Example #1 DataSource for FireStore.

RestaurantTableViewDataSource.swift

//
//  Copyright (c) 2018 Google Inc.
//
//  Licensed under the Apache License, Version 2.0 (the "License");
//  you may not use this file except in compliance with the License.
//  You may obtain a copy of the License at
//
//  http://www.apache.org/licenses/LICENSE-2.0
//
//  Unless required by applicable law or agreed to in writing, software
//  distributed under the License is distributed on an "AS IS" BASIS,
//  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
//  See the License for the specific language governing permissions and
//  limitations under the License.
//

import UIKit
import FirebaseFirestore

/// A class that populates a table view using RestaurantTableViewCell cells
/// with restaurant data from a Firestore query. Consumers should update the
/// table view with new data from Firestore in the updateHandler closure.
@objc class RestaurantTableViewDataSource: NSObject, UITableViewDataSource {

    private let query: Query
    private var listener: ListenerRegistration?
    private let updateHandler: ([DocumentChange]) -> ()
    
  private var restaurants: [Restaurant] = []

  public init(query: Query,
              updateHandler: @escaping ([DocumentChange]) -> ()) {
    self.query = query
    self.updateHandler = updateHandler
  }


  // Pull data from Firestore

  /// Starts listening to the Firestore query and invoking the updateHandler.
  public func startUpdates() {
    guard listener == nil else { return }
    listener = query.addSnapshotListener { [unowned self] (querySnapshot, error) in
        guard let snapshot = querySnapshot else {
            if let error = error {
                print("Error fetching snapshot results: \(error)")
            }
            else {
                print("Unknown error fetching snapshot data")
            }
            return
        }
        
        let models = snapshot.documents.compactMap({ (document) -> Restaurant? in
            if let model = Restaurant(document: document) {
                return model
            } else {
                print("Unable to initialize Restaurant with dictionary \(document.data())")
                return nil
            }
        })
        self.restaurants = models
        
        // An array of document changes
        self.updateHandler(snapshot.documentChanges)
    }
  }

  /// Stops listening to the Firestore query. updateHandler will not be called unless startListening
  /// is called again.
  public func stopUpdates() {
    listener?.remove()
    listener = nil
  }

  /// Returns the restaurant at the given index.
  subscript(index: Int) -> Restaurant {
    return restaurants[index]
  }

  /// The number of items in the data source.
  public var count: Int {
    return restaurants.count
  }

  // MARK: - UITableViewDataSource

  func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
    return count
  }

  func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCell(withIdentifier: "RestaurantTableViewCell",
                                             for: indexPath) as! RestaurantTableViewCell
    let restaurant = restaurants[indexPath.row]
    cell.populate(restaurant: restaurant)
    return cell
  }

}