Question

In an iOS application I implemented some application logic in the AppDelegate.

Several view controllers etc. need to call the AppDelegate, so i placed the AppDelegate in Interface Builder and gave the ViewControllers an IBOutlet AppDelegate* and drew a link to it.

In the app I see now that there seem to be two instances of the AppDelegate created, one that is called and that gets all the notifications and one that is linked to the ViewControllers.

So i changed my app to use in the ViewControllers:

app_del = (AppDelegate*)[[UIApplication sharedApplication] delegate].

Then I’m calling this appDelegate. But this also is a different instance than the one that gets the notifications. Can anybody explain why?

Can anybody tell me what is going wrong? My suspicion from googling is that the iOS creates the AppDelegate instance and Interface Builder does not know about that one and creates another one? Is that correct?

Can anybody give me some hint on how to best create/plan instances in Interface Builder and in what order they are created in application at startup and in what notification I can rely that they are all created?

Thanks for any hints,
Torsten.

Answer 1

Your suspicion is correct. iOS creates an instance of your application delegate when launching your app, this instance is the one that is registered to receive all of the delegate events and so forth. See here.

Anything you add in to a xib or storyboard will be a new instance, and will not work.

To obtain a pointer to the application delegate, use this method:

[[UIApplication sharedApplication] delegate];

There is a school of thought that says you shouldn’t overuse the application delegate as a conveniently available global data or method store, but that is outside the scope of this answer.