消息内部有折叠、展开的时候消息位置固定，当展开的内容很长的时候 standby 位置固定无效

[wksmile]

大佬你好，我按照
Originally posted by @wksmile in #103 的方法确实能实现展开位置固定，但是有一个问题，就是当展开的内容很长超过一屏的时候展开收起位置就不能固定了。

如图，展开很长位置固定失效。

写个一个 demo 问题复现代码如下：
scroll_list_page.dart 文件

import './expand_item.dart';

class MessageItem {
  final String content;
  final List<String> children;

  const MessageItem({required this.content, required this.children});
}

@RoutePage()
class ScrollListPage extends StatefulWidget {
  const ScrollListPage({super.key});

  @override
  State<ScrollListPage> createState() => _scrollListPage();
}

class _scrollListPage extends State<ScrollListPage> with WidgetsBindingObserver {
  final ScrollController scrollController = ScrollController();
  late ListObserverController observerController;
  late ChatScrollObserver chatObserver;

  List<MessageItem> messages = [
    MessageItem(
        content:
            '''监测**   - **尽可能上传最近的数据**，例如体温、心率血压、呼吸频率等，这有助于更精确地分析头痛原因。   - 记录头痛的发生时间、频率和持续时间，以及是否伴随其他症状。#### **4. 改善睡眠**   - 保持规律睡眠时间，晚上尽量避免在屏幕前时间过长。   - 可在睡前喝些温牛奶，或通过泡脚及放松呼吸训练，帮助身体进入更深的睡眠状态。---### **需要特别关注的情况**如果头痛伴有以下症状，请及时反馈或联系医生：- 持续加剧的头痛，并伴有明显的视力模糊、呕吐或晕厥。- 任何异常变化，比如剧烈恶心、无法言语或肢体无力。请根据建议逐步调节，如果头痛持续存在或频繁加重，及时向我反馈最新的症状与监测数据！''',
        children: [
          '''onize the data.- restingHeartRates:Data is not available for 2025-01-06 - 2025-01-13. Here is the last valid data on 2024-11-18 17:25:00: 30.0 BPMYou can kindly remind the user to synchronize the data.Reference: Apple Health[Retrieve respiratory health metrics to assess any breathing-related issues linked to headaches.]:- respiratoryRates:Data is not available for 2025-01-06 - 2025-01-13. Here is the last valid data on 2024-11-18 17:20:00: 12.0 BPMYou can kindly remind the user to synchronize the data.- oxygenSaturations:Data is not available for 2025-01-06 - 2025-01-13. Here is the last valid data on 2024-11-18 17:24:00: 1.0 %You can kindly remind the user to synchronize the data.Reference: Apple Health[Query lab results for indicators related to metabolic or systemic conditions that might cause headaches.]:No related information was found. Please verify if the relevant data has been uploaded or synchronized''',
        ]),
    MessageItem(
        content:
            '''这条消息第一个展开内容很多长展开有问题，位置不能固定''',
        children: [
          '''Identify the cause of the user's headache and provide possible solutions or recommendations.</summary>[Query vital signs data to check for any abnormalities that could be associated with headaches.]:- bodyTemperatures:Data is not available for 2025-01-06 - 2025-01-13. Here is the last valid data on 2024-11-18 17:23:00: 37.78 °CYou can kindly remind the user to synchronize the data.- restingHeartRates:Data is not available for 2025-01-06 - 2025-01-13. Here is the last valid data on 2024-11-18 17:25:00: 30.0 BPMYou can kindly remind the user to synchronize the data.- bloodGlucoses:Data is not available for 2025-01-06 - 2025-01-13. Here is the last valid data on 2024-11-18 17:23:00: 100.0 mg/dLYou can kindly remind the user to synchronize the data.- systolicPressures:Data is not available for 2025-01-06 - 2025-01-13. Here is the last valid data on 2024-11-18 17:24:00: 100.0 mmHgYou can kindly remind the user to synchronize the data.- diastolicPressures:Data is not available for 2025-01-06 - 2025-01-13. Here is the last valid data on 2024-11-18 17:24:00: 60.0 mmHgYou can kindly remind the user to synchronize the data.- oxygenSaturations:Data is not available for 2025-01-06 - 2025-01-13. Here is the last valid data on 2024-11-18 17:24:00: 1.0 %You can kindly remind the user to synchronize the data.- hrvDatas:Data is not available for 2025-01-06 - 2025-01-13. Here is the last valid data on 2024-11-18 17:25:00: 30.0 msYou can kindly remind the user to synchronize the data.- heartRates:Data is not available for 2025-01-06 - 2025-01-13. Here is the last valid data on 2024-11-18 20:22:00: 101.0 BPMYou can kindly remind the user to synchronize the data.- respiratoryRates:Data is not available for 2025-01-06 - 2025-01-13. Here is the last valid data on 2024-11-18 17:20:00: 12.0 BPMYou can kindly remind the user to synchronize the data.Reference: Apple Health[Retrieve heart-related metrics to identify any cardiovascular factors contributing to headaches.]:- hrvDatas:Data is not available for 2025-01-06 - 2025-01-13. Here is the last valid data on 2024-11-18 17:25:00: 30.0 msYou can kindly remind the user to synchronize the data.- heartRates:Data is not available for 2025-01-06 - 2025-01-13. Here is the last valid data on 2024-11-18 20:22:00: 101.0 BPMYou can kindly remind the user to synchronize the data.- restingHeartRates:Data is not available for 2025-01-06 - 2025-01-13. Here is the last valid data on 2024-11-18 17:25:00: 30.0 BPMYou can kindly remind the user to synchronize the data.Reference: Apple Health[Retrieve respiratory health metrics to assess any breathing-related issues linked to headaches.]:- respiratoryRates:Data is not available for 2025-01-06 - 2025-01-13. Here is the last valid data on 2024-11-18 17:20:00: 12.0 BPMYou can kindly remind the user to synchronize the data.- oxygenSaturations:Data is not available for 2025-01-06 - 2025-01-13. Here is the last valid data on 2024-11-18 17:24:00: 1.0 %You can kindly remind the user to synchronize the data.Reference: Apple Health[Query lab results for indicators related to metabolic or systemic conditions that might cause headaches.]:No related information was found. Please verify if the relevant data has been uploaded or synchronized''',
          '''respiratory health metrics to assess any breathing-related issues linked to headaches.]:- respiratoryRates:Data is not available for 2025-01-06 - 2025-01-13. Here is the last valid data on 2024-11-18 17:20:00: 12.0 BPMYou can kindly remind the user to synchronize the data.- oxygenSaturations:Data is not available for 2025-01-06 - 2025-01-13. Here is the last valid data on 2024-11-18 17:24:00: 1.0 %You can kindly remind the user to synchronize the data.Reference: Apple Health[Query lab results for indicators related to metabolic or systemic conditions that might cause headaches.]:No related information was found. Please verify if the relevant data has been uploaded or synchronized''',
          '''data.Reference: Apple Health[Query lab results for indicators related to metabolic or systemic conditions that might cause headaches.]:No related information was found. Please verify if the relevant data has been uploaded or synchronized''',
        ]),
    MessageItem(content: '''步数数据！''', children: [
      '''A relevant data has been uploaded or synchronized''',
    ]),
    MessageItem(content: '''步数数据！''', children: [
      '''A relevant data has been uploaded or synchronized''',
    ]),
    MessageItem(content: '''步数数据！''', children: [
      '''A relevant data has been uploaded or synchronized''',
    ]),
    MessageItem(content: '''步数数据！''', children: [
      '''A relevant data has been uploaded or synchronized''',
    ]),
    MessageItem(content: '''步数数据！''', children: [
      '''A relevant data has been uploaded or synchronized''',
    ]),
    MessageItem(content: '''步数数据！''', children: [
      '''A relevant data has been uploaded or synchronized''',
    ]),
    MessageItem(content: '''步数数据！''', children: [
      '''A relevant data has been uploaded or synchronized''',
    ]),
    MessageItem(content: '''步数数据！''', children: [
      '''A relevant data has been uploaded or synchronized''',
    ]),
    MessageItem(content: '''步数数据！''', children: [
      '''A relevant data has been uploaded or synchronized''',
    ]),
    MessageItem(content: '''步数数据！''', children: [
      '''A relevant data has been uploaded or synchronized''',
    ]),
    MessageItem(content: '''步数数据！''', children: [
      '''A relevant data has been uploaded or synchronized''',
    ]),
    MessageItem(content: '''步数数据！''', children: [
      '''A relevant data has been uploaded or synchronized''',
    ]),
  ];

  @override
  void initState() {
    super.initState();
    initChatObserver();
  }

  @override
  void dispose() {
    observerController.controller?.dispose();
    WidgetsBinding.instance.removeObserver(this);
    super.dispose();
  }

  @override
  void didChangeMetrics() {
    super.didChangeMetrics();

    chatObserver.observeSwitchShrinkWrap();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        backgroundColor: Colors.white,
        resizeToAvoidBottomInset: true,
        body: Align(
          alignment: Alignment.topCenter,
          child: ListViewObserver(
            controller: observerController,
            child: ListView.builder(
              physics: ChatObserverClampingScrollPhysics(observer: chatObserver),
              itemCount: messages.length,
              reverse: true,
              controller: scrollController, // 设置 ScrollController
              shrinkWrap: chatObserver.isShrinkWrap,
              padding: const EdgeInsets.only(left: 16, right: 16, top: 20, bottom: 10),
              itemBuilder: (context, index) {
                return _itemBuilder(
                  context,
                  index,
                );
              },
            ),
          ),
        ));
  }

  Widget _itemBuilder(BuildContext context, int index) {
    MessageItem item = messages[index];
    return Container(
        margin: EdgeInsets.only(bottom: 10),
        width: 150,
        color: Colors.grey[200],
        child: Column(crossAxisAlignment: CrossAxisAlignment.start, children: [
          ...item.children.map((e) => ExpandBox(childContent: e, callback: expandCb, index: index)),
          Text(item.content),
        ]));
  }

  void initChatObserver() {
    observerController = ListObserverController(
      controller: scrollController,
    )..cacheJumpIndexOffset = false;

    chatObserver = ChatScrollObserver(observerController)
      ..fixedPositionOffset = 5
      ..toRebuildScrollViewCallback = () {
        setState(() {});
      };
  }

  void expandCb(int index) {
    // 对比上一个 item 来定位
    final refItemIndex = index + 1;
    chatObserver.standby(
      mode: ChatScrollObserverHandleMode.specified,
      refIndexType: ChatScrollObserverRefIndexType.itemIndex,
      refItemIndex: refItemIndex,
      refItemIndexAfterUpdate: refItemIndex,
    );
  }
}

expand_item.dart 文件

class ExpandBox extends StatefulWidget {
  final String childContent;
  final Function? callback;
  final int index;

  const ExpandBox({
    super.key,
    required this.childContent,
    this.callback,
    required this.index,
  });

  @override
  State<ExpandBox> createState() => _ExpandBoxState();
}

class _ExpandBoxState extends State<ExpandBox> {
  bool isExpanded = false;

  @override
  Widget build(
    BuildContext context,
  ) {
    return Column(
      crossAxisAlignment: CrossAxisAlignment.start,
      children: [
        ElevatedButton(
            onPressed: () {
              setState(() {
                isExpanded = !isExpanded;
              });
              widget.callback!(widget.index);
            },
            child: isExpanded ? Text('收起') : Text('展开')),
        if (isExpanded) Text(widget.childContent),
        if (isExpanded) Text('Expanded content'),
      ],
    );
  }
}

[LinXunFeng]

问题的原因与 #111 (comment) 一致，请适度控制消息的长度或者 cacheExtent 的值，使被参照的 item 保持渲染，以确保该功能正常使用。

另外一些优化：

• 最后的一条消息的【展开/收起】也支持保持位置
• 处理中间内容太长的 item 【收起】时列表抖动

请升级至 1.25.0 版本，并结合如下代码调整

void expandCb(int index) {
  final isLastItem = index == messages.length - 1;
  // 默认是对比上一个 item 来定位
  // 最后一个 item，没有上一个可以对比，则对比自身
  // 此处对比自身无意义，因为由 customAdjustPosition 去完全控制保持位置的计算，忽略即可。
  final refItemIndex = isLastItem ? index : index + 1;
  chatObserver.standby(
    mode: ChatScrollObserverHandleMode.specified,
    refIndexType: ChatScrollObserverRefIndexType.itemIndex,
    refItemIndex: refItemIndex,
    refItemIndexAfterUpdate: refItemIndex,
    customAdjustPosition: (model) {
      // 仅处理最后一个 item 的情况
      if (!isLastItem) return null;
      // 使用 变化前后的底部间距差 来保持位置
      final delta =
          model.newPosition.extentAfter - model.oldPosition.extentAfter;
      return model.adjustPosition + delta;
    },
    customAdjustPositionDelta: (model) {
      // 仅处理不是最后一个 item 的情况
      if (isLastItem) return null;
      // 以 变化前后的 item 偏移差 来保持位置
      final adjustPosition = model.adjustPosition;
      final delta = model.currentItemModel.layoutOffset -
          model.observer.refItemLayoutOffset;
      if (delta < 0) {
        // 收起
        // 因消息的高度太大，在收起时，Flutter 内部对列表的偏移计算有问题
        // 所以这里调整计算方式，改为：视口的当前偏移量 - 内容变化量
        // 注：减去 adjustPosition，是因为保持位置功能的内部会加上 adjustPosition
        return model.currentItemModel.viewportPixels + delta - adjustPosition;
      }
      // 展开
      return delta;
    },
  );
}

[wksmile]

感谢！按照以上方式解决了我的问题。
但是customAdjustPosition 、 customAdjustPositionDelta 这两个属性的区别没太弄明白，希望能帮解释一下。

[LinXunFeng]

customAdjustPosition

• 被调用的前提： 当前列表视图满足需要保持位置的条件
• 作用：用于完全自定义列表视图新的偏移量，一般在没有参照 item 的情况下使用

flutter_scrollview_observer/lib/src/utils/src/chat/chat_observer_scroll_physics_mixin.dart

Lines 41 to 59 in 4f7b653

	// Customize the adjustPosition.
	double? customAdjustPosition = observer.customAdjustPosition?.call(
	ChatScrollObserverCustomAdjustPositionModel(
	oldPosition: oldPosition,
	newPosition: newPosition,
	isScrolling: isScrolling,
	velocity: velocity,
	adjustPosition: adjustPosition,
	observer: observer,
	),
	);
	if (customAdjustPosition != null) {
	_handlePositionCallback(ChatScrollObserverHandlePositionResultModel(
	type: ChatScrollObserverHandlePositionType.keepPosition,
	mode: observer.innerMode,
	changeCount: observer.changeCount,
	));
	return customAdjustPosition;
	}

如你的示例代码中的最后一个 item，当其高度发生变化时无可参照 item，但是视口底部距离 (extentAfter ) 是变化的，且知道差值是多少，这个时候就得使用使用 customAdjustPosition 回调来自定义列表视图新的偏移量，以达到保持位置的效果。

customAdjustPositionDelta

• 被调用的前提：
  • 当前列表视图满足需要保持位置的条件
  • 有参照 item 且被渲染
• 作用：用于在列表视图新的偏移量的基本上做调整，一般在有参照 item 的情况下使用

flutter_scrollview_observer/lib/src/utils/src/chat/chat_observer_scroll_physics_mixin.dart

Lines 61 to 98 in 4f7b653

	final model = observer.observeRefItem();
	if (model == null) {
	_handlePositionCallback(ChatScrollObserverHandlePositionResultModel(
	type: ChatScrollObserverHandlePositionType.none,
	mode: observer.innerMode,
	changeCount: observer.changeCount,
	));
	return adjustPosition;
	}
	_handlePositionCallback(ChatScrollObserverHandlePositionResultModel(
	type: ChatScrollObserverHandlePositionType.keepPosition,
	mode: observer.innerMode,
	changeCount: observer.changeCount,
	));
	// Customize the delta of the adjustPosition.
	double? customDelta = observer.customAdjustPositionDelta?.call(
	ChatScrollObserverCustomAdjustPositionDeltaModel(
	oldPosition: oldPosition,
	newPosition: newPosition,
	isScrolling: isScrolling,
	velocity: velocity,
	adjustPosition: adjustPosition,
	observer: observer,
	currentItemModel: model,
	),
	);
	// Calculate the final delta.
	//
	// If the customDelta is not null, use the customDelta.
	// Otherwise, use the layoutOffset minus innerRefItemLayoutOffset to get
	// the difference in the leading offset of the item.
	final delta =
	customDelta ?? (model.layoutOffset - observer.innerRefItemLayoutOffset);
	return adjustPosition + delta;

该回调在 #103 中被加入，允许在新偏移量的基础上去调整以达到保持位置的功能，返回 null 则按库中逻辑走，使用上可以看之前的例子 #103 (comment) 。

在上述代码中用来处理收起时 adjustPosition 计算异常的情况，当然，你可以继续选择 customAdjustPosition，看个人喜好了。

注：上述代码中 model.adjustPosition 是 Flutter 内部计算出来的列表视图的新偏移量。