根据两点计算延伸点

// 从两个点计算出指定延伸长度外的射线点,isNextPoint为YES则为向前点,否则为当前点的向后点
- (CGPoint)pointWithExtendLength:(CGFloat)length lastPoint:(CGPoint)pointA currentPoint:(CGPoint)pointB isNextPoint:(BOOL)isNext {
    CGFloat relativeX = pointB.x - pointA.x;
    CGFloat relativeY = pointB.y - pointA.y;
    CGFloat relativeZ = hypot(relativeX, relativeY);  // 直角三角形求斜边
    CGFloat x = relativeX / relativeZ * length;
    CGFloat y = relativeY / relativeZ * length;
    if (isNext) {
        return CGPointMake(pointB.x + x, pointB.y + y);
    }
    return CGPointMake(pointB.x - x, pointB.y - y);
}