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

Question

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()
  ],
)

Please explain issue before down voting – Farwa Sep 14 '19 at 10:02 — Farwa, Sep 14 '19 at 10:02

score 31 · Answer 1 · answered Sep 12 '19 at 06:39

31

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]

answered Sep 12 '19 at 06:39

Amit Prajapati

13,525
8
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post your all code here, let me look into this, may be you are doing wrong you are adding normal list, you have to first add that list into list widget and then try with it. – Amit Prajapati Sep 12 '19 at 07:00
1

@Farwa - as it said in error message - you are trying to put list of items where has to be `Widget` – Andrii Turkovskyi Sep 12 '19 at 07:45
1

Everything was fine but my Flutter SDK was too old. @AndreyTurkovsky – Farwa Sep 14 '19 at 12:00

score 15 · Answer 2 · answered Jun 12 '21 at 05:10

Dart 2.3 comes with the spread operator (...) and the null-aware spread operator (...?), which allows us to add multiple elements in Collections.

  List<String> values1 = ['one', 'two', 'three'];
  List<String> values2 = ['four', 'five', 'six'];
  var output = [...values1,...values2];
  print(output); // [one, two, three, four, five, six]

For Flutter, we can use this inside column

   Column(
          children: [
            ...values.map((value) {
              return Text(value);
            }),
          ],
        ),

Output:

score 13 · Answer 3 · answered Feb 11 '22 at 21:16

var x = [20,30,40];
var list = [1,2,3,x];

// Using the spread operator, adds the items individually not as a list
var separatedList = [1,2,3,...x];

print(list); // length = 4
print(separatedList); // length = 6

Results

[1, 2, 3, [20, 30, 40]]
[1, 2, 3, 20, 30, 40]

score 0 · Accepted Answer · edited Feb 02 '20 at 13:02

0

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)

edited Feb 02 '20 at 13:02

Ranvir

2,094
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answered Sep 13 '19 at 04:00

Farwa

6,156
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What's the usage of three dots (...) in dart List?

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