NSNotification & didReceiveMemoryWarning in iOS 6

Question

As of iOS 6.0 , viewDidUnload method is deprecated. Before iOS 6 I used to removeObserver of NSNotification in viewDidUnload method. But since it is deprecated , I have moved it to didReceiveMemoryWarning. Now if my app receives the low memory waring , notification are removed. So my code written in NSNotification doesn't work.

Can anybody tell me how can I solve this issue ?

Thanks in advance.

Answer 1

I assume that you added the observer in viewDidLoad. The problem is that on iOS 6 views are not unloaded, even in a low memory situation. Therefore, if you remove the observer in didReceiveMemoryWarning, viewDidLoad will not be called again.

There are two relative simple alternatives that you can choose from:

• Add the observer in viewWillAppear and remove it in viewWillDisappear.
• Add the observer in your initXXX method and remove it in dealloc.

I is possible to add the observer in viewDidLoad and remove it in didReceiveMemoryWarning. But then you have to "manually unload" the view in didReceiveMemoryWarning, so that viewDidLoad is later called again. See e.g. https://stackoverflow.com/a/15805715/1187415 for sample code how to forcibly unload the view.

Answer 2

The problem is you are completely misunderstanding what viewDidUnload means. viewDidUnload is called when the view is unloaded. In pre-iOS 6, when you are done with a view controller, its view does NOT get unloaded. In pre-iOS 6, 99% of the time you will never see viewDidUnload being run, because views are usually not unloaded. In pre-iOS 6, views are only unloaded in response to a memory warning and the view is not visible. In iOS 6, the only change is that views are never unloaded, even in a memory warning.

The behavior of viewDidUnload is identical in pre-iOS 6 and iOS 6 -- it is called when the view is unloaded. Because of this, you should NOT have to change any code for iOS 6. If you do, you did something wrong.

Before iOS 6 I used to removeObserver of NSNotification in viewDidUnload method.

If what you mean is that you added the observer in viewDidLoad, then you must have it removed in dealloc. Otherwise your code will crash. As I said above, 99% of the time, in pre-iOS 6, views are not unloaded. The typical flow is init -> viewDidLoad -> dealloc. The view does not get unloaded in the middle.

You can optionally also remove the observer in viewDidUnload. It is a good idea because it returns it to the state before the view was loaded. However, it is not strictly necessary, because when the view loads again, viewDidLoad will be called, and overwrite whatever state there was before anyway.

So the answer is, you should not have to change anything in response to iOS 6. Whatever you did in viewDidLoad should be undone in both viewDidUnload (optional) and dealloc (required). This is true in both iOS 6 and pre-iOS 6.