p1 code> p2 ：

Distance between p1 and p2:

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

背景：勾股定​​理

编辑：如果您只需要计算点之间的距离是增加还是减少， sqrt（），这将使它更快一点。

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.