如何对 firebase android studio 中的多个值求和

Question

I have carts collection and it contains CartItems as document, CartItems have quantity field I want to SUM them, I wrote this code but the return type should be List<CartItem> how I can change it to Int and return totalQuantity value

suspend fun getTotalQuantity(userID: String): Int {
    var totalQuantity = 0
    CartRepository.usersDocumentRef
        .document(userID)
        .collection("carts")
        .get()
        .addOnCompleteListener { task ->
            if (task.isSuccessful) {
                task.result.toObjects(CartItem::class.java).forEach {
                    totalQuantity += it.quantity
                }

            }
        }
}

Answer 1

Haven't test it but it should do the job:

suspend fun getTotalQuantity(userID: String): Int {
    var totalQuantity = 0
    CartRepository.usersDocumentRef
        .document(userID)
        .collection("carts")
        .get()
        .addOnCompleteListener { task ->
            if (task.isSuccessful) {
                for (i in task.result.documents) {
                  totalQuantity += i.toObject(CartItem::class.java)!!.quantity 
                }
                // print or use totalQuantity here
            }
        }
}