UIPageViewController/TextKit 在分页上回流文本

我正在开发一个由 TextKit 支持的多页阅读应用程序，该应用程序基于 WWDC 2013 的“Advanced Text Layouts and Effects with Text Kit”会议（但有些代码是根据不完整的示例重建的）。基本结构是预先计算文本所需的页面数，然后为每个页面创建一个 NSTextContainer 并将其添加到 NSLayoutManager 中。每当 UIPageViewController 请求下一页或上一页时，您都会创建一个新的 UITextView 并通过从 NLayoutManger 的 NSTextContainers 数组中选择正确的一个来设置其后台文本容器。

不幸的是，我遇到了一个问题，即文本在第一页上以及第一次翻回任何给定页面时都会重新排列。它看起来是这样的：

这不是最明显的效果（如果你错过了，请在回页时注意屏幕顶部），但它有点让人迷失方向，如果可能的话我想消除它。鉴于文本容器应该预先计算，我不明白为什么它要回流文本，或者如何防止它。有谁知道问题是什么？

EDIT：添加代码示例。

@interface ReaderViewController () <UIPageViewControllerDataSource>

@property (nonatomic, assign) NSUInteger numberOfPages;

@property (nonatomic, retain) UIPageViewController *pageViewController;
@property (nonatomic, retain) NSTextStorage *currentDocument;
@property (nonatomic, retain) NSLayoutManager *layoutManager;

@end

@implementation ReaderViewController

- (instancetype)initWithDocument:(NSTextStorage *)document {
  if ((self = [super init])) {
    _currentDocument = document;
  }
  return self;
}

- (void)viewDidLoad
{
  [super viewDidLoad];

  _layoutManager = [[NSLayoutManager alloc] init];
  [self.layoutManager setTextStorage:self.currentDocument];
  self.layoutManager.delegate = self;

  _pageViewController = [[UIPageViewController alloc] initWithTransitionStyle:UIPageViewControllerTransitionStyleScroll
                                                        navigationOrientation:UIPageViewControllerNavigationOrientationHorizontal
                                                                      options:nil];
  self.pageViewController.view.autoresizingMask = UIViewAutoresizingFlexibleHeight | UIViewAutoresizingFlexibleWidth;
  self.pageViewController.dataSource = self;
  [self addChildViewController:self.pageViewController];
  [self.view addSubview:self.pageViewController.view];

  // the numberOfPages accessor lazily calculates the number of pages needed to contain the document
  for (int i = 0; i < self.numberOfPages; ++i) {
    NSTextContainer *container = [[NSTextContainer alloc] init];
    container.size = [self _textFrame].size;
    [self.layoutManager addTextContainer:container];
  }

  [self.pageViewController setViewControllers:@[[self viewControllerForPageNumber:0]] direction:UIPageViewControllerNavigationDirectionForward animated:NO completion:nil];
}

- (UIViewController *)viewControllerForPageNumber:(NSUInteger)pageNumber {
  if (pageNumber >= self.numberOfPages) {
    return nil;
  }

  // SinglePageViewController is a lightweight view controller that has a UITextView and a page number
  SinglePageViewController *vc = [[SinglePageViewController alloc] init];

  UITextView *textView = [[UITextView alloc] initWithFrame:[self _textFrame] textContainer:[self.layoutManager.textContainers objectAtIndex:pageNumber]];
  textView.autoresizingMask = UIViewAutoresizingFlexibleHeight | UIViewAutoresizingFlexibleWidth;
  textView.scrollEnabled = NO;
  textView.editable = NO;
  [textView setFont:[UIFont fontWithName:@"IowanOldStyle-Roman" size:20.f]];
  [vc.view addSubview:textView];

  vc.textView = textView;
  vc.pageNumber = pageNumber;

  return vc;
}

#pragma mark - UIPageViewControllerDataSource

- (UIViewController *)pageViewController:(UIPageViewController *)pageViewController viewControllerAfterViewController:(UIViewController *)viewController {
  NSUInteger currentPage = [(SinglePageViewController *)viewController pageNumber];
  if (currentPage >= self.numberOfPages) {
    return nil;
  }

  return [self viewControllerForPageNumber:currentPage + 1];
}

- (UIViewController *)pageViewController:(UIPageViewController *)pageViewController viewControllerBeforeViewController:(UIViewController *)viewController {
  NSUInteger currentPage = [(SinglePageViewController *)viewController pageNumber];
  if (currentPage == 0) {
    return nil;
  }

  return [self viewControllerForPageNumber:currentPage - 1];
}

#pragma mark - Private

- (CGRect)_textFrame {
  return CGRectInset(self.view.bounds, 5., 0.);
}

@end

我遇到了同样的问题。我的解决方案是把scrollEnabled after addSubview将迫使textContainer重新计算尺寸。见下面的代码：

vc.view.addSubview(textView)
textView.scrollEnabled = false;

Update。最后，我想我找到了正确的答案......我们需要重置容器大小didChangeGeometryFromSize打回来。如果不正确请纠正：）

func layoutManager(layoutManager: NSLayoutManager, 
                   textContainer: NSTextContainer, 
                   didChangeGeometryFromSize oldSize: CGSize) {

    textContainer.size = self.textFrame().size;
    println(textContainer.size.width)
    println(textContainer.size.height)
}