/** Calculates the distance between point1 and point 2. */
CGFloat distanceBetweenPoints(CGPoint point1, CGPoint point2)
{
    CGFloat dx = point1.x - point2.x;
    CGFloat dy = point1.y - point2.y;
    return sqrt(dx*dx + dy*dy);
}

/** The method is a bit too generic - in our case both lines share the same start point. */
CGFloat angleBetweenLinesInDegrees(CGPoint beginLineA,
                                   CGPoint endLineA,
                                   CGPoint beginLineB,
                                   CGPoint endLineB)
{
    CGFloat a = endLineA.x - beginLineA.x;
    CGFloat b = endLineA.y - beginLineA.y;
    CGFloat c = endLineB.x - beginLineB.x;
    CGFloat d = endLineB.y - beginLineB.y;
 
    CGFloat atanA = atan2(a, b);
    CGFloat atanB = atan2(c, d);
 
    // convert radiants to degrees
    return (atanA - atanB) * 180 / M_PI;
}

- (void)reset;
- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event;
- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event;
- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event;
- (void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event;

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    [super touchesMoved:touches withEvent:event];
 
    if (self.state == UIGestureRecognizerStateFailed) return;
 
    CGPoint nowPoint  = [[touches anyObject] locationInView: self.view];
    CGPoint prevPoint = [[touches anyObject] previousLocationInView: self.view];
 
    // make sure the new point is within the area
    CGFloat distance = distanceBetweenPoints(midPoint, nowPoint);
    if (   innerRadius < = distance
        && distance    <= outerRadius)
    {
        // calculate rotation angle between two points
        CGFloat angle = angleBetweenLinesInDegrees(midPoint, prevPoint, midPoint, nowPoint);
 
        // fix value, if the 12 o'clock position is between prevPoint and nowPoint
        if (angle > 180)
        {
            angle -= 360;
        }
        else if (angle < -180)
        {
            angle += 360;
        }
 
        // sum up single steps
        cumulatedAngle += angle;
 
        // call delegate
        if ([target respondsToSelector: @selector(rotation:)])
        {
            [target rotation:angle];
        }
    }
    else
    {
        // finger moved outside the area
        self.state = UIGestureRecognizerStateFailed;
    }
}

@protocol OneFingerRotationGestureRecognizerDelegate <nsobject>
@optional
/** A rotation gesture is in progress, the frist argument is the rotation-angle in degrees. */
- (void) rotation: (CGFloat) angle;
/** The gesture is finished, the first argument is the total rotation-angle. */
- (void) finalAngle: (CGFloat) angle;
@end
</nsobject>

// calculate center and radius of the control
CGPoint midPoint = CGPointMake(image.frame.origin.x + image.frame.size.width / 2,
                               image.frame.origin.y + image.frame.size.height / 2);
CGFloat outRadius = image.frame.size.width / 2;
 
// outRadius / 3 is arbitrary, just choose something >> 0 to avoid strange 
// effects when touching the control near of it's center
gestureRecognizer = [[OneFingerRotationGestureRecognizer alloc] initWithMidPoint: midPoint
                                                            innerRadius: outRadius / 3 
                                                            outerRadius: outRadius
                                                                 target: self];
[someView addGestureRecognizer: gestureRecognizer];

- (void) rotation: (CGFloat) angle
{
    // calculate rotation angle
    imageAngle += angle;
    if (imageAngle > 360)
        imageAngle -= 360;
    else if (imageAngle < -360)
        imageAngle += 360;
 
    // rotate image and update text field
    image.transform = CGAffineTransformMakeRotation(imageAngle *  M_PI / 180);
    textDisplay.text = [NSString stringWithFormat: @"\u03b1 = %.2f", imageAngle];
}