為一個UIview 添加陰影：

UIView *shadow = [[UIView alloc] initWithFrame:CGRectMake(30, 30, 100, 100)];

shadow.backgroundColor = [UIColor whiteColor];

shadow.layer.shadowColor = [UIColor blackColor].CGColor;//shadowColor陰影顏色

shadow.layer.shadowOffset = CGSizeMake(4,0);//shadowOffset陰影偏移,x向右偏移4，y向下偏移4，默認(0, -3),這個跟shadowRadius配合使用

shadow.layer.shadowOpacity = 0.8;//陰影透明度，默認0

shadow.layer.shadowRadius = 4;//陰影半徑，默認3