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.