What's the usage of three dots (…) in dart List?

问题

I have my code written like this but it gives an error saying:

Error: A value of type 'List' can't be assigned to a variable of type 'Widget'.

Column(
  children: [
    Question(
      questions[_questionIndex]['questionText'],
    ),
    ...(questions[_questionIndex]['answers'] as List<String>)
        .map((answer) {
      return Answer(_answerQuestion, answer);
    }).toList()
  ],
)

回答1:

Dart 2.3 introduce Spread operator (…)

Reference link : https://medium.com/flutter-community/whats-new-in-dart-2-3-1a7050e2408d

    var a = [0,1,2,3,4];
    var b = [6,7,8,9];
    var c = [...a,5,...b];

    print(c);  // prints: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

回答2:

I've figured out my issue. My flutter SDK wasn't upgraded. Ran flutter doctor command and sorted out the missing updates. And the syntax seems to be working fine now.

flutter upgrade

Dart version (required) >= 2.3

Restart IDE (if the problem persists)

来源：https://stackoverflow.com/questions/57900901/whats-the-usage-of-three-dots-in-dart-list