スマホでVR写真表示アプリを作る(Xamarin) - OpenGLView OnTouchEvent

OGLView.cs

#if __ANDROID__
  public override bool OnTouchEvent(MotionEvent e)
  {
    base.OnTouchEvent(e);

    if (e.Action == MotionEventActions.Down)
    {
      prevx = e.GetX();
      prevy = e.GetY();
    }

    if (e.Action == MotionEventActions.Move)
    {
      float e_x = e.GetX();
      float e_y = e.GetY();

      float xdiff = (prevx - e_x);
      float ydiff = (prevy - e_y);

      _viewAngleX += 90.0f * xdiff / (float)Size.Width;
      _viewAngleY -= 90.0f * ydiff / (float)Size.Height;

      while (_viewAngleX >= 360) _viewAngleX -= 360;
      while (_viewAngleX < 0) _viewAngleX += 360;

      _viewAngleY = Math.Max(-80, _viewAngleY);
      _viewAngleY = Math.Min(80, _viewAngleY);

      prevx = e_x;
      prevy = e_y;

      SetupProjection();
      RenderSphere();
    }

    return true;
  }
#endif

#if __IOS__
  public override void TouchesBegan(NSSet touches, UIEvent e)
  {
    var touch = (UITouch)e.TouchesForView(this).AnyObject;
    CGPoint pt = touch.LocationInView(this);
    prevx = (float)pt.X;
    prevy = (float)pt.Y;
  }

  public override void TouchesMoved(NSSet touches, UIEvent e)
  {
    var touch = (UITouch)e.TouchesForView(this).AnyObject;

    CGPoint pt = touch.LocationInView(this);
    float e_x = (float)pt.X;
    float e_y = (float)pt.Y;

    float xdiff = (prevx - e_x);
    float ydiff = (prevy - e_y);

    _viewAngleX += 90.0f * xdiff / (float)Size.Width;
    _viewAngleY -= 90.0f * ydiff / (float)Size.Height;

    while (_viewAngleX >= 360) _viewAngleX -= 360;
    while (_viewAngleX < 0) _viewAngleX += 360;

    _viewAngleY = Math.Max(-80, _viewAngleY);
    _viewAngleY = Math.Min(80, _viewAngleY);

    prevx = e_x;
    prevy = e_y;

    SetupProjection();
    RenderSphere();
  }
#endif