Flutter ScrollController抛出异常

Question

我有一个ScrollController连接到一个Listview，但当我滚动时它会抛出异常：

@override
  initState() {
    super.initState();

    _mainCategoriesScrollController = ScrollController();
    WidgetsBinding.instance.addPostFrameCallback((timeStamp) {
      _mainCategoriesScrollController
          .addListener(_mainCatergoriesScrollListener());
    });
  }



_mainCatergoriesScrollListener() {
    if (_mainCategoriesScrollController.offset >=
            _mainCategoriesScrollController.position.maxScrollExtent &&
        !_mainCategoriesScrollController.position.outOfRange) {
      print("reach the bottom");
    }
    if (_mainCategoriesScrollController.offset <=
            _mainCategoriesScrollController.position.minScrollExtent &&
        !_mainCategoriesScrollController.position.outOfRange) {
      print("reach the top");
    }
  }

以及构建方法

 @override
  Widget build(BuildContext context) {
    _setCurrentMainCategory();
    SystemChrome.setEnabledSystemUIOverlays([]);
    return Container(
        child: Column /*or Column*/ (
          crossAxisAlignment: CrossAxisAlignment.stretch,
          children: <Widget>[
            Expanded(
                flex: 9,
                child: Container(
                  child: Row(
                    children: <Widget>[
                      Expanded(
                        flex: 3,
                        child: ListView.builder(
                            controller: _mainCategoriesScrollController,
                            shrinkWrap: true,
                            itemCount: _mainCategories.length,
                            itemBuilder: (context, index) {
                              return MainCategoryEntry(
                                mainCategory: _mainCategories[index],
                                mainCategorySelected: () {
                                  MyApp.setActivity(context);
                                  setState(() {
                                    currentMainCategory =
                                        _mainCategories[index];
                                  });
                                },
                                isSelected: _mainCategories[index] ==
                                    currentMainCategory,
                              );
                            }),
                      ),

以下是异常情况：

════════ 基础库捕获的异常 ════════════════════════════════ 在 ScrollController 的通知分发期间发现了 NoSuchMethodError 异常： 调用 null 上的方法 'call'。 接收器：null 尝试调用：call()

When the exception was thrown, this was the stack
#0      Object.noSuchMethod  (dart:core-patch/object_patch.dart:53:5)
#1      ChangeNotifier.notifyListeners 
package:flutter/…/foundation/change_notifier.dart:207
#2      ChangeNotifier.notifyListeners 
package:flutter/…/foundation/change_notifier.dart:207
#3      ScrollPosition.notifyListeners 
package:flutter/…/widgets/scroll_position.dart:775
#4      ScrollPosition.setPixels 
package:flutter/…/widgets/scroll_position.dart:244
...
The ScrollController sending notification was: ScrollController#7153b(one client, offset 0.7)
════════════════════════════════════════════════════════════════════════════════

有人能告诉我我做错了什么吗？非常感谢！

- helmut1978

Answer 1

与其将_mainCategoriesScrollListener方法添加到监听器中，这一行似乎是调用该方法：

_mainCategoriesScrollController.addListener(_mainCatergoriesScrollListener());

我猜测当客户端未被分配时，您尝试访问_mainCategoriesScrollController方法时会出现错误。

即使您尝试添加一个后帧回调，_mainCatergoriesScrollListener()方法仍会在initState()中被调用，因为您正在调用它而不是将其添加到监听器中。

这可能是错误的原因。

尝试将其更改为：

_mainCategoriesScrollController.addListener(_mainCatergoriesScrollListener);

或者

_mainCategoriesScrollController.addListener(() => _mainCatergoriesScrollListener());

- Afridi Kayal