Why does my Flutter screen not update after I change a ChangeNotifier value?

Cause

Two things are missing: notifyListeners() is never called, and the badge widget may be reading with read instead of watch.

1. Mutate inside the model and notify

class CartModel extends ChangeNotifier {
  final List<Item> _items = [];
  List<Item> get items => List.unmodifiable(_items);
  int get count => _items.length;

  void add(Item item) {
    _items.add(item);
    notifyListeners();
  }
}

Expose an unmodifiable view so widgets cannot change the list behind the model's back (that is exactly the bug in the snippet).

2. Use read for actions, watch for display

// in onPressed (an action, no rebuild needed)
context.read<CartModel>().add(item);

// in build (must rebuild when it changes)
final count = context.watch<CartModel>().count;

read never subscribes, watch subscribes and rebuilds. Calling watch inside onPressed throws in debug; calling read inside build silently stops updating.

3. Rebuild less with select or Consumer

final count = context.select<CartModel, int>((c) => c.count);

The widget now rebuilds only when count changes.

Common pitfalls

• The provider is created inside a widget that rebuilds (so a fresh model is made each time). Create it once above MaterialApp with ChangeNotifierProvider(create: (_) => CartModel(), child: ...).
• Using ChangeNotifierProvider.value for a new instance on each build leaks and resets state.
• Calling notifyListeners() during build raises "setState() or markNeedsBuild() called during build".

There is a worked example in the DEV3 snippet library.