CGFloat xDist = (p2.x - p1.x);
CGFloat yDist = (p2.y - p1.y);
CGFloat distance = sqrt((xDist * xDist) + (yDist * yDist));

http://stackoverflow.com/questions/1906511/how-to-find-the-distance-between-two-cg-points

Distance between p1 and p2:

CGFloat xDist = (p2.x - p1.x);
CGFloat yDist = (p2.y - p1.y);
CGFloat distance = sqrt((xDist * xDist) + (yDist * yDist));

Background: Pythagorean theorem

Edit: if you only need to calculate if the distance between the points increases or decreases, you can omit the sqrt() which will make it a little faster.

How to find the distance between two CG points?

Objective C