flutter method checking if item is added to cart in firestore - flutter

Question

I am having trouble with a method that is checking if the item is stored in the firestore database.
void add(BuildContext context, CartItem item) {
_items.add(item);
AuthService authService = Provider.of<AuthService>(context, listen: false);
Map<String, dynamic> cartMap = Map();
_items.forEach((CartItem item) {
cartMap['title'] = (item.product as Product).title;
cartMap['name'] = (item.product as Product).name;
});
_instance = FirebaseFirestore.instance;
_instance!
.collection('cart')
.doc(authService.getCurrentUser()) //need to get logged in account's id
.update({
'cartProduct': FieldValue.arrayUnion([cartMap])
}).then((value) {
print(_items.length);
notifyListeners();
});}
The add Method adds the item to the firestore in a way like the example image.
However, after I delete the data using the remove method,
void remove(BuildContext context, CartItem item) {
_items.remove(item);
AuthService authService = Provider.of<AuthService>(context, listen: false);
Map<String, dynamic> cartMap = Map();
// _items.forEach((CartItem item) {
cartMap['title'] = (item.product as Product).title;
cartMap['name'] = (item.product as Product).name;
// });
_instance = FirebaseFirestore.instance;
_instance!.collection('cart').doc(authService.getCurrentUser()).update({
'cartProduct': FieldValue.arrayRemove([cartMap]),
}).then((value) {
print(_items.length);
notifyListeners();
}); }
I check if the data is added to the cartProduct using isProductAddedToCart method and the result is still true. Also, when I print the _items.length, it doesn't decrease after I use the remove method.
bool isProductAddedToCart(Product? pro) {
return _items.length >= 0 ? _items.any(
(CartItem item) => item.product!.title == pro!.title) : false;
}
This is the code where I want to use the isProductAddedToCart method.
Consumer<CartService>(
builder: (context, cart, child) {
Widget renderedButton;
if (cart.isProductAddedToCart(widget.product) == false) {
renderedButton = DefaultButton(
text: "Participate",
press: () {
print(cart.isProductAddedToCart(widget.product));
cartService.add(context, CartItem(product: widget.product));
print(cart.isProductAddedToCart(widget.product));
},
);
} else {
renderedButton = DefaultButton(
text: "Delete",
press: () {
print(cart.isProductAddedToCart(widget.product));
cartService.remove(
context, CartItem(product: widget.product));
print(cart.isProductAddedToCart(widget.product));
},
);
}
return renderedButton;