UIView *view =[[UIView alloc] initWithFrame:CGRectMake(0,0,20,20)];
view.layer.shadowColor = [UIColor blackColor].CGColor;//阴影的颜色
view.layer.shadowOffset = CGSizeMake(5,5);//阴影偏移,默认为(0, -3)
view.layer.shadowRadius = 4;//阴影半径,默认为3
view.layer.shadowOpacity = 0.5;//阴影透明度,默认为0