Since my previous post, I have been able to get the functionality I
was hoping for with the following code in my "Department" subclass. I
would greatly appreciate some feedback as to whether this is an
appropriate way to implement the functionality or if there is a more
efficient or cleaner way.
- (void)awakeFromFetch {
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(managedObjectContextDidChange:) name:
NSManagedObjectContextObjectsDidChangeNotification object:[self
managedObjectContext]];
}
- (void)awakeFromInsert {
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(managedObjectContextDidChange:) name:
NSManagedObjectContextObjectsDidChangeNotification object:[self
managedObjectContext]];
}
- (void)managedObjectContextDidChange:(NSNotification *)notification {
// Get a set containing ALL objects which have been changed
NSSet *insertedObjects = [[notification userInfo]
objectForKey:NSInsertedObjectsKey];
NSSet *updatedObjects = [[notification userInfo]
objectForKey:NSUpdatedObjectsKey];
NSSet *deletedObjects = [[notification userInfo]
objectForKey:NSDeletedObjectsKey];
//this method of gathering changed objects is because the final set
was always null if the first set was null
NSSet *changedObjects;
if([insertedObjects count] > 0){
changedObjects = [insertedObjects
setByAddingObjectsFromSet:updatedObjects];
changedObjects = [changedObjects
setByAddingObjectsFromSet:deletedObjects];
}else{
if([updatedObjects count] > 0){
changedObjects = [updatedObjects
setByAddingObjectsFromSet:deletedObjects];
}else{
changedObjects = [NSSet setWithSet:deletedObjects];
}
}
if([changedObjects intersectsSet:[self employees]]){
//if an employee in this department changed, indicate
that the totalSalary attribute should be refreshed
[self willChangeValueForKey:@"totalSalary"];
[self didChangeValueForKey:@"totalSalary"];
}
}
Thank you,
John
On Mon, Oct 5, 2009 at 1:18 PM, John McIntosh
<johntmcint...@googlemail.com> wrote:
> Hi all.
> I am working on a Core Data application which contains an entity that
> needs to be notified of changes in entities that it has a relationship
> to. Let's talk about this in terms of Apple's example with Departments
> and Employees. As described here
> (http://developer.apple.com/mac/library/DOCUMENTATION/Cocoa/Conceptual/KeyValueObserving/Concepts/DependentKeys.html)
> they talk about the example where each Department has a "totalSalary"
> attribute and each Employee has a "salary" attribute.
>
> In regards to Core Data, the documentation linked above says this:
> "If you're using Core Data, you can register the parent with the
> application's notification center as an observer of its managed object
> context. The parent should respond to relevant change notifications
> posted by the children in a manner similar to that for key-value
> observing."
>
> I assume this means that something similar to the code posted
> previously in the article should be done. I understand the statement
> conceptually, but do not know how to go about implementing it. Here is
> my start in the Department entity.
>
> - (void) awakeFromInsert{
> [[NSNotificationCenter defaultCenter] addObserver:self
> selector:@selector(methodToCallOnNotification:) name:???
> object:managedObjectContext];
> }
>
> If this is the correct start, what should happen in
> methodToCallOnNotification?
> Do I need to remove each Department entity from being an observer when
> it is deleted?
> What else do I need to be sure to do upon Department or Employee
> creation or deletion?
>
> Thank you,
>
> John
>
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