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2013-06-28T18:43:18.130
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There's a couple of reasons why the notification for <code>count</code> isn't firing, both having to do with Key-Value Coding. The first reason is because the <code>count</code> property is not updated when calling <code>addTask:</code>. There isn't a direct link between the <code>count</code> property and the count of the _tasks array. Any code manipulating the array has to be wrapped between <code>-willChangeValueForKey:</code> and <code>-didChangeValueForKey:</code> calls. However, the KVO protocol provides the ability to set dependent keys. Since the count property is affected by the array, you can set a dependency between these two keys by using <pre><code>+ (NSSet*) keyPathsForValuesAffectingCount { return [NSSet setWithObject:@"tasks"]; } </code></pre> Or the more generic <code>+ keyPathsForValuesAffectingValueForKey:(NSString *)key</code>. Implementing either will fire notifications for <code>count</code> when <code>tasks</code> is modified. The second reason why this won't work, is because you're not updating the <code>tasks</code> array in a Key-Value compliant way as defined in the <a href="https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/KeyValueCoding/Articles/AccessorConventions.html#//apple_ref/doc/uid/20002174-BAJEAIEE" rel="nofollow">Key-Value Coding Programming Guide</a>. Basically, objects in ordered collections have to be added with either <pre><code>-insertObject:in<Key>AtIndex: -insert<Key>:atIndexes: </code></pre> Or be wrapped with the <code>will|didChangeValueForKey:</code> calls. In your example, if you provide the following implementation after setting the key dependency, you should observe a change. <pre><code>- (void)addTask:(JHTask *)task { [self insertObject:task inTasksAtIndex:[_tasks count]]; } </code></pre>
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1. COIdeal keypath that affects the count would be `"tasks.count"`, but **1.** he does not have `tasks` property, just a simple ivar and **2.** count of array is not observable. But in general your answer is the best approach to specify KVO dependency.
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