KVO With NSMutableArray - objective-c

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

The catch is that NSArrays don't respect KVO, so observing the key path count won't work.
If this is MacOSX, use NSArrayController. otherwise implement a wrapper class for the array that triggers the KVO calls when adding/removing contents of the array, and passes across all other calls.

Answer 2

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];

Answer 3

Did you try
- (void)insertObject:(id)obj inBlocksAtIndex:(NSInteger)index
{
[[self mutableArrayValueForKey:#"blocks"] insertObject:obj atIndex:index];
}

Answer 4

I just open-sourced a very small Objective-C library that adds a delegate to NSMutableArray. It might help you achieve what you were trying to do. Check out FCMutableArray on GitHub

Answer 5

It's not that NSMutableArray is not KVO-compliant in some respects, it's that all KVO-compliant properties must be NSObjects. "count" is an NSUInteger; you can't observe that property directly. If it was an NSNumber object, you could.
From the NSArray header file:
#interface NSArray<__covariant ObjectType> : NSObject <NSCopying, NSMutableCopying, NSSecureCoding, NSFastEnumeration>
#property (readonly) NSUInteger count;
Why was the answer accepted, by the way? It's clear the answerer did not test his answer.