class clsTarget : ImageView, ImageView.IOnTouchListener
{
    private ScaleGestureDetector mScaleDetector;
    private ScaleListener sListener;
    private static float mScaleFactor = 0.6F;
    private static float scalePointX;
    private static float scalePointY;

    private static int INVALID_POINTER_ID = -1;
    private int mActivePointerId = INVALID_POINTER_ID;
    private static float mPosX, mPosY, mLastTouchX, mLastTouchY;

    public clsTarget(Context context)
        : base(context)
    {
        this.SetOnTouchListener(this);

        sListener = new ScaleListener();
        mScaleDetector = new ScaleGestureDetector(context, sListener);
        mScaleFactor = 800F / (float)Math.Min(Resources.DisplayMetrics.WidthPixels, Resources.DisplayMetrics.HeightPixels);
    }

    protected override void OnDraw(Android.Graphics.Canvas canvas)
    {
        canvas.Save();

        canvas.Scale(mScaleFactor, mScaleFactor, scalePointX, scalePointY);
        canvas.Translate(mPosX, mPosY);

        Paint p = new Paint(PaintFlags.AntiAlias);
        p.Color = Color.Orange;
        p.SetStyle(Paint.Style.Fill);
        p.StrokeWidth = 1F;
        canvas.DrawCircle(400, 400, 200, p);

        canvas.Restore();
    }

    public bool OnTouch(View v, MotionEvent e)
    {
        mScaleDetector.OnTouchEvent(e);

        switch (e.Action & MotionEventActions.Mask)
        {
            case MotionEventActions.Down:
                float x = (e.GetX() - scalePointX) / mScaleFactor;
                float y = (e.GetY() - scalePointY) / mScaleFactor;

                mLastTouchX = x;
                mLastTouchY = y;

                mActivePointerId = e.GetPointerId(0);
                break;

            case MotionEventActions.Move:
                int pointerIndex = e.FindPointerIndex(mActivePointerId);

                float x2 = (e.GetX(pointerIndex) - scalePointX) / mScaleFactor;
                float y2 = (e.GetY(pointerIndex) - scalePointY) / mScaleFactor;

                float dx = (x2 - mLastTouchX);
                float dy = (y2 - mLastTouchY);

                if (!mScaleDetector.IsInProgress)
                {
                    mPosX += dx;
                    mPosY += dy;

                    mLastTouchX = x2;
                    mLastTouchY = y2;
                }

                this.Invalidate();

                break;

            case MotionEventActions.Up:
                mActivePointerId = INVALID_POINTER_ID;
                break;

            case MotionEventActions.Cancel:
                mActivePointerId = INVALID_POINTER_ID;
                break;

            case MotionEventActions.PointerUp:
                int pointerIndex2 = (int)(e.Action & MotionEventActions.PointerIndexMask) >> (int)MotionEventActions.PointerIndexShift;
                int pointerID = e.GetPointerId(pointerIndex2);

                if (pointerID == mActivePointerId)
                {
                    int newPointerIndex = pointerIndex2 == 0 ? 1 : 0;

                    mLastTouchX = (e.GetX(newPointerIndex) - scalePointX) / mScaleFactor;
                    mLastTouchY = (e.GetY(newPointerIndex) - scalePointY) / mScaleFactor;

                    mActivePointerId = e.GetPointerId(newPointerIndex);
                }

                break;
        }

        return true;
    }

    private class ScaleListener : ScaleGestureDetector.SimpleOnScaleGestureListener
    {
        public override bool OnScale(ScaleGestureDetector detector)
        {
            scalePointX = detector.FocusX;
            scalePointY = detector.FocusY;

            mScaleFactor *= detector.ScaleFactor;
            mScaleFactor = Math.Max(0.5f, Math.Min(mScaleFactor, 7.0f));

            return true;
        }
    }
}