Firestore UI provides a handful of classes that allow developers to easily bind UI elements to Cloud Firestore queries, and to update their UI elements when those queries change.

FUIFirestoreTableViewDataSource | Binds a Firestore query to a table view. FUIFirestoreCollectionViewDataSource | Binds a Firestore query to a collection view. FUIBatchedArray | Maintains a local array containing the contents of a Firestore query. FUISnapshotArrayDiff | Describes an array update in a manner friendly to table and collection views.

FUIFirestoreTableViewDataSource is responsible for observing a Firestore query and updating a UITableView as the query changes, suitable for single-section table views dependent on a single query. The query can be re-assigned while active, and the data source will generate an update from the two queries' contents and pass it to its table view. To get started, use the bind(to:populateCell:) method on UITableView. Usage is almost exactly the same as it is in Firebase Database UI.

self.dataSource = tableView.bind(to: query) { tableView, indexPath, snapshot in
  // Dequeue cell
  let cell = tableView.dequeueReusableCell(withIdentifier: "reuseIdentifier", for: indexPath)
  /* populate cell */
  return cell
}

Like its table view counterpart, FUIFirestoreCollectionViewDataSource keeps a Firestore query in sync with a collection view instance, suitable for single-section collection views dependent on a single query. To get started, use the bind(to:populateCell:) method on UICollectionView.

self.dataSource = collectionView.bind(to: query) { collectionView, indexPath, snap in
  let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "reuseIdentifier", for: indexPath)
  /* populate cell */
  return cell
}

Firestore UI binds the results of a Firestore query to a table or collection view. It does not provide client-side map / filter / sort transforms on those results (Firebase Database UI's FUISortedArray has no Firestore counterpart). Shape data for display in one of these places instead:

1. In the query — filter, order, and limit with Firestore query APIs before binding. Prefer this for anything that affects which documents appear or in what order.
2. In populateCell — map fields onto cell UI when dequeuing. Prefer this for presentation-only changes (labels, formatting, hiding empty fields).

// Newest-first list
let query = Firestore.firestore()
  .collection("posts")
  .order(by: "createdAt", descending: true)
  .limit(to: 50)

self.dataSource = tableView.bind(to: query) { tableView, indexPath, snapshot in
  let cell = tableView.dequeueReusableCell(withIdentifier: "reuseIdentifier",
                                           for: indexPath)
  let data = snapshot.data()
  cell.textLabel?.text = data?["title"] as? String
  return cell
}

For a chat-style feed (latest N messages, oldest → newest in the list), use limit(toLast:) with ascending order:

let query = Firestore.firestore()
  .collection("rooms").document(roomId).collection("messages")
  .order(by: "timestamp")
  .limit(toLast: 30)

self.dataSource = tableView.bind(to: query) { tableView, indexPath, snapshot in
  let cell = tableView.dequeueReusableCell(withIdentifier: "reuseIdentifier",
                                           for: indexPath)
  /* populate cell */
  return cell
}

Keep queries bounded. When a query changes, FUIBatchedArray falls back to a Longest Common Subsequence (LCS) algorithm with O(N^2) complexity on the main thread to compute the diff. Unbounded queries can easily block the main thread and cause UI hangs.

FUIBatchedArray powers all of the updating logic in the data source classes by generating diffs from the document change data in Firestore query snapshot updates. The query assigned to a batched array is mutable, and may be changed while the array is observing its query. In this event, the array will compute an update by diffing the contents of the old and new query and pass an update to its delegate. This operation is relatively expensive, so try to avoid diffing large or unbounded queries.

If you're creating a more complex UI, chances are you'll have to use FUIBatchedArray directly.

let array = FUIBatchedArray(query: query, delegate: self)
array.observeQuery()

This class and its helper classes are responsible for the diffing logic in FirestoreUI. You should never have to use this directly, though all of the operations here are pure and most have no dependencies on Firestore, so if you need to diff arbitrary data you can use the functions provided here.