chapter method
Implementation
Future<UEpubChapter> chapter(int index) async {
final UEpubChapter? cached = _chapters[index];
if (cached != null) return cached;
final UEpubBook? book = _book;
if (book == null) return UEpubChapter(spineIndex: index, href: "", blocks: const <UEpubBlock>[], title: "", text: "");
final UEpubChapter chapter = await book.chapter(index);
if (_chapters.length > 12) _chapters.clear();
_chapters[index] = chapter;
unawaited(_index?.writePage(index, chapter.text) ?? Future<void>.value());
return chapter;
}