How does iOS decide which objects get sent didReceiveMemoryWarning message?

Question

I am working on an iPhone application in which a number of UIViews are dynamically added to and removed from the main UIWindow.

When simulating low memory errors in the simulator, I have discovered that not all view controllers receive the didReceiveMemoryWarning notification. Unfortunately, these are the controllers that would most benefit from implementing this method.

I cannot seem to find good information about where and how the method gets called. I have read mentions that it gets sent to "all UIViewControllers", but this is evidently not the case. Adding a breakpoint in one of the classes that do receive the notification wasn't particularly enlightening either.

This is a complex project but one way these views get added is:

- (void) showMyView
{
  if(!myViewController){
    myViewController = [[MyViewController alloc]init];
    [window addSubview:myViewController.view];
  }
}

MyViewController is a subclass of another class, MySuperViewController, which is itself a subclass of UIViewController. None of those classes have corresponding NIBs; view hierarchies are created programatically.

I am looking for pointers to how I can go about diagnosing the problem.

Answer 1

When you are using .view of the view controller directly, there's a high chance that your view controller won't receive many notifications because it's not the correct way of using view controller. The UIWindow is special case, because the window can automagically know the controller of the view and direct the message to the controller correctly.

However, when you detach your view from UIWindow, the view controller is also detached and not managed by UIWindow any more. I think this is the source of the problem.

I would suggest that you add a navigation controller or tab bar controller as your root view controller, and use that view controller functionality to switch between your child controllers. Note that you should not remove your view controllers when switching so they will be able to receive the messages appropriately.

You might also considering releasing your view controller when not used if initialization of your view controller is trivial and not consuming too much time.

Answer 2

Somewhere in your code you are probably doing something like this:

[[NSNotificationCenter defaultCenter] removeObserver:self];

The only safe place to do this is in -dealloc.

Everywhere else, you should specify the notification that you want to unregister for (this will still potentially break if you register for the same notification as a superlcass).