SkiaSharp Xamarin Coach Marks Overlay

CoachMarks.cs

using System.Collections.Generic;
using System.Drawing;
using SkiaSharp;

#if __IOS__
using SkiaSharp.Views.iOS;
using UIKit;
#elif __ANDROID__
using SkiaSharp.Views.Android;
#endif

namespace some
{
    public class CoachMarks : ICoachMarks
    {
        private List<CoachMark> Marks { get; } = new List<CoachMark>();
        
        SKPaint paint = new SKPaint {
            IsAntialias = true,
            Color = 0xFF0000FF,
            Style = SKPaintStyle.Fill,
        };

        SKPaint bgPaint = new SKPaint {
            IsAntialias = false,
            Color = 0xAA000000,
            Style = SKPaintStyle.Fill,
        };

        private SKPaint textPaint = new SKPaint
        {
            IsAntialias = true,
            Color = 0xFF00FF00,
            TextSize = 40,
            FakeBoldText = true
            //Typeface = 
        };

        private SKCanvasView skiaCanvasView = null;

        public ICoachMarks Add(RectangleF rect, string text, CoachMarkPosition textPosition = null)
        {
            this.Marks.Add(new CoachMark(rect, text, textPosition));
            ReDraw();
            return this;
        }

        public ICoachMarks Create()
        {
            Marks.Clear();
#if __IOS__
            if (skiaCanvasView == null)
            {
                skiaCanvasView = new SKCanvasView(UIApplication.SharedApplication.KeyWindow.Bounds);
                skiaCanvasView.BackgroundColor = UIColor.Clear;
                skiaCanvasView.Opaque = false;
                UIApplication.SharedApplication.KeyWindow.AddSubview(skiaCanvasView);
                skiaCanvasView.FillParent();
                skiaCanvasView.UserInteractionEnabled = true;
                skiaCanvasView.AddGestureRecognizer (new UITapGestureRecognizer(skiaCanvasView.RemoveFromSuperview));
                skiaCanvasView.PaintSurface += (sender, e) =>
                {
                    var canvas = e.Surface.Canvas;
                    var size = e.Info.Size;
    
                    Draw(canvas, size, (float)skiaCanvasView.ContentScaleFactor);
                };
            }
#elif __ANDROID__
            //TODO
#endif
            ReDraw();
            return this;
        }

        private void ReDraw()
        {
#if __IOS__
            skiaCanvasView.SetNeedsDisplay();
#elif __ANDROID__
            skiaCanvasView.Invalidate();
#endif
        }

        private void Draw(SKCanvas canvas, SKSize size, float scale = 1)
        {
            canvas.Clear();

            // clipping
            foreach (var mark in Marks)
            {
                var pos = mark.Pos.Scale(scale);
                canvas.ClipRect(new SKRect(pos.Left, pos.Top, pos.Right, pos.Bottom), SKClipOperation.Difference);
            }

            // clipping reset
            canvas.ResetMatrix();
            
            // background 
            canvas.DrawRect(0, 0, size.Width, size.Height, bgPaint);

            // mark texts
            foreach (var mark in this.Marks)
            {
                var pos = mark.Pos.Scale(scale);
                if (mark.TextPosition.V == -1)
                    canvas.DrawText(mark.Text, pos.X, pos.Y, textPaint);
                else if (mark.TextPosition.V == 0)
                    canvas.DrawText(mark.Text, pos.X, pos.Y + pos.Height, textPaint);
                else if (mark.TextPosition.V == +1)
                {
                    SKFontMetrics metrics;
                    textPaint.GetFontMetrics(out metrics, scale);
                    canvas.DrawText(mark.Text, pos.X, pos.Y + pos.Height + metrics.XHeight, textPaint);
                }
            }
        }

        private class CoachMark
        {
            public RectangleF Pos { get; }
            public string Text { get; }
            public CoachMarkPosition TextPosition { get; }

            public CoachMark(RectangleF pos, string text, CoachMarkPosition textPosition = null)
            {
                Pos = pos;
                Text = text;
                TextPosition = textPosition ?? CoachMarkPosition.Below;
            }
        }
    }
}

_coachMarks
            .Create()
            .Add(passButton, "press meeeee!", CoachMarkPosition.Above)
            .Add(forgotPasswordButton, "u forgot bro?")
            ;