Clone a List, Map or Set in Dart

Question

Use of clone() in Java is tricky and questionable^1,2. Effectively, clone() is a copy constructor and for that, the Dart List, Map and Set types each have a named constructor named .from() that perform a shallow copy; e.g. given these declarations

  Map<String, int> numMoons, moreMoons;
  numMoons = const <String,int>{ 'Mars' : 2, 'Jupiter' : 27 };
  List<String> planets, morePlanets;

you can use .from() like this:

  moreMoons = new Map<String,int>.from(numMoons)
    ..addAll({'Saturn' : 53 });
  planets = new List<String>.from(numMoons.keys);
  morePlanets = new List<String>.from(planets)
    ..add('Pluto');

Note that List.from() more generally accepts an iterator rather than just a List.

For sake of completeness, I should mention that the dart:html Node class defines a clone() method.

---

¹ J. Bloch, "Effective Java" 2nd Ed., Item 11.
²B. Venners, "Josh Bloch on Design: Copy Constructor versus Cloning", 2002. Referenced from here³. Quote from the article:

If you've read the item about cloning in my book, especially if you read between the lines, you will know that I think clone is deeply broken. ---J.Bloch

³Dart Issue #6459, clone instance(object).

With the new version of dart cloning of a Map or List become quite easy. You can try this method for making a deep clone of List and Map.

For List

List a = ['x','y', 'z'];
List b = [...a];

For Maps

Map mapA = {"a":"b"};
Map mapB = {...mapA};

For Sets

Set setA = {1,2,3,};
Set setB = {...setA};

I hope someone find this helpful.

If you are using dart > 2.3.0, You can use spread operator something like:

List<int> a = [1,2,3];
List<int> b = [...a]; // copy of a

For lists and sets, I typically use

List<String> clone = []..addAll(originalList);

The caveat, as @kzhdev mentions, is that addAll() and from()

[Do] not really make a clone. They add a reference in the new Map/List/Set.

That's usually ok with me, but I would keep it in mind.

For deep copy (clone), you can use :

Map<String, dynamic> src = {'a': 123, 'b': 456};
Map<String, dynamic> copy = json.decode(json.encode(src));

but there may be some concerns about the performance.

Map.from() only works for 1D map.

To copy multi dimensional map without reference in dart use following method

    Map<keyType, valueType> copyDeepMap( Map<keyType, valueType> map )
    {
        Map<keyType, valueType> newMap = {};

        map.forEach
        (
            (key, value)
            {
                newMap[key] =( value is Map ) ? copyDeepMap(value) : value ;
            }
        );

        return newMap;
    }

This solution should work:

  List list1 = [1,2,3,4]; 

  List list2 = list1.map((element)=>element).toList();

It's for a list but should work the same for a map etc, remember to add to list if its a list at the end