iOS开发中，根据三个点获得之间的夹角

+ (CGFloat)getAnglesWithThreePoint:(CGPoint)pointA pointB:(CGPoint)pointB pointC:(CGPoint)pointC {

CGFloat x1 = pointA.x - pointB.x;
CGFloat y1 = pointA.y - pointB.y;
CGFloat x2 = pointC.x - pointB.x;
CGFloat y2 = pointC.y - pointB.y;

CGFloat x = x1 * x2 + y1 * y2;
CGFloat y = x1 * y2 - x2 * y1;

CGFloat angle = acos(x/sqrt(x*x+y*y));

return angle;
}

if (pointC.x < pointB.x) {
angle = M_PI*2 - angle;
}