Question

I have a navigation based iPhone application. in the Middle of the application sometimes i am receiving the memory warning level 1 and 2. Suddenly all the values i fetched from database and created based on the user input in root viewcontroller including the values i saved in the nsuserdefaults losed totally. When i try come back to the root view controller i have empty table view without any data.

How to handle this issue, Please suggest any solution for this problem?

Answer 1

This is how it works, "unfortunately". When the didReceiveMemoryWarning comes in, your views are released and this explain why you don’t find any value in the table.

What one should do is be prepared to create again all the views, according to Apple Docs.

The default implementation of this method checks to see if the view controller can safely release its view. This is possible if the view itself does not have a superview and can be reloaded either from a nib file or using a custom loadView method. If the view can be released, this method releases it and calls the viewDidUnload method.

You can override this method (as needed) to release any additional memory used by your view controller. If you do, be sure to call the super implementation at some point to allow the view controller to release its view. In iOS 3.0 and later, if your view controller holds references to objects in the view hierarchy, you should release those references in the viewDidUnload method instead. In earlier versions of iOS, you should continue to release them from this method. See the discussion in the viewDidUnload method for information about how to safely release outlets and other objects.

If you want to implement those guidelines, a way to go is:

1. use viewDidLoad/viewDidUnload to "save/restore" your data;

2. store your data in a model independent from your views;

3. in viewDidUnload, called after a warning`, store in your controller enough information about the view state (i.e., if it is displaying data about a customer, his customer/ID), so that you can recreate that information by getting the data back from the model;

4. in viewDidLoad, called also when the view is restored after a memory warning, use the information that you saved in your view controller (if it is there), to recreate the view in the exact state where it was left (otherwise, put it in a default state).

This should make it pretty easy.

One trick that you could deploy if it makes sense to you is the following:

1. override -didReceiveMemoryWarning in your view controllers;

2. for any view that you really don’t want to be automatically released, don’t call [super didReceiveMemoryWarning] in your -didReceiveMemoryWarning override.

Beware, this will make your life easier now, but will also defeat an important mechanism Apple has put in there to recover memory. This will not produce a crash in itself, but could make your device memory to fill up and your app be closed abruptly if you manage many views or your views are memory intensive (lots of graphics, etc.). So, your judgement here…