How to not duplicate same item in a list dart?

Question

I have created a listView and button and when I click the button it adds an item to listView.

The problem is I don't want actually to repeat the same item in the list.

I've tried the .contains method but it didn't work.

I want a good solution please,

Solution 1

There are different ways to achieve this:

1) Iterate the list and check if every element doesn't have the properties you consider equal:

items = [Item(id: 1), Item(id: 2)];
newItem = Item(id: 2);
if (items.every((item) => item.id != newItem.id)) {
  items.add(newItem);
}

2) Use contains() and override == operator (and override hashCode too) in the object class with the properties you consider equal.

items = [Item(id: 1), Item(id: 2)];
newItem = Item(id: 2);
if (!items.contains(newItem)) {
  items.add(newItem);
}

// inside Item class
@override
bool operator ==(other) {
  return this.id == other.id;
}

@override
int get hashCode => id.hashCode;

3) Instead of List use Set, where each element can occur only once. Its default implementation is LinkedHashSet that keeps track of the order.

Solution 2

Instead of List, Use Set.

void main() {
  Set<String> currencies = {'EUR', 'USD', 'JPY'};
  currencies.add('EUR');
  currencies.add('USD');
  currencies.add('INR');
  print(currencies);
}

output: {EUR, USD, JPY, INR} // unique items only

Reference: Set<E> class