How to update Map flutter/dart? Does the map updates by reference in flutter or by value?

user_15321156 0
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I have a map:

{0: {0:"a",1:"b"}}

Now, on click of button in flutter I want to copy the map to a new key. So it should be

{0: {0:"a",1:"b"},1: {0:"a",1:"b"}}

Now, if I add a new value to key 1, it gets reflected to key 0 also.

So instead of

{0: {0:"a",1:"b"},1: {0:"a",1:"b",2:"c"}}

it becomes

{0: {0:"a",1:"b",2:"c"},1: {0:"a",1:"b",2:"c"}}

 Map<int,dynamic> tempMap=new HashMap();
 tempMap={...pageMap};
 var tempValue=pageMap[pageIndex];
 tempMap[(pageIndex+1)]= tempValue;
 pageMap.clear();
for (var key in tempMap.keys) {
    pageMap.putIfAbsent(key, () => tempMap[key]);
}

Does the map copies by reference in flutter/dart?

flutter-web dart flutter
3 Answers

In general, var x = y makes both x and y refer to the same object. If you now send methods to x, it will also affect y, because they aren't two different objects. So adding a value to a map referenced by y will also make that same element be visible via x.

user_224830

In Dart, maps update by reference, not by value. But you can create a new copy of your first map and assign the copy to a second map. That way you can modify the second without modifying the first.

    final firstMap = {1: 'test'};
    var secondMap = new Map<int,dynamic>.from(firstMap);
    // You can modify secondMap now without firstMap changing

Have you tried updating directly the first map? It might be a simpler approach

    pageMap.putIfAbsent(pageIndex + 1, () => pageMap[pageIndex]);
user_119327220

This problem can be resolved with this code:

void main(List<String> args) {
  Map getClone(Map data) {
    return data.map((k, v) => MapEntry(k, v is Map ? getClone(v) : v));
  }
  
  var map1 = {0: {0:'a',1:'b'}};
  var map2 = <int, dynamic>{};
  map2[0] = getClone(map1);
  map2[1] = getClone(map1);
  print(map2);
  map2[0][0][2] = 'c';
  print(map2);
}

Result:

{0: {0: {0: a, 1: b}}, 1: {0: {0: a, 1: b}}}
{0: {0: {0: a, 1: b, 2: c}}, 1: {0: {0: a, 1: b}}}
user_9555940
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