iOS 控件阴影

阴影的属性都支持动画显示:    UIButton *button = [UIButton buttonWithType:UIButtonTypeCustom];    button.frame = CGRectMake(100, 100, 100, 30);    button.backgroundColor = [UIColor greenColor];button.layer.shadowColor = [UIColor grayColor].CGColor;    button.layer.shadowOffset = CGSizeMake(6, 6);    //需要设置透明度才显示    button.layer.shadowOpacity = 1;    //扩散半径, 默认为3, 支持动画显示    button.layer.shadowRadius = 2;