Update previously pushed view controllers via delegation IOS6 - ios

Question

I'm still getting the hang of IOS delegation, so I hope this question makes sense as I explain what I want to do...
What I want to do is download some JSON Data on a background thread as soon as my app first runs. The data will then be parsed and then update a global variable. Once that happens I want all previously pushed view controllers to update their content based on the data that has been downloaded and parsed.
My proposed way of solving this problem would be to have either my app delegate or my custom Navigation Controller subclass be a delegate for a custom JSON object that will parse the data. The delegate will run a protocol method that updates a global variable once the parsing is complete.
Now once this variable has been set, it will be available to any view controller that is pushed on to the navigation stack. I also want to update the view controllers that have been previously pushed so that their content can be updated.
I know I can make the top view controller an active delegate that will run a protocol but what about the previously pushed view controllers? Is this even possible or is there another way to make previously pushed view controller update their content?

Answer 1

In this case you don't really want to use a delegate. You want several objects to listen to a specific event, so use NSNotificationCenter instead.
When your parser finished parsing the JSON do the following:
[[NSNotificationCenter defaultCenter] postNotificationName: #"FinishedDataParsing" object:self userInfo:nil;
This way you also won't need a global variable. You could either make your parsed data accessable in your parser object or use the userInfo dictionary to pass some information to the notification receiver.
Everywhere you want to do something when your parsing finishes you first have to register as an observer (you could do that in viewDidLoad):
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(dataParsed:)
name:#"FinishedDataParsing"
object:nil];
And obviously you have to implement your callback method to do what ever you want to do with your parsed data.
- (void)dataParsed:(NSNotification *)notification {
// Do this to access the user info.
NSDictionary *userInfo = notification.userInfo;
// Or access your data parser object.
DataParser *parser = (DataParser *)notification.object;
}
Also you should deregister as observer when you no longer need to get notified (e.g. in dealloc)
- (void) dealloc
{
[[NSNotificationCenter defaultCenter] removeObserver:self];
[super dealloc];
}