Flutter获取ListView当前正在显示的Widget信息(应用场景)

BuildContext? _sliverListViewContext;
...
ListView _buildListView() {
  return ListView.separated(
    itemBuilder: (ctx, index) {
      // 在 builder 回调中，将 BuildContext 记录起来
      if (_sliverListViewContext != ctx) {
        _sliverListViewContext = ctx;
      }
      return _buildListItemView(index);
    },
    separatorBuilder: (ctx, index) {
      return _buildSeparatorView();
    },
    itemCount: 50,
  );
}

ListViewObserver(
  child: _buildListView(),
  sliverListContexts: () {
    return [if (_sliverListViewContext != null) _sliverListViewContext!];
  },
  onObserve: (resultMap) {
    final model = resultMap[_sliverListViewContext];
    if (model == null) return;

    // 打印当前正在显示的第一个子部件
    print('firstChild.index -- ${model.firstChild.index}');

    // 打印当前正在显示的所有子部件下标
    print('displaying -- ${model.displayingChildIndexList}');
  },
)

// 导航栏半透明
flutter: firstChild.index -- 0
flutter: displaying -- [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]
flutter: firstChild.index -- 0
flutter: displaying -- [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
// 导航栏完全不透明
flutter: firstChild.index -- 2
flutter: displaying -- [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]

ListViewObserver(
  ...
  dynamicLeadingOffset: () {
    if (_navBgAlpha < 1) {
      return 0;
    }
    return _safeAreaPaddingTop + _navContentHeight;
  },
  ...
),

ListViewOnceObserveNotification().dispatch(_sliverListViewContext);

class ListViewObserveModel {
  /// 第一个子部件模型数据
  final ListViewObserveDisplayingChildModel firstChild;

  /// 正在显示的所有子部件模型数据
  final List<ListViewObserveDisplayingChildModel> displayingChildModelList;

  /// 正在显示的所有子部件下标
  List<int> get displayingChildIndexList =>
      displayingChildModelList.map((e) => e.index).toList();
}

class ListViewObserveDisplayingChildModel {
  /// 子部件下标
  final int index;

  /// 子部件的 RenderObject
  final RenderBox renderObject;
}