Year: 2008

As we saw in the previous post, your view controller’s view can be released at any time because the device needs memory. One of the things you’ll want to look at in your own code is how you cleanup when when the memory warning occurs.

Here’s an interface for a simple view controller class that embeds a web view. The web view is retained because our controller will need to send it messages:

@interface IFWebViewController : UIViewController
{
  UIWebView *webView;
}

// webView is retained because we'll want to do things like this:
//   [self.webView loadRequest:[NSURLRequest requestWithURL:newUrl]]
@property (nonatomic, retain) UIWebView *webView;

@end

We setup and retain the web view when the controller’s loadView method is called:

- (void)loadView
{
  ...

  UIView *contentView = [[[UIView alloc] initWithFrame:applicationFrame] autorelease];
  self.webView = [[[UIWebView alloc] initWithFrame:CGRectMake(0.0f, 0.0f, applicationFrame.size.width, applicationFrame.size.height - 44.0f)] autorelease];
  [self.webView setScalesPageToFit:YES];
  [self.webView setDelegate:self];
  [contentView addSubview:self.webView];

  self.view = contentView;
}

Eventually this method will be called:

- (void)didReceiveMemoryWarning
{
  // default behavior is to release the view if it doesn't have a superview.

  // remember to clean up anything outside of this view's scope, such as
  // data cached in the class instance and other global data.
  [super didReceiveMemoryWarning];
}

The view and its sub-views are automatically released and set to nil by the default implementation. But there’s a problem here: the primary consumer of memory in this view has not been released. The web view is still being retained by the controller.

Since UIViewController doesn’t notify you that the view is being freed, you’re left holding onto a web view that you can’t do anything with since it’s not a part of the view hierarchy.

You can’t solve this problem by using a non-retained reference to the view because you’ll be left with stale reference and a crash if you try to use it. Instead, you’ll want to override the method used to set the view in UIViewController:

- (void)releaseSubviews
{
  self.webView = nil;
}

- (void)setView:(UIView *)view
{
  if (view == nil)
  {
    NSLog(@"setView: releasing subviews");
    [self releaseSubviews];
  }
  [super setView:view];
}

Of course, you’ll also want to release the retained views when the controller is freed:

- (void)dealloc
{
  [self releaseSubviews];
  [super dealloc];
}

When you’re debugging this code, make sure to force the memory warnings in the simulator. You can do this easily with the Hardware > Simulate Memory Warning menu item.

I’ve also found it helpful to poke around in the view controllers instance variables to see what’s allocated, check retain counts, etc. Since you can’t access self.view or [self.view superview] without instantiating a new view instance, you’ll need get at the private property using KVC:

- (void)didReceiveMemoryWarning
{
  // use this to look at view (without creating an instance with self.view)
  NSLog(@"didReceiveMemoryWarning: view = %@, superview = %@", [self valueForKey:@"_view"], [[self valueForKey:@"_view"] superview]);

  [super didReceiveMemoryWarning];
}

- (void)viewDidLoad
{
  NSLog(@"viewDidLoad: view = %@", self.view);
}

A lot of the complexity in this memory cleanup could be mitigated by UIViewController supporting a unloadView that is called prior to setView:nil in the default didReceiveMemoryWarning. If you agree, feel free to submit a duplicate bug on Radar ID# 5834347.

At present, there are no metrics for memory usage on the device, so there’s a bit of guesswork in determining how much of your view controller’s object graph should be released. If in doubt, free your data and let it be re-instantiated lazily. You’ll find the overall user experience to be much better.

Note: If Apple feels that sharing this information is outside of the bounds of the NDA, I’ll be forced to remove this post. So just add this to your ever growing collection of furbo PDF files.

Now that we’re beta testing and able to symbolicate our crash logs, let’s look into one of the crashes that they helped me solve in version 1.0 of Twitterrific.

The crash was happening during the posting of a notification. The backtrace looked something like this:

Program received signal:  “EXC_BAD_ACCESS”.
#0	0x300c87ec in objc_msgSend
#1	0x30675b0e in _nsnote_callback
#2	0x3025380c in _CFXNotificationPostNotification
#3	0x30673f46 in -[NSNotificationCenter postNotificationName:object:userInfo:]
#4	0x3067aa00 in -[NSNotificationCenter postNotificationName:object:]
#5	0x000029fe in -[TwitterrificTouchAppDelegate startTweetRefreshWithMessagesAndFavorites:] at TwitterrificTouchAppDelegate.m:183

The method that initiated the crash was sending IFStartTweetsNotification. This notification allows several views in the UI to reconfigure themselves while the application is refreshing.

One of those views is the toolpad that you see at the bottom of the detailed tweet view. It is a subclass of UIView and is instantiated like this:

- (id)initWithFrame:(CGRect)frame;
{
  self = [super initWithFrame:frame];
  if (self)
  {
    ...
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(refreshStarted) name:IFStartTweetsNotification object:nil];
  }
}

When the view was being freed, this code was being used:

- (void)dealloc
{
  ...
  [super dealloc];
}

I had looked at all of my code and was so convinced that this was a UIKit problem that I filed a Radar. But it turns out that it really was my fault: I had overlooked this little bit of code that gets defined when you create a new view controller:

- (void)didReceiveMemoryWarning {
 [super didReceiveMemoryWarning]; // Releases the view if it doesn't have a superview
 // Release anything that's not essential, such as cached data
}

Simple enough to understand: whenever the system determines that it is running low on memory, it starts to call these methods. The default implementation is to free a view if it’s not a part of the current view hierarchy.

With a desktop application, views are never released without an explicit action. (And it’s particularly hard to free them if they are instantiated via a NIB.) As a result, some of them us have gotten a bit sloppy with our view cleanup code. And that sloppiness is what bit me.

When my toolpad was out of view, as it is when you are looking at the main list, and a memory warning was issued, the detail view was released. Because the toolpad was a subview of that view, it was also released.

So what happens when you send a notification to an object that no longer exists? That backtrace I showed you above.

The fix was painfully easy:

- (void)dealloc
{
  [[NSNotificationCenter defaultCenter] removeObserver:self name:IFStartTweetsNotification object:nil];
  ...
  [super dealloc];
}

The lesson to be learned here: if you’re doing any kind of registration or allocation in your init methods, make sure that you have corresponding action in your dealloc method. Even if you don’t really need them on the desktop.

In my next essay, I’ll show you how to clean up memory when getting one of these warnings. It’s not as straightforward as you might think.

Note: If Apple feels that sharing this information is outside of the bounds of the NDA, I’ll be forced to remove this post. If you want, pretend like it’s 1995 and print this out instead of bookmarking it.