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POCore Data: Emitting KVO notifications for transient, derived properties
 

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  1. POCore Data: Emitting KVO notifications for transient, derived properties
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    <p>I have <code>Parent</code> entity with a custom class that has a transient and derived (read only) property called <code>DerivedProperty</code>.</p> <p>The value of <code>DerivedProperty</code> is dependent on the value of <code>Parent.IndependentProperty1</code> and so whenever <code>IndependentProperty1</code> changes, the value of <code>DerivedProperty</code> will change. However, <code>Parent</code> has a to-many relationship to <code>Child</code> (called <code>children</code>) and <code>DerivedProperty</code> is also dependent on the value of <code>IndependentProperty2</code> in all of <code>Parent</code>'s <code>Child</code> objects.</p> <p>So whenever <code>IndependentProperty1</code> of <code>Parent</code> or <code>IndependentProperty2</code> of any of the <code>Child</code> objects changes, I would like to notify any observers that <code>DerivedProperty</code> has also changed.</p> <p>I've arrived at the following code so far. The only problem is that no KVO notifications are emitted for <code>DerivedProperty</code> since if I try to do this in <code>objectContextDidChange:</code> then the code will end up in a loop.</p> <pre><code>- (void) awakeFromInsert { [super awakeFromInsert]; [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(objectContextDidChange:) name:NSManagedObjectContextObjectsDidChangeNotification object:self.managedObjectContext]; } - (void) awakeFromFetch { [super awakeFromFetch]; [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(objectContextDidChange:) name:NSManagedObjectContextObjectsDidChangeNotification object:self.managedObjectContext]; } - (void) objectContextDidChange: (NSNotification *) notification { NSSet *updatedObjects = [[notification userInfo] objectForKey:NSUpdatedObjectsKey]; if ([updatedObjects containsObject:self] || [updatedObjects intersectsSet:self.children]) { //clear caches _derivedProperty = nil; } } - (void) didTurnIntoFault { [[NSNotificationCenter defaultCenter] removeObserver:self]; } - (NSString *) DerivedProperty { if (_derivedProperty == nil) { _derivedProperty = [self someExpensiveCalculation]; } return _derivedProperty ; } </code></pre> <p>I'm sure I need a total rethink on my approach here. I've tried using KVO to observe <code>IndependentProperty1</code> and <code>IndependentProperty2</code> of the to-many relation but I just can't seem to get it working right. If the derived property wasn't dependent on a to-many relationship then I'm sure I could just use <code>keyPathsForValuesAffectingValueForKey:</code> but of course that won't work across a to-many relationship.</p> <p>How to I get KVO notifications working with a Core Data transient, derived property that is dependent on a to-many relationship?</p>
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    1. COI cannot test this at the moment, but what about working the other way around: The setter function for `IndependentProperty2` of the Children entity calls a method on its parent that computes and sets the `DerivedProperty` ?
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    2. COThat would work but it doesn't play nice with my domain model. The `Child` objects shouldn't know anything about their `Parent` nevermind be responsible for updating it. I might end up having to go down this route though.
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