How to copy List in dart without old reference?

Question

Code will explain all:

//modal for list
class MyModal
{
 int myField1;
 List<MyModal> adjacentNodes;
 MyModal(this.myField1)
 {
  adjacentNodes= new List<MyModal>();
 }
}

//pre code
List<MyModal> originalList = new List<MyModal>();
originalList.add(new MyModal(1,"firstBuddy"));

//copying list
List<MyModal> secondList = new List<MyModal>();
secondList.addAll(originalList);

//Modifing copy list
secondList.adjacentNodes.add(new MyModal(2,"anotherBuddy"));

//Also modifies original list
print(originalList[0].childs.length); //prints 1, it should prints 0

How can I perform changes in the second list without affecting the original list?

What kind of changes do you want to make in the second list? Only add/remove or also changes to `MyModal` instances that should not have an effect in the original list? — Günter Zöchbauer, May 22 '18 at 05:27
changes in the member fields of instance to as I explained in above code. — pallav bohara, May 22 '18 at 05:36

Günter Zöchbauer · Accepted Answer · 2018-05-22T07:09:31.980

36

class MyModal {
  int myField1;
  String myField2;
  List<MyModal> adjacentNodes;
  MyModal(this.myField1,this.myField2);

  MyModal.clone(MyModal source) : 
      this.myField1 = source.myField1, 
      this.myField2 = source.myField2,
      this.adjacentNodes = source.adjacentNodes.map((item) => new MyModal.clone(item)).toList();
}

var secondList = originalList.map((item) => new MyModal.clone(item)).toList();

If a member of MyModal is of a non-primitive type like String, int, double, num, bool, then the clone() method needs to clone the instances references point to as well.

I think for your use case using immutable values is a better approach, for example with https://pub.dartlang.org/packages/built_value

edited May 22 '18 at 07:09

answered May 22 '18 at 05:26

Günter Zöchbauer

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1

still it is taking reference and changes are reflecting – pallav bohara May 22 '18 at 05:35
Depends on what references you are talking about. There is no way to automatically create copies of the content of a list, only a copy of the list itself. – Günter Zöchbauer May 22 '18 at 05:37
Updated code is working for members which are of String, bool etc but not for user-defined types(I updated my question, added List as member). – pallav bohara May 22 '18 at 07:00
3

That's what I wrote in my answer. If you have properties of objects, then you need to clone recursively. See my updated answer. – Günter Zöchbauer May 22 '18 at 07:03

score 20 · Answer 2 · answered Oct 28 '19 at 03:22

20

you can use List.from() function. try this code:

//modal for list
class MyModal {
  int myField1;
  String name;
  List<MyModal> adjacentNodes;
  MyModal(this.myField1, this.name) {
    adjacentNodes = new List<MyModal>();
  }
}

void runCopy() {
//pre code
  List<MyModal> originalList = new List<MyModal>();
  originalList.add(new MyModal(1, "firstBuddy"));

//copying list
  List<MyModal> secondList = List.from(originalList);
  secondList.addAll(originalList);
  print(originalList);
  print(secondList);
}

answered Oct 28 '19 at 03:22

mosleim

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6
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20

This will only perform a shallow copy of objects, try to change the value of name in original list then print it from both of them, it will be the same. This will work fine only for simple types like int or string. So, no it should not be selected as correct answer. – hydeparkk Jan 17 '20 at 10:17

score 13 · Answer 3 · edited Aug 27 '21 at 06:14

13

The recommended way is to use .toList or a list literal.

List<int> source = [1];

List<int> copied = source.toList();
List<int> copied2 = [...source];

edited Aug 27 '21 at 06:14

Suragch

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answered May 26 '20 at 02:02

CopsOnRoad

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score 7 · Answer 4 · answered May 20 '20 at 10:22

7

I just used thelist2 = thelist1.toList(); and it work

answered May 20 '20 at 10:22

Malek Tubaisaht

1,170
14
16

Luiz Filipe Medeira · Answer 5 · 2020-07-26T21:11:44.777

7

I was having the same problem, List.from() and List.of() didn't work. I had to map the current list, thus creating a new instance.

List<CompanyBranchesDTO> get companyBranchesCopy {
  return companyBranches.map((e) =>
          CompanyBranchesDTO(id: e.id, name: e.name, isChecked: e.isChecked))
      .toList();
}

That way I have a new instance to inject where it is needed.

edited Jul 26 '20 at 21:11

answered Jul 26 '20 at 21:01

Luiz Filipe Medeira

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This is the only correct solution for List, I tried all other solutions but this is the only one working – Hassan M. Said Feb 17 '22 at 19:32

score 2 · Answer 6 · answered Jan 21 '20 at 03:02

2

Change it to JSON then parse it again to object. I know it's dirty way.

(+) it works and less code. (-) it not parsing method.

answered Jan 21 '20 at 03:02

Rico AW

59
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score 1 · Answer 7 · answered Jan 05 '23 at 18:32

Make this extension, put it somewhere.

import 'dart:convert';

extension ListExtensions on List {
  get() {
    return jsonDecode(jsonEncode(this));
  }
}

Basically we need to encode and decode the List so it doesn't refer to the previous List. You can use it like this:

//import your extension

List productList = ["GG FILTER 12","SK KRETEK 12"];
List newProductList = productList.get();

score 0 · Answer 8 · answered Jun 03 '21 at 10:18

0

Spread operator allow you to insert values from another list into a collection as explained in the dart language tour: https://dart.dev/guides/language/language-tour#lists So you can simply do that to avoid shallow copying:

final List<MyType> newList = [...oldList];

See this dartpad as POC:

https://dartpad.dev/191c4767abc9540c63103044038562da?null_safety=true

answered Jun 03 '21 at 10:18

Arnaud Delubac

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Spread operator does copy object from list with its original reference. – Corentin Houdayer Jun 28 '21 at 10:49

score -1 · Answer 9 · edited Sep 05 '20 at 14:48

-1

List<T>.addall(T) worked for me

i.e.

List<mylist>.addall(oldlist)

edited Sep 05 '20 at 14:48

Markus

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answered Sep 04 '20 at 18:09

Xavier

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The answer is lacking explanation and has syntax errors which means your code will not compile. Please, before posting an answer make sure you: 1) have an explanation enough to understand your solution; 2) your solution is working; 3) good, but not required - point out the reason of the error OP gets, if possible. – Jenea Vranceanu Sep 05 '20 at 17:53

How to copy List in dart without old reference?

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