How to declare a function callback that accepts multiple parameters in Dart/Flutter?

Question

final ValueSetter<int> setVal;

can be assigned a function callback that accepts one parameter like

setVal=(i){
     //assign i to some variable
            }

What if I wanted the callback to accept 2 parameters like

  setVal=(i,j){
         //assign i and j to some variable
                }

?

score 4 · Answer 1 · answered Nov 17 '19 at 18:33

4

ValueSetter is typedef'ed as:

typedef ValueSetter<T> = void Function(T value);

As in, a function that takes one parameter. So, no, you can't hand it a function that takes two parameters.

answered Nov 17 '19 at 18:33

Randal Schwartz

39,428
4
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70

mezoni · Accepted Answer · 2019-11-18T06:30:21.157

4

Variables of type Function can be declared as follows:

void main() {
  void Function(int) setVal1;
  void Function(int, int) setVal2;

  setVal1 = (int i) => print('$i');
  setVal2 = (int i, int j) => print('$i, $j');
  final String Function(int, int, [double]) setVal3 =
      (int i, int j, [double k]) => '$i, $j, $k';

  final i = 0;
  final j = 1;
  final double k = 2.0;
  setVal1(i);
  setVal2(i, j);
  print(setVal3(i, j));
  print(setVal3(i, j, k));
}

edited Nov 18 '19 at 06:30

answered Nov 17 '19 at 19:27

mezoni

10,684
4
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54

Okay so how do I make the second parameter optional? – Michel Thomas Nov 17 '19 at 19:37
@MichelThomas you accepted this answer even it does not answer your question? – fajar ainul Mar 10 '20 at 10:32
@MichelThomas Ha-ha. This was an example of `How to declare a function callback(s) that accepts multiple parameters in Dart/Flutter`. Including an additional answer to the question asked in the comments about `how to make the second parameter optional`. Yeah... – mezoni Mar 11 '20 at 12:15

score 2 · Answer 3 · answered Nov 17 '19 at 19:13

2

final Function(int, String) = (int a, String b){
  /* do stuff */
}

answered Nov 17 '19 at 19:13

Caffo17

513
2
10

score 1 · Answer 4 · answered May 29 '21 at 15:38

1

Just put whatevery you want in a map and pass the map. Something like this:

  Map<String, dynamic> updateMap = {
    'id': id,
    'value': value


  widget.onUpdate(updateMap);

answered May 29 '21 at 15:38

Akram

56
5

score 0 · Answer 5 · answered Feb 16 '22 at 01:50

0

You can declare your own type, just add this def in your helpers.dart file:

typedef ValueSetter2<T, E> = void Function(T value, E value2);

answered Feb 16 '22 at 01:50

Eran Ravid

139
1
3

How to declare a function callback that accepts multiple parameters in Dart/Flutter?

5 Answers5