CanvasView | Android Application Library

using System;
using System.Collections.Generic;

using Xamarin.Forms;

using TouchTracking;

using SkiaSharp;
using SkiaSharp.Views.Forms;

namespace SkiaSharpFormsDemos.Paths
{
    public partial class FingerPaintPage : ContentPage
    {
        Dictionary inProgressPaths = new Dictionary();
        List completedPaths = new List();
        SKBitmap bitmap;

        SKPaint paint = new SKPaint
        {
            Style = SKPaintStyle.Stroke,
            Color = SKColors.Blue,
            StrokeWidth = 10,
            StrokeCap = SKStrokeCap.Round,
            StrokeJoin = SKStrokeJoin.Round
        };

        SKPath testPath = SKPath.ParseSvgPathData("M851.953 371.006L865.957 438.527L874.922 462.995L880.957 453.997L884.941 441.011L893.965 427.874L895.957 418.233L900.938 415.006L902.93 405.02L898.945 403.998");

        public FingerPaintPage()
        {
            InitializeComponent();
            DrawTestLine();
        }

        private void DrawTestLine()
        {
            canvasView.InvalidateSurface();
        }

        void OnTouchEffectAction(object sender, TouchActionEventArgs args)
        {
            switch (args.Type)
            {
                case TouchActionType.Pressed:
                    if (!inProgressPaths.ContainsKey(args.Id))
                    {
                        SKPath path = new SKPath();
                        path.MoveTo(ConvertToPixel(args.Location));
                        inProgressPaths.Add(args.Id, path);
                        canvasView.InvalidateSurface();
                    }
                    break;

                case TouchActionType.Moved:
                    if (inProgressPaths.ContainsKey(args.Id))
                    {
                        SKPath path = inProgressPaths[args.Id];
                        path.LineTo(ConvertToPixel(args.Location));
                        canvasView.InvalidateSurface();
                    }
                    break;

                case TouchActionType.Released:
                    if (inProgressPaths.ContainsKey(args.Id))
                    {
                        completedPaths.Add(inProgressPaths[args.Id]);
                        inProgressPaths.Remove(args.Id);
                        canvasView.InvalidateSurface();
                    }
                    break;

                case TouchActionType.Cancelled:
                    if (inProgressPaths.ContainsKey(args.Id))
                    {
                        inProgressPaths.Remove(args.Id);
                        canvasView.InvalidateSurface();
                    }
                    break;
            }
        }

        void OnCanvasViewPaintSurface(object sender, SKPaintSurfaceEventArgs args)
        {           
            bitmap = new SKBitmap((int)canvasView.CanvasSize.Width, (int)canvasView.CanvasSize.Height);
            var canvas = new SKCanvas(bitmap);
            canvas.Clear();
            canvas.DrawPath(testPath, paint);

            args.Surface.Canvas.DrawBitmap(bitmap, 0, 0);

            canvas.Clear();

            foreach (SKPath path in completedPaths)
            {
                canvas.DrawPath(path, paint);
            }

            foreach (SKPath path in inProgressPaths.Values)
            {
                canvas.DrawPath(path, paint);
            }
        }

        SKPoint ConvertToPixel(Point pt)
        {
            return new SKPoint((float)(canvasView.CanvasSize.Width * pt.X / canvasView.Width),
                               (float)(canvasView.CanvasSize.Height * pt.Y / canvasView.Height));
        }
    }
}