Suggested code to get rid of objects with no references except for their presence in a specific set

ANSession+discardUnusedResources.m

// self.resources is an NSMutableSet of ANResource objects.
// I want to remove the objects that have no other references in the app.
// (This is intended to be called in response to an iOS memory warning.)
// Will this do the trick?

- (void)discardUnusedResources {
    // We pass the objects through a weak reference; if they're not 
    // referenced anywhere else in the app, they won't survive the journey.
    NSMutableSet * newResources = [NSMutableSet new];
    NSUInteger oldResourceCount = self.resources.count;
    
    __weak ANResource * weakResource;
    while((weakResource = [self.resources anyObject])) {
        [self.resources removeObject:weakResource];
        
        __strong ANResource * strongResource = weakResource;
        if(strongResource) {
            [newResources addObject:strongResource];
        }
    }
    
    self.resources = newResources;
    NSLog(@"-[ANSession discardUnusedResources]: started with %u resources, ended with %u", oldResourceCount, self.resources.count);
}

ok how about this - use both a weak NSHashTable and a strong NSSet. When you set objects into your cache, add them to both sets. When you get memory pressure, call removeAllObjects on the NSSet. When you read from your cache, always read from the hash table. You never really need to remove objects from the NSHashTable since whether the key doesn't exist or it has been release, nil will be the result, and you don't care about exactly when weak objects will be reaped. (If you're worried about your hash table getting massive, you could just prune any nil values either periodically or on every X calls to your cache api).

No one here's heard of NSCache?

@belkadan, NSCache drops the entire cache on the floor under memory pressure. That's not what I want—it would lose references to objects that are still active. The main purpose of this set is not caching, but uniquing—ensuring that each of the external resources is represented by exactly one ANResource object. (Think of the way a Core Data managed object context always has exactly one object for each record—that's what I'm doing.) If I used NSCache, then a cache drop could cause a new object to be created for a resource which already has an old object, and the old object would slowly become staler and staler.

I'm using an NSHashTable for now, but I'm not really happy with that solution, because I want this code to run on iOS 5 as well as 6...