Duplicate Array list value automatically changed in Flutter

Question

I'm very surprised after this issue. First, inform all things I have used in my project. I have used Getx in my project. I have called the API using the Getx controller file.

Below code used in getx controller file. "PLTimeSlotModel" is model and. it has two params (name, isselect).

 var futureTimeSlot_PL_C_D = Future.value(<PLTimeSlotModel>[]).obs;

 callTimeSlotAPI() async {
   futureTimeSlot_PL_C_D.value = FetchTimeSlotList();
 }
Future<List<PLTimeSlotModel>> FetchTimeSlotList() async {
     // Fetching data with API calling
 }

Screen A:

 List<PLTimeSlotModel> listA = [];
 List<PLTimeSlotModel> listB = [];
 @override
 void initState() {
 super.initState();
 _plController.callTimeSlotAPI();
 }

Another method is to create two lists using the future list:

List<PLTimeSlotModel> temp1 = await _plController.futureTimeSlot_PL_C_D.value;
temp1.forEach((element) {
  listA.add(element);
  listB.add(element);
});

onclick:

for(int i =0;i<listA.length;i++){
     listA[i].isselect = false;
       print(listA[i].isselect);
        print(listB[i].isselect);
     }

Now the issue is I have changed/updated the only "listA" value, So why automatically set the same value to "listB"? The two list is based on the one list.

Answer 1

A List in Dart contains references objects, not the objects themselves. So what you are doing is copying references to objects into two lists. But since they are pointing at the same objects, you will see any modification in one list also happen in the other list. You need to copy each PLTimeSlotModel object and put the copy into your new list.

One way to copy objects is to create an constructor which takes an object of the same type and creates a new object based on this first object. So something like this:

class Person {
  String name;

  Person(this.name);
  Person.fromPerson(Person other) : this(other.name);

  @override
  String toString() => 'Person($name)';
}

void main() {
  final persons = [
    Person('Adam'),
    Person('Bob'),
  ];

  // Make new list based on persons
  final otherPersonList = [
    ...persons.map((person) => Person.fromPerson(person))
  ];

  otherPersonList[0].name = 'Carl';

  print(persons);         // [Person(Adam), Person(Bob)]
  print(otherPersonList); // [Person(Carl), Person(Bob)]
}