renderPage method
Implementation
@override
Future<ui.Picture?> renderPage(int pageIndex, {double scale = 1, Rect? clip}) {
final String key = UDocPictureCache.key(documentId, pageIndex, 1);
final ui.Picture? cached = UDocPictureCache.instance.get(key);
if (cached != null) return Future<ui.Picture?>.value(cached);
final Future<ui.Picture?>? pending = _pending[pageIndex];
if (pending != null) return pending;
final Future<ui.Picture?> request = _renderInternal(pageIndex, key);
_pending[pageIndex] = request;
return request;
}