Why is my array iteration not updating the subtotal?

I am making a simple app for a small business using Firebase. So far the data from the cart gets read without any issues, but when I try to iterate through it to add the subtotal I get nothing. This is the code for this part. Let me know if you need any more information.

@Published var data = [cart]()
@Published var subtotal: Float = 0
init() {
    
    let db = Firestore.firestore()
    
    db.collection("cart").addSnapshotListener { snap, error in
        if error == nil {
            if let snap = snap {

                DispatchQueue.main.async {
                    self.data = snap.documents.map { doc in
                        return cart(
                            id: doc.documentID,
                            item_name: doc["name"] as? String ?? "",
                            item_price: doc["price"] as? NSNumber ?? 0,
                            item_description: doc["description"] as? String ?? "",
                            item_image: doc["image"] as? String ?? "")
                    }
                }  
            }
        }
    }
    
        for i in self.data {
            self.subtotal += Float(i.item_price)
        }