我想实现图中的效果，下面一层UIView是黑色半透明的，上面一层的UIImageView是不透明的，该怎么实现？

soeasy, 你只要设置你的半透明的黑色背景的background.

blackView.backgroundColor = [UIColor colorWithRed:0 green:0 blue:0 alpha:0.5];

就OK啦。

    UIView *blackView = [UIView new];
    blackView.backgroundColor = [UIColor colorWithRed:0 green:0 blue:0 alpha:0.5];
    [self.view addSubview:blackView];

    [blackView mas_makeConstraints:^(MASConstraintMaker *make) {
        make.size.equalTo(self.view);
        make.center.equalTo(self.view);
    }];

    UIImageView *topImageView = [[UIImageView alloc]initWithImage:[UIImage imageNamed:@"fenxiang"]];
    [blackView addSubview:topImageView];

    [topImageView mas_makeConstraints:^(MASConstraintMaker *make) {
        make.edges.equalTo(blackView).insets(UIEdgeInsetsMake(200, 50, 100, 50));
    }];