1 
 Expanded(
            flex: 10,
            child: Container(
                child: CupertinoPicker(
              itemExtent: 50,
              onSelectedItemChanged: (int i) {},
              scrollController: FixedExtentScrollController(
                initialItem: isIndex,
              ),
              useMagnifier: true,
              children: appwidget,
            ))),

我有这个代码，children 是每个更改的列表小部件。当我更改列表小部件的“appwidget”时，我可以设置初始项索引吗？

我不能调用 FixedExtentScrollController。我不知道。

4

1 回答 1

2

首先，您需要创建一个FixedExtentScrollController，它允许您方便地使用项目索引，而不是使用标准要求的原始像素滚动偏移量ScrollController（来源来自Flutter 文档）：

FixedExtentScrollController? _scrollWheelController;
final String? value;
final String values = ['male','female','other'];
@override
  void initState() {
    super.initState();
    _scrollWheelController = FixedExtentScrollController(
      /// Jump to the item index of the selected value in CupertinoPicker
      initialItem: value == null ? 0 : values.indexOf(value!),
    );
  }

然后将其连接到，CupertinoPicker以便您可以通过编程方式控制滚动视图：

CupertinoPicker.builder(scrollController: _scrollWheelController);

ModalBottomSheet如果您想在弹出时立即跳转到所选值的项目索引，以下代码将帮助您实现此目的：

showModalBottomSheet<String>(
  context: context,
  builder: (_) {
    /// If the build() method to render the
    /// [ListWheelScrollView] is complete, jump to the
    /// item index of the selected value in the controlled
    /// scroll view
    WidgetsBinding.instance!.addPostFrameCallback(
      /// [ScrollController] now refers to a
      /// [ListWheelScrollView] that is already mounted on the screen
      (_) => _scrollWheelController?.jumpToItem(
        value == null ? 0 : values.indexOf(value!),
      ),
    );

    return SizedBox(
      height: 200,
      child: Column(
        crossAxisAlignment: CrossAxisAlignment.end,
        children: [
          Padding(
            padding: const EdgeInsets.symmetric(horizontal: 16),
            child: TextButton(
              onPressed: () => Navigator.pop(context),
              child: Text('Done'),
            ),
          ),
          const Divider(thickness: 1),
          Expanded(
            child: CupertinoPicker.builder(
              /// if [itemExtent] is too low, the content for each
              /// item will be squished together
              itemExtent: 32,
              scrollController: _scrollWheelController,
              onSelectedItemChanged: (index) => setState(() => values[index]),
              childCount: values.length,
              itemBuilder: (_, index) => Center(
                child: Text(
                  valueAsString(values[index]),
                  style: TextStyle(
                    color: Theme.of(context).accentColor,
                  ),
                ),
              ),
            ),
          ),
        ],
      ),
    );
  },
);

WidgetsBinding.instance!.addPostFrameCallback()将确保build()当前帧中要渲染的所有小部件的所有方法都是完整的。如果_scrollWheelController指的ListWheelScrollView是尚未完全构建的，jumpToItem()则将不起作用。

阅读此线程以获取有关如何在 Widget build complete 上运行方法的更多信息

于 2021-08-11T11:39:44.247 回答