Understanding Objective-C Notifications and Delegates

When working with iOS development, one of the common patterns used for communication between objects is the use of notifications and delegates. In this article, we will explore how to use these mechanisms to achieve a specific goal: calling viewDidAppear when the app comes to foreground.

Introduction to iOS App Life Cycle

Before diving into the specifics of notifications and delegates, it’s essential to understand the iOS app life cycle. The life cycle refers to the series of methods that an iOS app goes through as it is launched, runs in the background, or is terminated. The following are some key methods in the iOS app life cycle:

• viewDidLoad: Called after the view is loaded into memory.
• viewWillAppear:bool: Called before the view is displayed.
• viewDidAppear(bool): Called after the view is displayed.
• viewWillDisappear:bool: Called before the view is about to be removed from display.
• didDisappear:bool: Called after the view has been removed from display.

Using Notifications

Notifications are a mechanism used for asynchronous communication between objects in iOS development. An object can register itself as an observer of a notification and, when the notification is posted, it will receive a call to its designated selector (method).

In our case, we want to call viewDidAppear when the app comes to foreground. We can achieve this by posting a notification with the name UIApplicationDidBecomeActiveNotification.

Adding an Observer

To post a notification and have our code executed, we need to add ourselves as observers of that notification.

[[NSNotificationCenter defaultCenter] addObserver:self 
selector:@selector(appReturnsActive) name:UIApplicationDidBecomeActiveNotification 
object:nil];

In this line of code, self refers to the current object (our app), @selector(appReturnsActive) is the method we want to call when the notification is posted, and name:UIApplicationDidBecomeActiveNotification specifies that we’re interested in notifications with the name UIApplicationDidBecomeActiveNotification.

Handling Notifications

When a notification is posted, our code will receive a call to appReturnsActive.

- (void)appReturnsActive{

    // Code here
}

However, this method does not automatically call viewDidAppear. Instead, we need to manually call it.

Manual Call to viewDidAppear

To make the notification work as desired, we should never directly call viewDidAppear from our observer method. This is because viewDidAppear is called by the system and not by us when we register ourselves as observers of a notification.

Instead, we can define a new method that calls viewDidAppear.

- (void)callViewDidAppear {

    [self viewDidAppear:YES];
}

Then, in our observer method, we call this new method instead of directly calling viewDidAppear.

- (void)appReturnsActive{

    [self callViewAppear];
}

Conclusion

In conclusion, to achieve the desired behavior of calling viewDidAppear when the app comes to foreground, we need to register ourselves as observers of the UIApplicationDidBecomeActiveNotification and then manually call a method that calls viewDidAppear.

The correct way to do this is by defining a new method that calls viewDidAppear and using this method in our observer code.

Example Use Case

Here’s an example of how you might use these concepts:

- (void)viewDidLoad {

    [super viewDidLoad];

    [[NSNotificationCenter defaultCenter] addObserver:self 
                                               selector:@selector(appReturnsActive) name:UIApplicationDidBecomeActiveNotification 
                                                 object:nil];
}

- (void)appReturnsActive{

    [self callViewAppear];
}

- (void)callViewAppear {

    [self viewDidAppear:YES];
}

In this example, we’re using the viewDidLoad method to register ourselves as observers of the notification and then calling our new callViewAppear method when the notification is posted.

By following these steps and defining a new method that calls viewDidAppear, you can achieve your desired behavior without directly calling viewDidAppear from your observer code.

Last modified on 2025-02-06