UINavigationController + UIScrollView组合，视图尺寸的设置探秘（二）

承接上文，我想把view布局修改为如下模式，让ScrollView长在NavigationBar的下方，这总不会有遮挡的问题了吧：

story board内容如下，主要是右侧视图蓝色区域添加了ScrollView：

ViewController的代码如下：

@interface ViewController ()
@property (weak, nonatomic) IBOutlet UIScrollView *scrollView;
@property (nonatomic, strong) UIView* viewA;
@property (nonatomic, strong) UIView* viewB;
@end
……
@implementation ViewController
……
- (void)viewDidLoad {
    [super viewDidLoad];
        
    CGRect rect = self.scrollView.bounds;
    rect.size.width *= 2;
    self.scrollView.contentSize = rect.size;
    self.scrollView.pagingEnabled = YES;
    // 在self.scrollView内添加两个view
    CGRect rtViewA = self.scrollView.bounds;
    self.viewA = [[UIView alloc]initWithFrame:rtViewA];
    self.viewA.backgroundColor = [UIColor redColor];
    [self.scrollView addSubview:self.viewA];
    
    CGRect rtViewB = self.scrollView.bounds;
    rtViewB.origin.x += rtViewA.size.width;
    self.viewB = [[UIView alloc]initWithFrame:rtViewB];
    self.viewB.backgroundColor = [UIColor blueColor];
    [self.scrollView addSubview:self.viewB];
}

得到的结果更加诡异了，我把ScrollView的背景色设为黄色，为什么我给viewA、viewB的origin.y的初始值应该为0，可是进入scrollView之后却靠下一条？而且滚动条的问题还是没有解决：

当我把viewA向上拖动到origin.y=0的位置后，还能在往上拖，可是viewA的高度已经和contentSize一样高了，这到底是怎么回事？

查看Debug View Hierarchy，scroll View确实长在navigation bar的下面了：

打印出scrollview 、其内部view以及他们的上层view信息，如下。scrollview 和其子view宽高分别为414、672，scrollview的contentsize宽高分别为828、672，看数据也都是没问题的。

<UIScrollView: 0x7f81b9021400; frame = (0 64; 414 672); clipsToBounds = YES; autoresize = RM+BM; gestureRecognizers = <NSArray: 0x7f81b8c9e680>; layer = <CALayer: 0x7f81b8c67f50>; contentOffset: {0, -64}; contentSize: {828, 672}>
Printing description of $4:
<UIView: 0x7f81b8c7e030; frame = (0 0; 414 672); layer = <CALayer: 0x7f81b8ca1be0>>
Printing description of $5:
<UIView: 0x7f81b8ca0ee0; frame = (0 0; 414 736); autoresize = W+H; layer = <CALayer: 0x7f81b8c9ef10>>