Implement velocity in custom UIGestureRecognizer - iphone

Question

I have written a custom UIGestureRecognizer which handles rotations with one finger. It is designed to work exactly like Apples UIRotationGestureRecognizer and return the same values as it does.
Now, I would like to implement the velocity but I cannot figure out how Apple defines and calculates the velocity for the gesture recognizer.
Does anybody have an idea how Apple implements this in the UIRotationGestureRecognizer?

Answer 1

You would have to keep reference of last touch position and it's timestamp.
double last_timestamp;
CGPoint last_position;
Then you could do something like:
-(void) touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event{
last_timestamp = CFAbsoluteTimeGetCurrent();
UITouch *aTouch = [touches anyObject];
last_position = [aTouch locationInView: self];
}
-(void) touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
double current_time = CFAbsoluteTimeGetCurrent();
double elapsed_time = current_time - last_timestamp;
last_timestamp = current_time;
UITouch *aTouch = [touches anyObject];
CGPoint location = [aTouch locationInView:self.superview];
CGFloat dx = location.x - last_position.x;
CGFloat dy = location.y - last_position.y;
CGFloat path_travelled = sqrt(dx*dx+dy*dy);
CGFloat sime_kind_of_velocity = path_travelled/elapsed_time;
NSLog (#"v=%.2f", sime_kind_of_velocity);
last_position = location;
}
This should give you some kind of speed reference.