Question

I am working on an app that has a rotating image (the user tapps and drags and the image rotates in a circle tracking their finger). What I am trying to keep track of is how many times the user makes a complete circle. An additional “hitch” is that I also need to know if the user is circling clockwise vs counter clockwise.

Here is the Code that is rotating the image… Please feel free to request additional information.

   - (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event{

    UITouch *touch = [[event allTouches] anyObject];
    CGPoint touchPoint = [touch locationInView:self.view];

    long double rotationNumber = atan2(touchPoint.y - originY, touchPoint.x - originX);

    totalRotationCount ++;

    schedulingWheel.transform = CGAffineTransformMakeRotation(rotationNumber);

    offset = (rotationNumber * 100)/14;

    dateRibbon.center = CGPointMake(offset, 24);  
}

Thanks for the help!

Answer 1

My solution isn’t elegant and there might be a cleaner solution I’m missing but this is what I did recently. The trick is to keep track of the angle from the last time touchesMoved: is called. Then, add the delta of your current angle and the stored angel value to your total.

The problem is the “boundaries” that atan2 creates needed ugly code to overcome. Say your lastAngle is 359 and you cross the origin so your next angle is 1. The difference will not be 2 but -358, so whenever you cross that boundary your total will be reset to 0.

Here is what I did:

 CGFloat angle = atan2f(center.y - point.y, point.x - center.x);

 //Translate to Unit circle
 if (angle > 0) {
     angle = (M_PI - angle) + M_PI;
 } else {
     angle = fabsf(angle);
 }

 CGFloat delta = angle - lastAngle;

 //Adjust for boundaries
 if (fabsf(delta) > (2*M_PI)-fabsf(delta)) {
      BOOL greaterThanZero = (delta > 0);
      delta = (2*M_PI)-fabsf(delta);
      if (greaterThanZero) {
           delta = -1 * delta;
      }
  }

  totalAngle += delta;
  lastAngle = angle;

The big/ugly conditional under “Adjust for boundaries” basically just looks to see if there is a shorter angle to get to the new point (So, 2 instead of -258) & assumes that if there is it probably means you crossed that origin and adjusts the delta accordingly.

I translated the Atan2 results so that it represents a full unit circle from 0 to 2π. Bonus side affect, it then accounts for clockwise/counter clockwise movement better than the standard -π to π of Atan2.