如何将Singleton应用于课程？

Question

在Flutter中，我试图使下面的类A singleton。

class ChatMessagesListView extends StatefulWidget {
  /// Creates a chat list widget
  ChatMessagesListView(
      {Key? key,
      this.isLastPage,
      required this.itemBuilder,
      required this.items,
      this.onEndReached,
      this.onEndReachedThreshold,
      required this.tempMessageIndicators})
      : super(key: key);

  /// Used for pagination (infinite scroll) together with [onEndReached].
  /// When true, indicates that there are no more pages to load and
  /// pagination will not be triggered.
  final bool? isLastPage;

  final List<TempMessageIndicator> tempMessageIndicators;

  /// Items to build
  final List<Object> items;

  /// Item builder
  final Widget Function(Object, int? index) itemBuilder;

  /// Used for pagination (infinite scroll). Called when user scrolls
  /// to the very end of the list (minus [onEndReachedThreshold]).
  final Future<void> Function()? onEndReached;

  /// Used for pagination (infinite scroll) together with [onEndReached].
  /// Can be anything from 0 to 1, where 0 is immediate load of the next page
  /// as soon as scroll starts, and 1 is load of the next page only if scrolled
  /// to the very end of the list. Default value is 0.75, e.g. start loading
  /// next page when scrolled through about 3/4 of the available content.
  final double? onEndReachedThreshold;

  /// Scroll controller to control the scrolling
  final ScrollController scrollController = ScrollController();
  final ScrollController scrollController2 = ScrollController();

  @override
  _ChatMessagesListViewState createState() => _ChatMessagesListViewState();
}

这是我尝试的方法

class ChatMessagesListView extends StatefulWidget {
  static ChatMessagesListView? _instance;

  factory ChatMessagesListView(
          {Key? key,
    isLastPage,
    required itemBuilder,
    required items,
    onEndReached,
    onEndReachedThreshold,
    required tempMessageIndicators,}) =>
      _instance ??=
          ChatMessagesListView._(key, isLastPage, itemBuilder, items, onEndReached, onEndReachedThreshold, tempMessageIndicators);

  /// Used for pagination (infinite scroll) together with [onEndReached].
  /// When true, indicates that there are no more pages to load and
  /// pagination will not be triggered.
  late final bool? isLastPage;

  late final List<TempMessageIndicator> tempMessageIndicators;

  /// Items to build
  late final List<Object> items;

  /// Item builder
  late final Widget Function(Object, int? index) itemBuilder;

  /// Used for pagination (infinite scroll). Called when user scrolls
  /// to the very end of the list (minus [onEndReachedThreshold]).
  late final Future<void> Function()? onEndReached;

  /// Used for pagination (infinite scroll) together with [onEndReached].
  /// Can be anything from 0 to 1, where 0 is immediate load of the next page
  /// as soon as scroll starts, and 1 is load of the next page only if scrolled
  /// to the very end of the list. Default value is 0.75, e.g. start loading
  /// next page when scrolled through about 3/4 of the available content.
  late final double? onEndReachedThreshold;

  /// Scroll controller to control the scrolling
  final ScrollController scrollController = ScrollController();
  final ScrollController scrollController2 = ScrollController();

  @override
  _ChatMessagesListViewState createState() => _ChatMessagesListViewState();
}

，但是它给我带来了一个错误，该方法'_'并未定义为'chatmessageslistview'.try将名称纠正到现有方法的名称，或定义名为<<的方法< /代码>。因此，基本上，我无法执行以下行

_instance ??=
              ChatMessagesListView._(key, isLastPage, itemBuilder, items, onEndReached, onEndReachedThreshold, tempMessageIndicators);

加，如果您查看我的第一组代码，您会看到我将构造函数数据分配给变量。我相信这也不会在这里发生。

如何使此类A Singleton？

Answer 1

当代码和消息显示您缺少构造函数时。
代码必须如下：

class ChatMessagesListView extends StatefulWidget {
  ChatMessagesListView._(Key? key,
      isLastPage,
      required itemBuilder,
      required items,
      onEndReached,
      onEndReachedThreshold,
      required tempMessageIndicators);

  static ChatMessagesListView? _instance;

  factory ChatMessagesListView(
      {Key? key,
        isLastPage,
        required itemBuilder,
        required items,
        onEndReached,
        onEndReachedThreshold,
        required tempMessageIndicators,}) =>
      _instance ??=
          ChatMessagesListView._(key, isLastPage, itemBuilder, items, onEndReached, onEndReachedThreshold, tempMessageIndicators);
//..............
//.......
}