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.