Firebase Realtime Database, how do I know when dataset has finished loading

Question

How do I know when the dataset is loaded?

I have been using Firebase for over five years and love it. But, I have one question which I hope you can help with, but has been driving me mad for years.

I have many datasets stored on Firebase. What I need is a way to know when the dataset has been loaded. So, if there are 1000 records at the node /Main/PNG/flight/airstrip I want to know when I have received all leaves.

I have tried using .value and subscribe to the whole node using

Database.database().reference(withPath: key).observe(.value) { [weak self, key] snapshot ...}

And then I will know when the dataset comes in because I will get just one snapshot containing a dictionary of all the leaves, but I don't get the deltas (changes and additions).

and I have subscribed to all the leaves, using

[.childChanged, .childRemoved, .childAdded].forEach { eventType in
Database.database().reference(withPath: key).observe(eventType) { [weak self, storeEventType] snapshot in ...}

But, then I don't know when I have received all of the dataset.

My datamodel is based on an Initial Snapshot followed by listening to any changes to the leaves. The initial snapshot is easy using

Database.database().reference(withPath: key).observe(.value) { [weak self, key] snapshot ...}

but when I follow this with

[.childChanged, .childRemoved, .childAdded].forEach { eventType in
Database.database().reference(withPath: key).observe(eventType) { [weak self, storeEventType] snapshot in ...}

I get the complete dataset again.

What I would like is either :

1. to .observe(.childAdded) and somehow know when all the leaves have completed, at least for this dataset,
2. or to use .observe(.childAdded) without receiving an initial dataset of all the leaves, but then I need to work out if I missed anything between the .observe(.value) and .observe(.childAdded) calls.

I know that I could view the dataset as an endless number of deltas and changes, so therefore there isn't an END OF SET , but updating a UITableView in Swift is easier if I know lots of records are queued, in which case I can call a reloadData() on the UITableview. But, if there are just a few records queued, I prefer to let the tableView animate nicely, a row at a time.

Answer 1

When an entire snapshot is loaded, the Firebase SDK fires the value event. So even if you use the child* events for actually handling the data, you can implement a value listener to detect when the loading is done.

Implementing both child* and value event listeners does not double the amount of data that gets load. The Firebase SDK deduplicates the listeners behind the scenes.