Understanding NSNotifications in View Controllers

Introduction

In Objective-C, notifications are a way to communicate between objects without having a direct reference to one another. The NSNotificationCenter class provides a centralized mechanism for posting and observing notifications. In this article, we’ll explore how view controllers interact with notifications and why sometimes they may not receive them.

Creating a Navigation Controller and Adding Observers

Let’s start by creating a simple navigation controller with two views: View 1 and View 2. We’ll add a UIBarButtonItem called “Add” to the navigation bar, which will post a notification when clicked.

// Create the navigation controller
self.navigationController = [[UINavigationController alloc] init];

// Add view controllers as children of the navigation controller
[self.navigationController addChildViewController:self.viewController1];
[self.navigationController addChildViewController:self.viewController2];

// Set up the add button
UIBarButtonItem *addButton = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonItemTypeAdd target:self action:@selector(addData)];
self.navigationItem.rightBarButtonItem = addButton;

In this example, we’re creating two view controllers: viewController1 and viewController2. We’ll add these view controllers as children of the navigation controller using addChildViewController:. We also create an add button with a target-action pair to post a notification when clicked.

Posting Notifications

To post a notification from the “Add” button, we use the postNotificationName:object: method of the NSNotificationCenter class.

- (IBAction)addData {
    [[NSNotificationCenter defaultCenter] postNotificationName:@"DidAddNotification" object:self];
}

In this example, we’re posting a notification with the name "DidAddNotification" and the object as itself (self). The receiver of this notification will be any object that is currently observing for this notification.

Observing Notifications

To receive notifications, we need to add ourselves as observers using addObserver:selector:name:object:. We do this in the viewDidLoad method of both view controllers.

- (void)viewDidLoad {
    [super viewDidLoad];
    
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(didPressAdd:) name:@"DidAddNotification" object:nil];
}

In this example, we’re adding ourselves as an observer for the "DidAddNotification" notification. The selector method is the one that will be called when the notification is received.

What’s Happening Behind the Scenes?

Now, let’s take a closer look at what’s happening behind the scenes when notifications are posted and received.

When you post a notification using postNotificationName:object:, the following happens:

1. The NSNotificationCenter creates a notification with the specified name and object.
2. The notification is added to the notification queue, which contains all pending notifications that need to be processed.

When an observer is added for a notification using addObserver:selector:name:object:, the following happens:

1. The observer is stored in the notification’s observer dictionary.
2. When a notification with the specified name is posted, the notification will check if there are any observers for this notification that match the object and selector.

The Problem: View Controllers Don’t Exist Forever

Now, let’s talk about why view controllers sometimes don’t exist forever and why notifications might not be received by them.

In Objective-C, objects don’t exist indefinitely. When an object is created, it exists until it’s deallocated using dealloc. This means that if you post a notification from one view controller, the receiver of this notification will only be able to receive it while that view controller still exists.

When you pop a navigation controller using popViewControllerAnimated:, the following happens:

1. The topmost view controller is removed from the stack.
2. Any notifications posted by the removed view controller are discarded.
3. The view controller is deallocated, which means its objects are no longer alive and can’t receive notifications.

Solving the Problem

So, how can we solve this problem? One solution is to use a weak reference to the observer in the notification post. A weak reference is an object that doesn’t retain another object, so it won’t affect the lifetime of the observed object.

Here’s an example:

- (IBAction)addData {
    __weak typeof(self) weakSelf = self;
    [[NSNotificationCenter defaultCenter] postNotificationName:@"DidAddNotification" object:weakSelf];
}

By using a weak reference to self, we ensure that the notification is only posted while our view controller still exists.

Another solution is to use a singleton or a manager class that observes notifications and posts them when necessary. This way, you can decouple the posting of notifications from the lifetime of specific view controllers.

Conclusion

Notifications are an important mechanism for communicating between objects in Objective-C. However, they require careful consideration of object lifetimes and observer dictionaries. By understanding how notifications work behind the scenes, you can write more efficient and effective code that takes advantage of this powerful feature.

Last modified on 2024-10-22