objective c - NSMutableDictionary KVO

Question

I'm trying to observe changes in dictionary using KVO.

Example:

dictionary = [NSMutableDictionary new];
[dictionary setObject:@"test1" forKey:@"key1"];
[dictionary setObject:@"test2" forKey:@"key2"];    
[dictionary setObject:@"test3" forKey:@"key1"];

I'd love to be able to hook an observer for whenever a value is added to the dictionary. removed, or replaced (ie in the above cases, whenever any of the setObject methods are called)

So in conclusion: I want a function to have

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context

called when I ADD a any new entry to a dictionary, or Remove any entry, or REPLACE any entry.

NOT: I do NOT want to have to specify which keys I'm observing for. (eg observe only when @"key1" is added) as this solution doesn't scale.

Answer 1

Subclassing NSMutableDictionary is a bit annoying, due to the fact that NSDictionary and its friends are class clusters. It's certainly doable, and if you have to pass the object itself to another set of classes, then you may want to do exactly that. Otherwise, it might be easier to create a composite class which has the same basic API and uses NSMutableDictionary object internally for storage. There's a pretty good write-up as CocoaWithLove.com, Ordered Dictionary Subclassing, which goes into doing this.

However, that doesn't completely solve your problem. What I would suggest is that you begin with a subclass or decorator class such as the one above, then add support explicitly for -(NSArray*)allKeys, which is a standard accessor in NSDictionary itself. Then, you can add support to pass along change messages for allKeys, which will make it observable.

This can be done by adding the following code around the -setObject:forKey: and -removeObjectForKey: methods.

- (void)setObject:(id)anObject forKey:(id)aKey
{
    BOOL addKey=NO;
    if (![dictionary objectForKey: aKey]) {
        addKey=YES;
        [self willChangeValueForKey: @"allKeys"];   
    }
    [dictionary setObject:anObject forKey:aKey];
    if (addKey)
        [self didChangeValueForKey: @"allKeys"];
}

- (void)removeObjectForKey:(id)aKey
{
    [self willChangeValueForKey: @"allKeys"];
    [dictionary removeObjectForKey:aKey];
    [self didChangeValueForKey: @"allKeys"];
}

What is being done here is that we're adding explicit KVO notification to the class when the dictionary's keys are changed to mark a change in the array.

This will take care of adds and removes. If you want changes to be notified on the same basis, you can remove the if statements, and just have allKeys notify on either set or remove, like this:

- (void)setObject:(id)anObject forKey:(id)aKey
{
    [self willChangeValueForKey: @"allKeys"];   
    [dictionary setObject:anObject forKey:aKey];
    [self didChangeValueForKey: @"allKeys"];
}

Then, in your code, you put in a single observer for the key @"allKeys" on this object and you'll be receiving notifications whenever an item changes.