Let's build a countdown app using diffable data source.

Our countdown app will start from 10 and decrement the initial value by 1 every second and add the new value as a row in the table view. Throughout the app we will make use of the snapshot as we update the table view and apply the changes.

1. Create an Xcode project named Countdown.
2. Navigate to the ViewController.swift file and add the following:

enum Section {
  case main 
}

private var tableView: UITableView! 
private var dataSource: UITableViewDiffableDataSource<Section, Int>! 
private var timer: Timer! 
private var startInterval = 10 // seconds 

3. Configure the table view.
4. Configure the data source.
5. Configure the timer.
6. Add the startCountdown method.
7. Add the decrementCounter method.
8. Add the ship method.
9. Add a refresh bar button item to restart countdown.