Call Selector method from another Class - NSNotificationCenter - iphone

Question

I would like to know how I can call the selector which is in another class when notification is posted.I am on tabbarcontroller.
The FirstViewController, SecondViewController are tab bar items
Inside `FirstViewController` I have the following
-(void)viewdidload
{
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(productPurchased:) name:kProductPurchasedNotification object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self selector: #selector(productPurchaseFailed:) name:kProductPurchaseFailedNotification object: nil];
}
- (void)productPurchased:(NSNotification *)notification {
[NSObject cancelPreviousPerformRequestsWithTarget:self];
NSString *productIdentifier = (NSString *) notification.object;
NSLog(#"Purchased: %#", productIdentifier);
[appDelegate.myDownloadablePoemsArray addObject:productIdentifier];
[self.tabBarController setSelectedIndex:3];
}
- (void)productPurchaseFailed:(NSNotification *)notification {
[NSObject cancelPreviousPerformRequestsWithTarget:self];
SKPaymentTransaction * transaction = (SKPaymentTransaction *) notification.object;
if (transaction.error.code != SKErrorPaymentCancelled) {
UIAlertView *alert = [[[UIAlertView alloc] initWithTitle:#"Error!"
message:transaction.error.localizedDescription
delegate:nil
cancelButtonTitle:nil
otherButtonTitles:#"OK", nil] autorelease];
[alert show];
}
}
The above code is working fine. Now what the issue is, I want to call the same selector method from my another view say for example I have a view controller named SecondViewController, in that I am adding the same notification observer.
but the selector method is not called in the FirstViewController.
Inside SecondViewController I have the following
-(void)viewdidload
{
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(productPurchased:) name:kProductPurchasedNotification object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self selector: #selector(productPurchaseFailed:) name:kProductPurchaseFailedNotification object: nil];
}
But I want to call the selecor methods from FirstViewController;
Please let me know , is that possible ? And how can I do that?
Thanks a lot

Answer 1

in the SecondViewController change the self as observer to the pointer of the FirstViewController, because the instance of FirsViewController has the methods.
inside SecondViewController.m you must use these lines:
- (void)viewdidload {
[[NSNotificationCenter defaultCenter] addObserver:firstViewController selector:#selector(productPurchased:) name:kProductPurchasedNotification object:nil];
[[NSNotificationCenter defaultCenter] addObserver:firstViewController selector: #selector(productPurchaseFailed:) name:kProductPurchaseFailedNotification object: nil];
}
BUT! AND THIS IS THE POINT.
if the FirstViewController is already a valid and loaded view controller in the memory with the methods as you've mentioned above, and it is an observer already for these notifications in the NSNotificatioCenter, you don't need to add again it to the NSNotificationCenter because the FirstViewController can receive and will receive the desired notification still. (it is just not shown, because an other view controller covers it.)
if the FirstViewController is not exists yet when the SecondViewController is, you cannot reach any instance method called from an another class because the FirstViewController was not instantiated before, and you cannot add it to the NSNotificationCenter as well.
CONCLUSION
it would be better to isolate the purchase callbacks into a third class what you can use for every independent view controller, according to the spirit of the OOP and MVC.

Answer 2

If your view controllers are the roots of tab-bar controllers, once they are loaded the first time, they stay around unless manually replaced.
Thus, when you install the notification handler in the first controller, unless you remove the notification handler, it will still get them, even when the second controller is onscreen.
Now, it may get unloaded due to memory pressure, or by the custom tab-bar-controller code. However, it's highly unusual for a tab-bar controller to deallocate one of its view controllers, so your installed notification handlers will be around until you cancel them.
In fact, if both view controllers register for the notifications, then they will both get them.
You are registering in viewDidLoad so the first one will get registered immediately, as it will be loaded and displayed as the initial controller. It will continue to receive those notifications.
When the second one loads, it will also register. Now both view controllers are receiving the notifications. When you go back to the first view controller, they will both still be getting the notifications.