Question

I’m trying to use this code but Xcode returns an error because the method I’m trying to call in the selector:@selector() is in another class. Thanks for your help!

AppDelegate.m:

-(void)applicationDidBecomeActive:(UIApplication *)application{
 [..]
        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(myMethodHere) name:UIApplicationDidBecomeActiveNotification object:nil];
}

MainViewController.m:

-(void)myMethodHere{
    [..]
}

Answer 1

Update
Add the call to the init method of MainViewController

// MainViewController.m
- (id)init;
{
  self = [super init];
  if (self) {

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

  }
  return self;
}

Make sure to remove yourself in dealloc

- (void)dealloc;
{
  [[NSNotificationCenter defaultCenter] removeObserver:self];
  [super dealloc];
}

By doing it this way from the very moment the object comes in to existence it is ready to receive notifications and then when it is being deallocated it will safely remove itself.

---

A good pattern to follow is to make the class that is doing the observing responsible for registering for notifications. This keeps encapsulation well and removes some risk of sending notification to deallocated instances.

Rationale
You need to balance your calls for registering for notifications and unregistering for notifications otherwise a message may be called on a deallocated object which could be hard to track down.

If I have a class that needs to be notified of an event the likely hood is I will register for the notifications in the init method and then unregister for the notifications in the dealloc (init and dealloc are just examples of times I often do this, not necessarily the best place in every example, do what makes sense in your case).