列出第一个坏状态:没有元素

Question

我正在运行list.firstWhere,这有时会引发异常:

Bad State: No element

When the exception was thrown, this was the stack:
#0      _ListBase&Object&ListMixin.firstWhere (dart:collection/list.dart:148:5)

我不明白这意味着什么也无法通过查看来源来识别问题.

我firstWhere看起来像这样:

list.firstWhere((element) => a == b);

Answer 1

当没有匹配的元素,即当会发生这种情况a == b是从来没有接触过的内容的真实list 和的可选参数orElse没有被指定.

您还可以指定orElse处理这种情况:

list.firstWhere((element) => a == b, orElse: () => print('No matching element.'));

Answer 2

现在您可以导入package:collection并使用扩展方法firstWhereOrNull

import 'package:collection/collection.dart';

void main() {

  final myList = [1, 2, 3];

  final myElement = myList.firstWhereOrNull((a) => a == 4);

  print(myElement); // null
}

Answer 3

firstWhere 返回根据条件生成的 newList

void main() {
  List<String> list = ['red', 'yellow', 'pink', 'blue'];
  var newList = list.firstWhere((element) => element == 'green', 
      orElse: () => 'No matching color found');
  print(newList);
}

输出：

No matching color found

或者

 void main() {
      List<String> list = ['red', 'yellow', 'pink', 'blue'];
      var newList = list.firstWhere((element) => element == 'blue', 
          orElse: () => 'No matching color found');
      print(newList);
    }

输出：

blue

如果代码中没有定义 orElse 并且错误的项目得到了列表中不存在的搜索，那么它会显示 BadStateException

 void main() {
      List<String> list = ['red', 'yellow', 'pink', 'blue'];
      var newList = list.firstWhere((element) => element == 'green');
      print(newList);
    }

输出：

Uncaught Error: Bad state: No element