iPhone Development - Simulate Memory Warning

Question

Background:

I have a tab bar application. Each tab contains navigation controller allowing the user to transition from one view to the other showing a drill down information of the data (each view is being handled by a view controller and each view controller class has didReceiveMemoryWarning method). Lists are populated by pulling the data from web services.

Problem:

When i use "Hardware > Simulate Memory Warning" option of iPhone Simulator, the didReceiveMemoryWarning method is called for ALL my view controllers - even the one which the user is viewing. I don't want to clear any content which is being used by the active view controller. How can I achieve that?

Which method should have the implementation to reload the data after the data was released because of memory warning? (I see that the view controller classes that contain a table view call viewDidLoad method when user comes back to that view, but if the view contains (say UIWebView) then viewDidLoad method is not called. Why is that?)

Edited (Friday 30 January 2009 - 03:10 PM)

(Note: I'm using Interface builder for creating views, and loadView method is commented out.)

So, when a view controller receives a memory warning message, these are the steps that are carried out:

1. Following method is called:

   - (void)didReceiveMemoryWarning {
       [super didReceiveMemoryWarning]; 
   }
   

2. As a result of call to [super didReceiveMemoryWarning], [self setView:nil] gets automatically called?

3. If any resources should be cleared, then setView method should be overwritten to clear local resources.

4. [self setView:nil] is not called if the view is currently active (By default). Right? - I'm really curious which method takes this decision and how?

Can you please confirm. Plus, I was getting an error following this approach but adding myObject = nil after releasing myObject in dealloc method of controller class fixed the issue. Thanks.

Answer 1

This is an old question, but I don't see a proper answer, so here goes:

When a memory warning is received, -didReceiveMemoryWarning gets called in ALL view controllers, whether they are the "current" one or not. The view controllers are simply listening for the memory warning event broadcast.

If the view controller's view isn't being used at the time of the memory warning, the controller will unload it by setting the property to nil. How does it know if the the view is used? By the view's -superview property. If view.superview is nil, the view isn't part of any tree and can be unloaded safely.

Once that happens, the controller's -viewDidUnload gets called. This is the correct place to unload any outlets, and anything that will get re-created in -viewDidLoad.

---

So what is -didReceiveMemoryWarning for? Your controller might have objects that don't get instanced until accessed. For example, you could have a controller that sometimes needs a big chunk of data from a file, but not always. You could have a property set for it like this:

- (NSData*)bigChunkOfData {
  // Get data from our instance variable _data, read from disk if necessary
  if (_data == nil) {
    _data = [[NSData alloc] initWithContentsOfFile:@"/path/to/data"];
  }
  return _data;
}

This will read the data from disk this first time, then keep it in an instance variable. Since the _data variable is created on demand, it's safe for us to unloaded it in low-memory situations: it'll just get created again next time we need it.

- (void)didReceiveMemoryWarning {
  [super didReceiveMemoryWarning];

  [_data release];
  _data = nil;  // <-- Very important: don't leave strong references dangling.
}

Answer 2

I do my clean up like this:

-(void)setView:(UIView*)view
{
    [super setView:view];
    if(view == nil)
    {
       // Our view has been cleared, therefore we should clean up everything 
       // we are not currently using
....

setView:nil is called by UIViewController in response to a memory warning, if that view is not currently visible - which is basically what you want to know.

EDITED

In answer to the follow ups:

1. Correct.
2. That's what I do, and it works for me.

3. Correct. The implementation of didReceiveMemoryWarning in UIViewController is what does this. If you don't override didReceiveMemoryWarning, then the base class implementation in UIViewController will be called - if you do override it, obviously you should call:

   [super didReceiveMemoryWarning]