Implementing touch-based rotation in cocoa touch

Question

I am wondering what is the best way to implement rotation-based dragging movements in my iPhone application.

I have a UIView that I wish to rotate around its centre, when the users finger is touch the view and they move it. Think of it like a dial that needs to be adjusted with the finger.

The basic question comes down to:

1) Should I remember the initial angle and transform when touchesBegan is called, and then every time touchesMoved is called apply a new transform to the view based on the current position of the finger, e.g., something like:

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

  UITouch *touch = [touches anyObject];
  CGPoint currentPoint = [touch locationInView:self]; //current position of touch   

 if (([touch view] == self) 
    &&  [Utility getDistance:currentPoint toPoint:self.middle] <= ROTATE_RADIUS //middle is centre of view
    && [Utility getDistance:currentPoint toPoint:self.middle] >= MOVE_RADIUS) { //will be rotation gesture

  //remember state of view at beginning of touch
  CGPoint top = CGPointMake(self.middle.x, 0);
  self.initialTouch = currentPoint;
  self.initialAngle = angleBetweenLines(self.middle, top, self.middle, currentPoint);   
  self.initialTransform = self.transform;
 }
}

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

  UITouch *touch = [touches anyObject];
  CGPoint currentPoint = [touch locationInView:self]; //current position of touch

  if (([touch view] == self) 
  &&  [Utility getDistance:currentPoint toPoint:self.middle] <= ROTATE_RADIUS
  && [Utility getDistance:currentPoint toPoint:self.middle] >= MOVE_RADIUS) { //a rotation gesture

  //rotate tile
  float newAngle = angleBetweenLines(self.middle, CGPointMake(self.middle.x, 0), self.middle, currentPoint); //touch angle
  float angleDif = newAngle - self.initialAngle; //work out dif between angle at beginning of touch and now.
  CGAffineTransform newTransform = CGAffineTransformRotate(self.initialTransform, angleDif); //create new transform
  self.transform = newTransform;  //apply transform.
}

OR

2) Should I simply remember the last known position/angle, and rotate the view based on the difference in angle between that and now, e.g.,:

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

      UITouch *touch = [touches anyObject];
      CGPoint currentPoint = [touch locationInView:self]; //current position of touch   

     if (([touch view] == self) 
        &&  [Utility getDistance:currentPoint toPoint:self.middle] <= ROTATE_RADIUS
        && [Utility getDistance:currentPoint toPoint:self.middle] >= MOVE_RADIUS) { //will be rotation gesture

      //remember state of view at beginning of touch
      CGPoint top = CGPointMake(self.middle.x, 0);
      self.lastTouch = currentPoint;
      self.lastAngle = angleBetweenLines(self.middle, top, self.middle, currentPoint);  
     }
    }

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

      UITouch *touch = [touches anyObject];
      CGPoint currentPoint = [touch locationInView:self]; //current position of touch

      if (([touch view] == self) 
      &&  [Utility getDistance:currentPoint toPoint:middle] <= ROTATE_RADIUS
      && [Utility getDistance:currentPoint toPoint:middle] >= MOVE_RADIUS) { //a rotation gesture

      //rotate tile
      float newAngle = angleBetweenLines(self.middle, CGPointMake(self.middle.x, 0), self.middle, currentPoint); //touch angle
      float angleDif = newAngle - self.lastAngle; //work out dif between angle at beginning of touch and now.
      CGAffineTransform newTransform = CGAffineTransformRotate(self.transform, angleDif); //create new transform

      self.transform = newTransform;  //apply transform.
      self.lastTouch = currentPoint;
      self.lastAngle = newAngle;
    }

The second option makes more sense to me, but it is not giving very pleasing results (jaggy updates and non-smooth rotations). Which way is best (if any), in terms of performance?

Cheers!

Answer 1

Have you considered using UIRotationGestureRecognizer? Seems like that has the logic already baked in, and might make things simpler.

Answer 2

It is actually much simpler than what you have tried.

You need three data points:

• The origin of your view.
• The location of the current touch
• The location of the previous touch

The Touch object passed to you actually contains the last touch location. So you don't need to keep track of it.

All you have to do is calculate the angle between two lines:

• Origin to Current Touch
• Origin to Previous Touch

Then convert that to radians and use that in your CGAffineTransformRotate(). Do that all in your touchesMoved handler.

Here is a function to calculate what you need just that:

static inline CGFloat angleBetweenLinesInRadians(CGPoint line1Start, CGPoint line1End, CGPoint line2Start, CGPoint line2End) {

    CGFloat a = line1End.x - line1Start.x;
    CGFloat b = line1End.y - line1Start.y;
    CGFloat c = line2End.x - line2Start.x;
    CGFloat d = line2End.y - line2Start.y;

    CGFloat line1Slope = (line1End.y - line1Start.y) / (line1End.x - line1Start.x);
    CGFloat line2Slope = (line2End.y - line2Start.y) / (line2End.x - line2Start.x);

    CGFloat degs = acosf(((a*c) + (b*d)) / ((sqrt(a*a + b*b)) * (sqrt(c*c + d*d))));


    return (line2Slope > line1Slope) ? degs : -degs;    
}

Courtesy of Jeff LaMarche at:

http://iphonedevelopment.blogspot.com/2009/12/better-two-finger-rotate-gesture.html

Example:

UITouch *touch = [touches anyObject];
CGPoint origin = [view center];
CGFloat angle = angleBetweenLinesInRadians(origin, [touch previousLocationInView:self.superview.superview], origin, [touch locationInView:self.superview.superview]);