判断某点是否在圆圈内

 - (BOOL) pointInside:(CGPoint)point withEvent:(UIEvent *)event
 {
 CGPoint pt;
 float HALFSIDE = SIDELENGTH / 2.0f;
 // normalize with centered origin
 pt.x = (point.x - HALFSIDE) / HALFSIDE;
 pt.y = (point.y - HALFSIDE) / HALFSIDE;
 // x^2 + y^2 = radius
 float xsquared = pt.x * pt.x;
 float ysquared = pt.y * pt.y;
 // If the radius < 1, the point is within the clipped circle
 if ((xsquared + ysquared) < 1.0) return YES;
 return NO;
 }