Flutter/Dart - Dynamic value in a variable

Question

I want to create a variable with dynamic value in it. Something like this:

String myAlphabet = 'c';
List myList = [
   'a',
   'b',
   getMyAlphabet()
];

String getMyAlphabet(){
   return myAlphabet;
}

When I access myList, I should get ['a','b','c']. But at one point of my program, the value of myAlpabet is changed to 'd'. At that point when I access myList I should be getting ['a','b','d'] but instead I'm still getting ['a','b','c'].

How do I make it so that everytime I access myList, I will be getting the latest value instead of the value when it is first initialized? Thank you in advance.

jamesdlin · Accepted Answer · 2021-02-15 06:58:13Z

List myList = [
   'a',
   'b',
   getMyAlphabet()
];

getMyAlphabet() will be evaluated at the time that this List is created, and the result will be stored in the List.

If you want myList to always return a List with the latest evaluation of getMyAlphabet(), you could make myList create and return a new List each time by making it a getter function:

List get myList => ['a', 'b', getMyAlphabet()];

Note that you wouldn't be able to effectively mutate myList, which potentially could be surprising to callers.

Another approach would be to make myList a list of thunks so that they would need an extra level of evaluation:

List<String Function()> myList = [
  () => 'a',
  () => 'b',
  getMyAlphabet,
];

var a = myList[0]();
var lastElement = myList.last();
var list = [for (var thunk in myList) thunk()];

To take it further, you could create your own List-like class whose operator[] implementation invokes each thunk automatically instead of making callers do it.

KuKu · Accepted Answer · 2021-02-15 03:29:21Z

1

Hm... How about you use making a method to get a my List?


//

//

void main() {
  String myAlphabet = 'c';
  String getMyAlphabet(){
    return myAlphabet;
  }
  
  List<String> getMyList() {
    return [
      'a',
      'b',
      getMyAlphabet()
    ];
  }
  
  List myList = [
    'a',
    'b',
    getMyAlphabet()
  ];


  print(myList);
  print(getMyList());
  
  myAlphabet = 'd';
  
  print(myList);
  print(getMyList());
  

}

answered Feb 15, 2021 at 3:29

KuKu

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Comments

Baker · Accepted Answer · 2021-02-15 04:09:49Z

0

I believe you'll have to edit the List rather than try to create a "pointer" in the list, which I don't believe is possible with Dart.

List has a method called replaceRange (docs) which you could use to replace a value at a certain index in the list. There are other methods you could combine to do the same thing.

answered Feb 15, 2021 at 4:09

Baker

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