PaintView, For Hand Drawing on a Drawable Image (Xamarin)

PaintView.cs

using System;
using Android.App;
using Android.Content;
using Android.Views;
using Android.Widget;
using Android.Graphics;
using Android.Util;

namespace ArborTrac.Droid.UI.Controls
{
    public class PaintView : ImageView
    {
        private Paint _Paint = new Paint();
        private const int THICKNESS = 10;
        private PointF _StartPt = new PointF();
        private PointF _EndPt = new PointF();
        private Bitmap _Bmp = null;
        private Canvas _Canvas = null;

        public PaintView(Context context, IAttributeSet attrs)
            : base(context, attrs)
        {
            this.Initialize();
        }

        void Initialize()
        {
            _Paint.Color = new Color(255, 0, 0);
            _Paint.StrokeWidth = THICKNESS;
            _Paint.StrokeCap = Paint.Cap.Round;

            _Bmp = BitmapFactory.DecodeResource(Resources, Resource.Drawable.tree_trunk);
            _Bmp = _Bmp.Copy(Bitmap.Config.Argb8888, true);
        }

        override protected void OnMeasure(int wSpec, int hSpec)
        {
            base.OnMeasure(wSpec, hSpec);
            int w = MeasureSpec.GetSize(wSpec);
            int h = MeasureSpec.GetSize(hSpec);

            _Bmp = Bitmap.CreateScaledBitmap(_Bmp, w, h, false);
            _Canvas = new Canvas(_Bmp);
            this.SetImageBitmap(_Bmp);
        }

        override public bool OnTouchEvent(MotionEvent e)
        { 
            switch (e.Action)
            {
                case MotionEventActions.Down: 
                    _StartPt.Set(e.GetX() - 1, e.GetY() - 1);// for just a tapping
                    DrawLine(e);
                    break;
                case MotionEventActions.Move: 
                    DrawLine(e);
                    break;
                case MotionEventActions.Up: 
                    DrawLine(e);
                    break;
            }
            return true;
        }

        private void DrawLine(MotionEvent e)
        {
            _EndPt.Set(e.GetX(), e.GetY());
            _Canvas.DrawLine(_StartPt.X, _StartPt.Y, _EndPt.X, _EndPt.Y, _Paint);
            _StartPt.Set(_EndPt);
            Invalidate();
        }

    }
}

i need to develop the xamarin android code please help me if anyone has done similar