How can I capture which direction is being panned using UIPanGestureRecognizer?

Question

Ok so I have been looking around at just about every option under the sun for capturing multi-touch gestures, and I have finally come full circle and am back at the UIPanGesture

Answer 1

On UIPanGestureRecognizer you can use -velocityInView: to get the velocity of the fingers at the time that gesture was recognised.

If you wanted to do one thing on a pan right and one thing on a pan left, for example, you could do something like:

- (void)handleGesture:(UIPanGestureRecognizer *)gestureRecognizer
{
    CGPoint velocity = [gestureRecognizer velocityInView:yourView];

    if(velocity.x > 0)
    {
        NSLog(@"gesture went right");
    }
    else
    {
        NSLog(@"gesture went left");
    }
}

If you literally want to detect a reversal, as in you want to compare a new velocity to an old one and see if it is just in the opposite direction — whichever direction that may be — you could do:

// assuming lastGestureVelocity is a class variable...

- (void)handleGesture:(UIPanGestureRecognizer *)gestureRecognizer
{
    CGPoint velocity = [gestureRecognizer velocityInView:yourView];

    if(velocity.x*lastGestureVelocity.x + velocity.y*lastGestureVelocity.y > 0)
    {
        NSLog(@"gesture went in the same direction");
    }
    else
    {
        NSLog(@"gesture went in the opposite direction");
    }

    lastGestureVelocity = velocity;
}

The multiply and add thing may look a little odd. It's actually a dot product, but rest assured it'll be a positive number if the gestures are in the same direction, going down to 0 if they're exactly at right angles and then becoming a negative number if they're in the opposite direction.