How to rotate image around center point automatically with finger touch

Question

On iPhone, how to implement rotating image around the center point using finger touch ?

Just like wheel, if you put finger on the iPhone screen , then move suddenly,

Answer 1

I was working with a "spin the bottle"-app yesterday. On the window I have a ImageView with an bottle that's suppose to response to touches and rotate the way the user swipes his finger. I struggled to get my ImageView to rotate during the touch-events (TouchesBegan, Touchesoved, TouchesEnd). I used this code in TouchesMoved to find out the angle in witch to rotate the image.

public override void TouchesMoved (NSSet touches, UIEvent evt)
{    
  PointF pt = (touches.AnyObject as UITouch).LocationInView(this);

  float x = pt.X  - this.Center.X;
  float y = pt.Y  - this.Center.Y;
  double ang =  Math.Atan2(x,y);

  // yada yada, rotate image using this.Transform
}

THIS IS IMPORTANT! When the ImageView rotates, even the x & y-coordinates changes. So touching the same area all the time would give me different values in the pt and prePt-points. After some thinking, googeling and reading I came up with an simple solution to the problem. The "SuperView"-property of the ImageView.

PointF pt = (touches.AnyObject as UITouch).LocationInView(this.SuperView);

Having that small change in place made it alot easier, no i can use the UITouch-metohs LocationInView and PreviousLocationInView and get the right x & y coordinates. Her is parts of my code.

float deltaAngle;

public override void TouchesMoved (NSSet touches, UIEvent evt)
{
    PointF pt = (touches.AnyObject as UITouch).LocationInView(this.Superview);

    float x = pt.X  - this.Center.X;
    float y = pt.Y  - this.Center.Y;
    float ang =  float.Parse(Math.Atan2(dx,dy).ToString());


    //do the rotation
     if (deltaAngle == 0.0) {
       deltaAngle = ang;
     }
else
    {
      float angleDif = deltaAngle - ang;
      this.Transform = CGAffineTransform.MakeRotation(angleDif);
    }   
}

Hope that helped someone from spending hours on how to figure out how to freaking rotate a bottle! :)