一、scrollView的位置确定

scrollView位置的计算还是通过其约束进行计算的

    UIScrollView *scrollview = [[UIScrollView alloc]init];
    scrollview.delegate = self;
    [self.view addSubview:scrollview];
    [scrollview mas_makeConstraints:^(MASConstraintMaker  *make) {
         make.left.right.equalTo(self.view);
         make.height.equalTo(@300);
    }];

二、scrollView的contentSize确定

    UIView *yellowView = [[UIView alloc]init];
    [scrollview addSubview:yellowView];
    yellowView.backgroundColor = [UIColor yellowColor];
    [yellowView mas_makeConstraints:^(MASConstraintMaker *make) {
        make.width.equalTo(@100);
        make.height.equalTo(@500);
        make.top.left.equalTo(scrollview).offset(50);
             make.bottom.equalTo(scrollview.mas_bottom).offset(100);
    }];

scrollView滚动内容大小由子控件的约束计算获得，对于yellowView的约束一方面决定了自己的frame,另一方面影响着contentsize的宽、高。yellowView的height决定了自身的高度，top+heigheight+bottom共同决定了contentsize的竖直滚动范围。而width+left 不能确定contentsize的水平滚动范围，视为0。因此scrollView的contentsize为CGSizeMake(0, 450)