使用自动化布局获取空间的frame

UIView *footerview = [[UIView alloc]initWithFrame:CGRectMake(0, 0, self.view.bounds.size.width, 1)];
    footerview.backgroundColor = [UIColor blackColor];
    
    UIView *redView = [UIView new];
    redView.backgroundColor = [UIColor redColor];
    [footerview addSubview:redView];
    [redView mas_makeConstraints:^(MASConstraintMaker *make) {
        make.right.offset(-300);
        make.top.left.offset(0);
        make.height.equalTo(@70);
    }];
    
    UIView *blueView = [UIView new];
    blueView.backgroundColor = [UIColor blueColor];
    [footerview addSubview:blueView];
    [blueView mas_makeConstraints:^(MASConstraintMaker *make) {
        make.right.offset(-300);
        make.top.equalTo(redView.mas_bottom).offset(0);
        make.left.offset(0);
        make.height.equalTo(@300);
//        make.bottom.offset(0);
    }];
    
    //一定要在获取frame之前调用layoutIfNeed
    [footerview layoutIfNeeded];
    footerview.frame = CGRectMake(0, 0, self.view.bounds.size.width, CGRectGetMaxY(blueView.frame));
    NSLog(@"%d",CGRectGetMaxY(blueView.frame));