按照Sketch设置阴影

参考：https://juejin.im/post/5dd4cd71f265da0bf80b5820

- (void)sketchSetShadow:(UIColor *)color alpha:(CGFloat)alpha x:(CGFloat)x y:(CGFloat)y blur:(CGFloat)blur spread:(CGFloat)spread
{
    self.shadowOffset = CGSizeMake(x, y);
    self.shadowRadius = blur * 0.5;
    self.shadowColor = color.CGColor;
    self.shadowOpacity = alpha;
    
    CGRect rect = CGRectInset(self.bounds, -spread, -spread);
    
    UIBezierPath *path = [UIBezierPath bezierPathWithRoundedRect:rect cornerRadius:self.cornerRadius];
    
    self.shadowPath = path.CGPath;
}