- (void)viewDidLoad
{
    [super viewDidLoad];
    
    CALayer *layer        = [CALayer layer];
    layer.frame           = CGRectMake(20, 20, 50, 50);
    layer.backgroundColor = [UIColor whiteColor].CGColor;
    layer.borderWidth     = 1.f;

    /* --- shadow相关属性都可以做动画 --- */
    
    layer.shadowOpacity   = 0.5f;                       // 透明度(默认值为0)
    layer.shadowOffset    = CGSizeMake(5, 5);           // 阴影偏移量
    layer.shadowRadius    = 0.f;                        // 阴影圆角
    layer.shadowColor     = [UIColor redColor].CGColor; // 阴影颜色
    
    /*
    layer.shadowPath      = path;                       // 阴影形状
     */
    
    [self.view.layer addSublayer:layer];
}