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Quis custodiet ipsos custodes? Going Heisenberg on iOS' NSNotificationCenters

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You may think your observers are well behaved, but are they? With modern block-based NSNotificationCenters, you may be leaking without ever realizing it. Erica Sadun shows how to clean things up.
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Mea culpa. I have been abusing and misusing iOS’s block-based notification handlers repeatedly. For years I’ve been working under the mistaken impression they operated like the classic target-action version. That is, I assumed I could clean up after myself in dealloc by removing myself as an observer. I was wrong. The following dealloc approach won’t work for these handlers.

- (void) dealloc
{
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}

If you’re playing along here, please feel free to make rude buzzing noises or squirt me with virtual seltzer water. This approach is perfectly fine for classic notifications, namely the ones you create with target-action. In the old-style system, you supplied a selector and a notification name and you implemented the handler in a separate method.

[[NSNotificationCenter defaultCenter] addObserver:self 
    selector:@selector(handleNotification:) 
    name:kNotificationName object:nil];

With blocks, however, you are not the observer. The observer is built on your behalf as a new object. If you’re calling the following method without catching the new observer, you’re leaving a trail of orphaned handlers behind you.

id observer = [[NSNotificationCenter defaultCenter] 
    addObserverForName:kNotificationName object:nil 
    queue:[NSOperationQueue mainQueue] 
    usingBlock:^(NSNotification *note) {
        // implementation here
    }];

Cleaning Up Observers

As with old-style handlers, it’s up to you to remove observers from the notification center when your object deallocates. I’ve started using a mutable observers array, adding each newly established observer to it whenever I create a notification handler. This array enables me to iteratively unload observers in my dealloc method, as simply as I used to remove myself as an observer:

- (void) dealloc
{
    for (id observer in observers)
        [[NSNotificationCenter defaultCenter] removeObserver:observer];
}

The observers array offers a simple and reliable approach for handlers that persist throughout a view controller’s lifetime. I establish observers, typically in viewDidLoad or loadView, and they function until the controller is dismissed. You’ll read about short-lived observers later in this write-up.

Working with Blocks

Using a block-based approach for notification handlers provides fundamental advantages. It encapsulates the notification handler implementation at the physical location where the notification observer is defined. That union simplifies both code design and review. Should you need to update or even remove functionality, all elements of the notification and its handler are centralized to a single position in your code.

Being block-based however, comes with costs. For one thing, you must take care not to create self-referential retain cycles. Any reference to self, whether directly stated or through property use (as in Figure 1) can prove problematic. This kind of development overhead isn’t an issue when using the older target-action approach.  Prepare to change your best practices when applying blocks.

Figure 1 Beware of block-based retain cycles

Although Xcode is getting better and better at spotting block issues (as in Figure 1), block hygiene still rests on your shoulders. Here are the key rules for working with notification handler blocks.

  • To avoid retain cycles, establish a weak self reference outside any block that uses self in its code.
__weak typeof(self) weakSelf = self
  • Create a strong reference to that weak version as the first line in your block. If self still exists when the block starts to execute and hasn’t fallen back to nil, this next line ensures it persists throughout the block’s execution lifetime.
__strong typeof(self) strongSelf = weakSelf;
  • Translate all direct and indirect self references to strongSelf. Instead of calling [self method], use [strongSelf method]. For properties avoid using calls like the _originalTransform you saw in Figure 1. Replace these with strongSelf.origialTransform dot notation.

Short-Term Observers

Many notification observers persist for long periods of time. Most are intended to provide background handlers for external events, so it makes sense to keep them active for as long as your object is around to handle them.

In other cases, you’ll want to remove the observer as soon as some event triggers. For example, you might enable some user-directed event and then respond to it by listening for a notification. Using notifications enables you to avoid using cross-class dependencies. For example, you might fire a notification when a gesture finishes recognition or when a dynamic animation completes.

After the handler block executes, clear the observer and its associated block from memory by cleaning up the trigger-then-dispose mechanism. This approach ensures stale handlers won’t repeatedly fire and prevents lingering handlers from producing extraneous results when you least expect them.

With this approach, as with long-term listeners, expect block-based complications. In this case, the challenge lies in ensuring you properly remove your observer. As you’ll see, transferring the observer reference to block code requires finesse. The following snippet listens for the end of a dynamic animator sequence. When complete, it re-enables the right bar button and removes a temporary observer from the notification center.

// Declare the temporary observer as a __block variable
id __block temporaryObserver = nil;

// Establish the weak self reference
__weak typeof(self) weakSelf = self;

// Create the notification handler
temporaryObserver = [[NSNotificationCenter defaultCenter] 
    addObserverForName:AnimationsDidPause 
    object:nil queue:[NSOperationQueue mainQueue] 
    usingBlock:^(NSNotification *note) {

    // Establish the strong self reference
    __strong typeof(self) strongSelf = weakSelf;

    // Perform the work (re-establish the bar button)
    strongSelf.navigationItem.rightBarButtonItem.enabled = YES;

    // Remove the observer
    [[NSNotificationCenter defaultCenter] removeObserver:temporaryObserver];
}];

Any variable marked by the __block storage attribute is shared between the calling method and the block that uses it. Here it’s vital to create your observer with that __block variable storage. This approach enables the block to use the value assigned to the temporaryObserver variable after the block has been established.

When running the previous snippet, the block is created first in the steps to establish the notification handler. Unless otherwise specified, it uses the initial value of the temporaryObserver variable. This is set to nil. Next, the block is added to the notification center. Only then is the new observer returned, to be stored into temporaryObserver.

Without the __block attribute, the value is used as-is, that is, as nil. By enabling the block to access the variable storage, you permit its code to see the value assigned after the add-observer method returns and whenever the handler executes.

You can best see this in action by logging your observer in the block. When using the __block storage type, the observer is properly established:

2014-03-24 10:44:54.382 Hello World[8800:60b] Removing observer <__NSObserver: 0x9b63ae0>

But without that attribute, you’ll inadvertently leak. The temporaryObserver remains nil and is never removed from the notification center.

2014-03-24 10:50:35.439 Hello World[8873:60b] Removing observer (null)

Wrapping Up

Never be afraid to use block-style notification handlers. Although you’ll encounter extra overhead, it’s not hard to apply best practices. The lessons here are simple. Here’s a quick summary of tips from this write-up:

  • When using block-based notification handlers, avoid self-referential retain cycles. Apply these to both explicit and implicit self references.
  • Keep track of all observer objects created by the notification center.
  • Release long-term observers in dealloc. Release short-term observers in the handler block via __block variable references.
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