Question

I have a NSMutableArray property in my AppDelegate called blocks.
I would like to observe whenever an object is added to this array.
I’ve read other posts, but I can’t understand why this isn’t working.

In my app delegate class, I implement

- (void)insertObject:(id)obj inBlocksAtIndex:(NSInteger)index
{
    [blocks insertObject:obj atIndex:index];
}

In my view controller’s init method, I add an observer to my AppDelegate reference.

boardModel = [[UIApplication sharedApplication] delegate];
[boardModel addObserver:self forKeyPath:@"blocks" options:0 context:NULL];

In my view controller’s viewDidLoad method, I try invoking the KVO Indexed array accessor I implemented previously,

[boardModel insertObject:[[Block alloc] init] inBlocksAtIndex:0];

Then I implement my observeValueForKeyPath method:

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    if ([keyPath isEqualToString:@"blocks"])
    {
        NSLog(@"ADDED");
    }
}

I’ve tried adding an NSLog statement before the if statement in observeValueForKeyPath, and it seems as if it’s never being called.

I’ve also tried NSLogging [[boardModel blocks] count], and it says the count is 1 (the object is being added).

I must be missing something.

Answer 1

You’re observing the blocks property of the app delegate, not the blocks array itself. Hopefully the following example will make the difference clear:

// This will fire KVO as you're changing the app delegate's `blocks` property.
appDelegate.blocks = [NSMutableArray array];

// This will not fire KVO as the app delegate's `blocks` property still points
// to the same object; from the app delegate's perspective, nothing's happened.
[appDelegate.blocks addObject:@"Object"];

If you want to be notified when the contents of the blocks array changes, observe a property on the array itself—something like count. Updating your code:

[boardModel.blocks addObserver:self forKeyPath:@"count" options:0 context:NULL];