Having trouble getting an Invalidate event on a Maui User Control

Charles Brauer 61 Reputation points
2022-10-26T19:33:58.447+00:00

Hey Guys,

I’m trying to test Maui Graphics in a Windows UserControl that I put on a Windows Form. My App looks like:

254449-mauitest.png

The problem I’m having is that when I click on the “Draw Lines” button, nothing happens. In other words, “skglControl1.Invalidate();” does not cause “skglControl1_PaintSurface” to get an Invalidate event.

Why not?

I put all my reproducible code on GitHub at:

CBrauer/Maui-Graphics-in-a-WIndows-UserControl (github.com)

Here is the code for the UserControl:

using System.Diagnostics;  
  
using Microsoft.Maui.Graphics;  
using Microsoft.Maui.Graphics.Skia;  
using SkiaSharp.Views.Desktop;  
  
namespace Maui_graphics_in_a_Windows_User_Control_on_a_Windows_form;  
  
public partial class MauiGraphicsUserControl:UserControl {  
  public static class Graphics {  
    public static void RandomLines(ICanvas canvas, Random rand, float width, float height, int lines = 100) {  
      canvas.FillColor = Colors.Navy;  
      canvas.FillRectangle(0, 0, width, height);  
  
      canvas.StrokeColor = Colors.White.WithAlpha(.1f);  
      canvas.StrokeSize = 2;  
      for (int i = 0; i < lines; i++) {  
        float x1 = (float) rand.NextDouble() * width;  
        float x2 = (float) rand.NextDouble() * width;  
        float y1 = (float) rand.NextDouble() * height;  
        float y2 = (float) rand.NextDouble() * height;  
        canvas.DrawLine(x1, y1, x2, y2);  
      }  
    }  
  }  
  
  readonly Random Rand = new();  
  readonly int LineCount = 1000;  
  private static int canvasWidth, canvasHeight;  
  
  private static ICanvas? canvas { get; set; }  
  private bool clearScreen { get; set; } = true;  
  public MauiGraphicsUserControl() {  
    InitializeComponent();  
  }  
  public void Clear() {  
    Debug.WriteLine("at Clear");  
    clearScreen = true;  
    skglControl1.Invalidate();  
    skglControl1.Update();  
  }  
  public void DrawLines() {  
    Debug.WriteLine("at DrawLines");  
    clearScreen = false;  
    skglControl1.Invalidate();  
    skglControl1.Update();  
  }  
  private void skglControl1_PaintSurface(object sender, SKPaintGLSurfaceEventArgs e) {  
    Debug.WriteLine("entered skglControl1_PaintSurface");  
    canvas = new SkiaCanvas() {  
      Canvas = e.Surface.Canvas  
    };  
    canvasWidth  = skglControl1.Width;  
    canvasHeight = skglControl1.Height;  
    if (clearScreen) {  
      Debug.WriteLine("doing Clear");  
      Debug.Assert(canvas is not null, "Oh crap, canvas is null");  
      canvas.FillColor = Colors.White;  
      canvas.FillRectangle(0, 0, canvasWidth, canvasHeight);  
      clearScreen = false;  
      return;  
    }  
    Debug.WriteLine("doing DrawLines");  
    Debug.Assert(canvas is not null, "Oh crap, canvas is null");  
    Graphics.RandomLines(canvas, Rand, canvasWidth, canvasHeight, LineCount);  
  }  
  private void skglControl1_SizeChanged(object sender, EventArgs e) {  
    skglControl1.Invalidate();  
  }  
}  
  
Windows Forms
Windows Forms
A set of .NET Framework managed libraries for developing graphical user interfaces.
1,863 questions
.NET MAUI
.NET MAUI
A Microsoft open-source framework for building native device applications spanning mobile, tablet, and desktop.
3,053 questions
{count} votes

Accepted answer
  1. Rob Caplan - MSFT 5,427 Reputation points Microsoft Employee
    2022-10-26T22:42:30.167+00:00

    Cut MAUI out of the picture and host SkiaSharp directly in your Windows desktop app. SkiaSharp supports WinForms and WPF directly, so MAUI won't add anything to this scenario other than complications. See https://github.com/mono/SkiaSharp and https://www.nuget.org/packages/SkiaSharp.Views.WindowsForms/

    There are other, platform specific, libraries that you could want to look at as well. I believe Win2D will work in your WinForms app via WPF interop.

    Hosting MAUI in a WinForms app won't work. MAUI on Windows is built on top of WinUI 3, and WinUI 3 is incompatible with WinForms. They use dramatically different windowing paradigms, and hosting a WinUI 3 control in a WinForm would require a complicated translation layer. The XAML islands feature provides that translation layer for the older, UWP version of WinUI, but that version isn't compatible with .Net 6 and MAUI.

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