem should be in the OpacityMask + VisualBrush.

Am I doing something wrong? Or is there a known problem? Is there a way to do the same (apply a shape to a picture..) in a different manner?

Thanks.

You might be able to try binding the MyMask.Data to an actual Path.Data, and setting the Path.Fill to an ImageBrush created from the image?

You need to freeze your VisualBrush ;)

I had this problem in a DataGrid's column template where I was using a <Canvas><Path /></Canvas> (as a static-resource) into a VisualBrush (also a static-resource) and using that as the OpacityMask for a Rectangle. Whenever the DataGrid was reloaded the Rectangle wouldn't release VisualBrush references to the OpacityMask, I used a memory-profiler tool to reveal that all the VisualBrush objects were using the bulk of memory.

I don't understand why or how this happened - but I'm glad I'm not alone (even if I had the same problem some 6.5 years later...).

My XAML was something like this:

<DataGrid.Resources>

    <Canvas x:Key="icon" ...>
        <Path ... />
    </Canvas>

    <VisualBrush x:Key="iconBrush" Stretch="Uniform" Visual="{StaticResource icon}" />

</DataGrid.Resources>

<DataGrid.Columns>

    <DataGridTemplateColumn>
        <DataGridTemplateColumn.CellTemplate>
            <DataTemplate>
                <Rectangle
                    Fill="{Binding Foreground, ElementName=myDataGrid}"
                    Width="14"
                    Height="14"
                    Margin="4"
                    Visibility="{Binding IconVisibility}"
                    OpacityMask="{StaticResource iconBrush}"
                />
            </DataTemplate>
        </DataGridTemplateColumn.CellTemplate>
    </DataGridTemplateColumn>

    ...

</DataGrid.Columns>

I read that setting IsFrozen = true (done using this technique: https://www.codeproject.com/Tips/72221/Freeze-brushes-directly-in-the-XAML-to-improve-you ) would help memory issues with Brushes, however this seemingly had no effect at all. Weird.

I thought I'd experiment and I reasoned that if the issue was leaking the VisualBrush then I wondered if having it as a StaticResource was messing with object-references, so I changed it to an "owned" object, like so:

    <DataGridTemplateColumn>
        <DataGridTemplateColumn.CellTemplate>
            <DataTemplate>
                <Rectangle
                    Fill="{Binding Foreground, ElementName=myDataGrid}"
                    Width="14"
                    Height="14"
                    Margin="4"
                    Visibility="{Binding IconVisibility}"
                >
                    <VisualBrush Stretch="Uniform" Visual="{StaticResource iconBrush}" />
                </Rectangle>
            </DataTemplate>
        </DataGridTemplateColumn.CellTemplate>
    </DataGridTemplateColumn>

This fixed the issue! And I still don't know why - I wonder if it's a bug in WPF?

On a related note, I came to realise that using a VisualBrush was overkill as I'm rendering just a simple Path - VisualBrush is expensive because it renders an entire WPF view - I also learned from other documentation that Path itself isn't necessary for rendering simple shapes because itself is a complete UIElement and FrameworkElement - which are "heavier" types.

I changed my code to store the path in a PathGeometry value inside a GeometryDrawing static-resource which is loaded into a DrawingBrush:

<GeometryDrawing x:Key="iconDrawing" Brush="Black" Geometry="..." /> 

<Rectangle
    Fill="{Binding Foreground, ElementName=myDataGrid}"
    Width="14"
    Height="14"
    Margin="4"
    Visibility="{Binding IconVisibility}"
    OpacityMask="{StaticResource iconBrush}"
>
    <DrawingBrush Stretch="Uniform" Drawing="{StaticResource iconDrawing}" />
</Rectangle>

Doing this also made a dent in memory usage, and hopefully, performance.

In your project I see you're not using the path information as a resource, but the same technique applies: load your path into a PathGeometry (or rather, StreamGeometry object, which is even faster and is meant for immutable geometry) and set that as the Drawing for a DrawingBrush.