最終的な、サンプルコードです。GraphicsViewコントロール内をクリックすると、円が描かれます。
MainPage.xaml:
<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:MauiTouch2"
             x:Class="MauiTouch2.MainPage">
    <ContentPage.Resources>
        <local:GraphicsDrawable x:Key="drawable" />
    </ContentPage.Resources>
    <ScrollView>
        <VerticalStackLayout
            Spacing="25"
            Padding="30,0"
            VerticalOptions="Center">
            <GraphicsView x:Name="graph" HorizontalOptions="Center" VerticalOptions="Start" HeightRequest="300" WidthRequest="300"
                      StartInteraction="OnStartInteraction" >
                <GraphicsView.Drawable>
                    <local:GraphicsDrawable x:Name="drawable" />
                </GraphicsView.Drawable>
            </GraphicsView>
            <Label
                Text="text"
                x:Name="Text"
                FontSize="10"
                HorizontalOptions="Start"
                VerticalOptions="End"
                />
        </VerticalStackLayout>
    </ScrollView>
</ContentPage>

MainPage.xaml.cs:

 

namespace MauiTouch2;

public partial class MainPage : ContentPage

{   

    public MainPage()

    {

        InitializeComponent();

    }

    private async void OnStartInteraction(object sender, TouchEventArgs evt)

    {

        PointF firstPoint = evt.Touches.FirstOrDefault();

        string msg = $"Touch/click at {firstPoint}";

        Text.Text = msg;

        await this.drawable.OnTouchAsync(this.graph, firstPoint, CancellationToken.None);

    }

}

public class HitItem

{

    public PointF pos { get; set; }

    //public float radius { get; set; }

}



public class GraphicsDrawable : IDrawable

{

    public List<HitItem> HitItems { get; } = new List<HitItem>();

    public async Task OnTouchAsync(IGraphicsView view, PointF point, CancellationToken token)

    {

        HitItem c = new HitItem();

        c.pos = point;

        //c.radius = 0;

        //lock (this.HitItems)

        //{

           this.HitItems.Add(c);

       // }

       // DateTime start = DateTime.Now;

       // DateTime end = start.AddDays(1);

       // while ( !token.IsCancellationRequested) //DateTime.Now <= end &&

       // {

            //c.radius = 20;  // (float)(start - DateTime.Now).TotalSeconds * 100;

            view.Invalidate();

            await Task.Delay(30);

       // }



        //lock (this.HitItems)

        //{

            this.HitItems.Remove(c);

        //}



        //view.Invalidate();

    }



    public void Draw(ICanvas canvas, RectF dirtyRect)

    {

        DrawBack(canvas, dirtyRect);

        DrawHitPoint(canvas, dirtyRect);

    }



    private void DrawHitPoint(ICanvas canvas, RectF dirtyRect)

    {

        if (HitItems.Count == 0)

        {

            return;

        }

        canvas.SaveState();

        canvas.StrokeColor = Colors.Black;

        canvas.StrokeSize = 2;

        canvas.ClipRectangle(dirtyRect);

        HitItem[] items;

        lock (this.HitItems)

        {

            items = this.HitItems.ToArray();

        }

        // foreach (var item in items)

        // {

        canvas.DrawCircle(items[0].pos, 20);

       // }

        canvas.ResetState();

    }

    private void DrawBack(ICanvas canvas, RectF dirtyRect)

    {

        int x = 300;

        canvas.StrokeColor = Colors.Black;

        canvas.StrokeSize = 3;

        canvas.DrawLine(0, 0, x, 0);

        canvas.DrawLine(0, x, x, x);

        canvas.DrawLine(0, 0, 0, x);

        canvas.DrawLine(x, 0, x, x);      

    }

}