Using NotificationCenter in Swift

Overview

In Swift, to notify different parts of your application about changes, we use the NotificationCenter and the NSNotification observer pattern. This allows for a "broadcast" style of communication, where one part of your app can post a notification, and any other interested parts can listen for it and react accordingly.

In our example project, after saving data from the DetailsViewController to Core Data, we want the main ViewController to refresh its data and update the UITableView.

Reference:

• Complete iOS Development Series 1-16

Step 1: Posting a Notification

First, let's create and post a notification from the DetailsViewController right after the data has been successfully saved. We will name our notification "dataEntered". After posting, we navigate back to the main ViewController.

// In DetailsViewController, inside the save function's do-catch block

do {
    try context.save()
    print("Successfully saved data.")
    
    // Post a notification to inform other parts of the app
    NotificationCenter.default.post(name: NSNotification.Name("dataEntered"), object: nil)
    
    // Go back to the previous screen
    self.navigationController?.popViewController(animated: true)
    
} catch {
    print("Failed to save data: \(error)")
}

Step 2: Observing a Notification

Now, let's switch to the main ViewController to listen for this notification.

The best place to start observing is in the viewWillAppear(_:) method. We use this method because we want to set up the listener every time this view is about to appear on the screen.

In viewWillAppear, we add an observer to the NotificationCenter. This observer listens for a specific notification name ("dataEntered") and, when it hears it, calls a specific function (#selector(getData)).

// In ViewController.swift

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    
    // Add an observer to listen for our custom notification
    NotificationCenter.default.addObserver(self, selector: #selector(getData), name: NSNotification.Name("dataEntered"), object: nil)
}

Step 3: Responding to the Notification

When the observer "hears" the notification, it executes the @objc function we specified in the selector. In this case, it calls getData().

The getData() function contains our logic to fetch the latest data from Core Data and reload the table view, ensuring the UI is always up-to-date.

// In ViewController.swift

@objc func getData() {
    // This is the same function we created in the previous tutorial
    // to fetch data from Core Data.
    
    // ... code to fetch data and populate your arrays ...
    
    // Reload the table view to display the new data
    tableView.reloadData()
}

With this pattern, you can easily inform different parts of your application about events and trigger corresponding actions, keeping your code decoupled and organized.