Is it necessary to use setState when removing items from a list in a Dismissible in Flutter?

Question

I'm building a Flutter app where I use a Dismissible widget to allow users to remove items from a list. I've noticed that the UI updates correctly when an item is dismissed, even if I don't call setState.

here is the code:

  @override
  _GroceryListScreenState createState() => _GroceryListScreenState();
}

class _GroceryListScreenState extends State<GroceryListScreen> {
  List<String> groceryItems = ['Apples', 'Bananas', 'Oranges', 'Milk'];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Grocery List'),
      ),
      body: ListView.builder(
        itemCount: groceryItems.length,
        itemBuilder: (context, index) {
          return Dismissible(
            key: ValueKey(groceryItems[index]),
            onDismissed: (direction) {
              groceryItems.removeAt(index); // UI updates even without setState
            },
            background: Container(color: Colors.red),
            child: ListTile(
              title: Text(groceryItems[index]),
            ),
          );
        },
      ),
    );
  }
}

Answer 1

It is necessary, groceryItems just removed an item after onDismissed, but ListView haven't noticed that, so you need to call setState to notify ListView, failing to do so will cause a RangeError when you scroll to the end.