In AvaloniaUI, how to render without clearing

Question

I am using a RenderTargetBitmap. I have a piece of code like this

using (var context = WorldImage.CreateDrawingContext()) {
    Pen pen = new Pen(Brushes.Black);
    context.DrawLine(pen, previousPoint, currentPoint);
}

The purpose of this code is to draw a curve by pairs of points. However, it doesn't work because it clears the image before each draw. How to disable this clearing?

Answer 1

How to disable this clearing?

You can't. You must instead keep all the points that should be drawn, e.g. in a collection of Points.

It is unclear what exactly you are trying to achieve. The following sample draws a polyline that follows the mouse cursor as long as the left mouse button is pressed.

With an Image element in XAML

<Grid>
    <Border BorderThickness="1" BorderBrush="Black"
            HorizontalAlignment="Center" VerticalAlignment="Center">
        <Image x:Name="image" Stretch="None"/>
    </Border>
</Grid>

the code below would draw the polyline. Note that in Avalonia there seems to be no automatic UI update when a RenderTargetBitmap is redrawn, hence the InvalidateVisual() on the Image element.

private readonly RenderTargetBitmap bitmap = new(new PixelSize(400, 400));
private readonly List<Point> polyline = [];

public MainView()
{
    InitializeComponent();

    image.Source = bitmap;
    image.PointerPressed += OnPointerPressed;
    image.PointerMoved += OnPointerMoved;
}

private void OnPointerPressed(object sender, PointerPressedEventArgs e)
{
    polyline.Clear();
    polyline.Add(e.GetPosition(image));
}

private void OnPointerMoved(object sender, PointerEventArgs e)
{
    var p = e.GetCurrentPoint(image);

    if (p.Properties.IsLeftButtonPressed)
    {
        polyline.Add(p.Position);

        using var context = bitmap.CreateDrawingContext();

        Pen pen = new(Brushes.Black);

        for (int i = 1; i < polyline.Count; i++)
        {
            context.DrawLine(pen, polyline[i - 1], polyline[i]);
        }

        image.InvalidateVisual();
    }
}