Re: Core Data
Re: Core Data
- Subject: Re: Core Data
- From: "I. Savant" <email@hidden>
- Date: Fri, 23 Mar 2007 19:24:42 -0400
On Mar 23, 2007, at 5:49 PM, Keith Penrod wrote:
I have a fairly simple question. I'm doing a Core Data app and I
have two columns where I can type in numbers and I'd like the third
column (in a table view) to be based on the first two (say a
difference or a sum or something). Is there a way to do that with
Core Data and IB or do I need to actually code that in somehow?
1: Create your own sublcass of NSManagedObject.
2: To your subclass, add a method with the name of the third
attribute that returns the correct type, then flesh it out with the
appropriate "this-is-based-on-that" code.
3: Set up proper dependency in your class using the
+setKeys:triggerChangeNotificationsForDependentKey: method so that
changes to attributes one and two trigger an update on the third,
dependent / transient attribute.
4: In your managed object model, set your entity's class to the
name of your subclass.
5: Add the third attribute to the entity, but check the
"Transient" checkbox and set the appropriate type.
6: Make sure you disallow editing in the table column since you
don't have a setter for this property.
The corresponding method in your subclass would look something
like this:
- (NSNumber *)totalNumberOfAttendees
{
int attendees = [[self numberOfAttendees] intValue];
int guests = [[self numberOfGuests] intValue];
return [NSNumber numberWithInt:(attendees + guests)];
}
You *could* implement a -setTotalNumberOfAttendees: method, but
you'd need a mechanism to figure out how many are attendees and how
many are guests, etc. This is highly dependent on your application's
logic.
Also, I've left the
+setKeys:triggerChangeNotificationsForDependentKey: code as an
exercise for the reader.
List, if I've missed missed or misstated anything, feel free to
chastise. :-)
--
I.S.
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| References: | |
| | >Core Data (From: Keith Penrod <email@hidden>) |