Question

我正在运行 list.firstWhere ，有时会抛出异常：

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at the source来识别问题。

我的firstWhere如下：

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

Answer 1

当没有匹配的元素时，即对于a == b 和可选参数{{ 1}} 未指定。

您还可以指定list来处理这种情况：

orElse

Answer 2

firstWhere重新调整根据条件生成的newList

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

输出：

No matching color found

OR

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

输出：

blue

如果在代码中未定义orElse，并且错误的项目得到了列表中不存在的搜索，则显示BadStateException

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

输出：

Uncaught Error: Bad state: No element

Answer 3

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 4

firstWhere()实现如下所示：

  E firstWhere(bool test(E element), {E orElse()?}) {
    for (E element in this) {
      if (test(element)) return element;
    }
    if (orElse != null) return orElse();
    throw IterableElementError.noElement();
  }

因此，如您所见，如果定义了orElse arg，则该方法将使用该方法，并且如果没有元素与条件匹配，则不会引发任何异常。因此，使用下面的代码来处理错误的情况：

list.firstWhere((element) => a == b, orElse: () => null); // To return null if no elements match
list.firstWhere((element) => a == b, orElse: () => print('No elements matched the condition'));

Answer 5

在飞镖 2.10 上有一个名为null-safety的新功能，当您有一系列对象并且想要对某个对象做某事时，它可以为您做魔术对象的属性：

class User {
   String name;
   int age;

  User(this.name, this.age);

   @override
   String toString() {
   return  'name:$name, age:$age';
   }

}

void main() {
  List<User?> users = [User("Ali", 20), User("Mammad", 40)];
  print(users.toString()); // -> [name:Ali, age:20, name:Mammad, age:40]
  // modify user age who it's name is Ali:
  users.firstWhere((element) => element?.name == "Ali")?.age = 30;
  print(users.toString()); // -> [name:Ali, age:30, name:Mammad, age:40]
  // modify a user that doesn't exist:
  users.firstWhere((element) => element?.name == "AliReza", orElse: ()=> null)?.age = 30; // doesn't find and works fine
  print(users.toString()); // -> [name:Ali, age:30, name:Mammad, age:40]

}

首先列出错误状态：无元素

5 个答案: