Check whether a list contain an attribute of an object in dart

Question

I need to check whether myItemsList contains myitem.itemId or not, If it exists need to add itemQuantity, if it not exists need to add myitem object to myItemsList.

List<MyItem> myItemsList = new List();

MyItem myitem = new MyItem (
  itemId: id,
  itemName: name,
  itemQuantity: qty,
);

if (myItemsList.contains(myitem.itemId)) {
  print('Already exists!');
} else {
  print('Added!');
  setState(() {
    myItemsList.add(myitem);
  });
}

MyItem class

class MyItem {
  final String itemId;
  final String itemName;
  int itemQuantity;

  MyItem ({
    this.itemId,
    this.itemName,
    this.itemQuantity,
  });
}

above code is not working as expected, please help me to figure out the issue.

Answer 1

As already said before, contains compares two Objects with the == operator. So you currently compare MyItem with String itemId, which will never be the same.

To check whether myItemsList contains myitem.itemId you can use one of the following:

myItemsList.map((item) => item.itemId).contains(myitem.itemId);

or

myItemsList.any((item) => item.itemId == myitem.itemId);

Answer 2

You're using contains slightly wrong.

From: https://api.dartlang.org/stable/2.2.0/dart-core/Iterable/contains.html

bool contains(Object element) {
  for (E e in this) {
    if (e == element) return true;
  }
  return false;
}

You can either override the == operator, see: https://dart-lang.github.io/linter/lints/hash_and_equals.html

@override    
bool operator ==(Object other) => other is Better && other.value == value;

Or you can loop over your list and search the normal way one by one, which seems slightly easier.