Swift 3, Firebase completion Handler is executed twice

Question

My Use case - I have list of items which I fetch from Firebase. Below is loadItems() function that I call from HomeViewController - viewDidLoad() and updating tableView with the fetched data.

func loadItems() {

    Database.database().reference().child("items").observe(.value, with: { snapshot in
        var fetchedItems = [Item]()
        guard let receivedvalue = snapshot.value as? [String: Any] else     {
            print("Received null")
            return
        }
        print(receivedvalue)

        for (key, value) in receivedvalue {
            let item = Item(id: Int(key)!, json: value as! [String : Any])
            fetchedItems.append(item!)
        }
        self.items = fetchedItems
        self.tableView.reloadData()
    })
}

I am saving an item and coming back from CreateViewController to HomeViewController, I am - Saving the item in Firebase, Appending the item to prefetched array, reloading tableView.

func addItem(item: Item?) {
    rootRef = Database.database().reference()

    let id = String(describing: item.id!)
    let itemRef = self.rootRef.child("items").child(id)
    itemRef.setValue(["name": item.name!, "type": item.type!])

    items.append(item!)
    self.tableView.reloadData()
}

After reloading tableView, its is going in the Firebase GET Call handler which is present in loadItems().

The handler is executed once when I am getting all items during viewDidLoad(). Is there any reason why the Firebase GET call handler is executed the second time even though I am not calling loadItems() in create workflow?

Answer 1

When .observe(.value is used, it adds an observer to that node and any changes to that node (add, change, remove) will fire the code in the closure.

If you want to leave the observer so you can be notified of changes, the the proper flow is to simply write the data to Firebase and let the closure load the data and populate the tableView.

However, the downside to this is that .value loads ALL of the data in the node. You may want to take a look at adding separate observers for .childAdded, .childChanged and .childRemoved. Those will only load the node that was modified.

If you want to only load the data once (to populate a dataSource on startup for example), use observeSingleEvent which fires once and does not leave an observer.

Then store the data in Firebase and manually add it to the array and reload the tableView.

See the documentation Read Data Once section.