无法访问数组内的对象数组及其大小

Question

我有一个简单的问题。请有人帮忙。

我是 Flutter 的绝对初学者。我无法访问列表中的对象及其大小。这是我想要达到的目标的一个简单示例。

final questions = [
    {
      'questionText': 'What's your favorite color?',
      'answers': [
        {'text': 'Black', 'score': 3},
        {'text': 'Blue', 'score': 1},
        {'text': 'White', 'score': 1}
      ]
    },
    {
      'questionText': 'What's your favorite animal?',
      'answers': [
        {'text': 'Lion', 'score': 3},
        {'text': 'Snake', 'score': 5},
        {'text': 'Cat', 'score': 1}
      ]
    },
    {
      'questionText': 'What's your favorite teacher?',
      'answers': [
        {'text': 'Ali', 'score': 3},
        {'text': 'Aslam', 'score': 3},
        {'text': 'Akram', 'score': 3}
      ]
    }
  ];


  print(questions.length);    // 3
  print(questions[0].length);    // 2
print(questions[0]['answers']);   //[{text: Black, score: 3}, {text: Blue, score: 1}, {text: White, score: 1}]

但是当我尝试通过以下方法获得问题中“答案”的长度时。我收到此错误（在 cmets 内部给出）。

print(questions[0]['answers'].length);  //Error: The getter 'length' isn't defined for the class 'Object'.

当我尝试在问题中打印 'answers' 数组的第一个元素（即“{'text': 'Black', 'score': 3}”）时，我得到了以下错误

print(questions[0]['answers'][0]); // The method '[]' isn't defined for the class 'Object'

也试过这些

print(questions[0]['answers']['text']); // Error: The method '[]' isn't defined for the class 'Object'.
print(questions[0]['answers'[0]]);  // null
print(questions[0]['answers'['text']]); // Error: The argument type 'String' can't be assigned //to the parameter type 'int'.

我该怎么做？另外，如果我只想打印“答案”的所有三个选项（“文本”），我该怎么办。我知道这是一个有点愚蠢的问题，但我完全被困在这里。任何帮助将不胜感激。

Answer 1

你可以使用这个技巧。

var answers = questions[0]['answers'] as List<Map<String, Object>>;
print('${answers.length}'); //3
print('${answers[0]}'); //{text: Black, score: 3}

或者一行。

print('${(questions[0]['answers'] as List<Map<String, Object>>).length}'); //3

Answer 2

通过您使用的符号，Dart 将类型推断为具有字符串键但混合值类型的映射列表（questionText 是字符串，answers 又是映射列表）。这迫使 Dart 使用 Object 作为值类型，这使得不强制转换就无法访问嵌套字段。

questions[0].runtimeType // Type (_InternalLinkedHashMap<String, Object>)

您还可以通过检查 questions 对象在调试器中轻松看到这一点。

我建议定义具有正确类型属性的类。

class Question {
  Question(this.text, this.answers);

  final String text;

  final List<Answer> answers;

  String toString() => 'Question { text: $text, answers: $answers }';
}

class Answer {
  Answer(this.text, this.score);

  final String text;

  final int score;

  String toString() => 'Answer { text: $text, score: $score }';
}

main(List<String> args) {
  final questions = [
    Question('What\'s your favorite color?', [
      Answer('Black', 3),
      Answer('Blue', 1),
      Answer('White', 1),
    ]),
    Question('What\'s your favorite animal?', [
      Answer('Lion', 3),
      Answer('Snake', 5),
      Answer('Cat', 1),
    ]),
    Question('What\'s your favorite teacher?', [
      Answer('Ali', 3),
      Answer('Aslam', 3),
      Answer('Akram', 3),
    ]),
  ];

  print(questions.length); // 3
  print(questions[0].text); // What's your favorite color?
  print(questions[0].answers); // [Answer { text: Black, score: 3 }, Answer { text: Blue, score: 1 }, Answer { text: White, score: 1 }]
  print(questions[0].answers.length); // 3
  questions[0].answers.forEach((answer) => print(answer.text)); // print all three options('text') of 'answers'
  print(questions[0].answers[0]); // Answer { text: Black, score: 3 }
  print(questions[0].answers[0].text); // Black
  print(questions[0].answers[0].score); // 3
}