KVO or custom accessor methods? - ios

Question

I got few entities for an iOS App which are linked by relations. To make a simple example, lets assume we got a relation like this (one-to-many):
company <--->> person
I'm using xcode 4.6 and the core data modelling tool for model generation, so I end up with
//Company.h
#class Person;
#interface Company : NSManagedObject
#property (nonatomic, retain) NSString * company_name;
#property (nonatomic, retain) NSNumber *has_changed;
#property (nonatomic, retain) NSSet *person;
#end
#interface Company (CoreDataGeneratedAccessors)
- (void)addPersonObject:(Person *)value;
- (void)removePersonObject:(Person *)value;
- (void)addPerson:(NSSet *)values;
- (void)removePerson:(NSSet *)values;
#end
//Company.m
#import "Company.h"
#import "Person.h"
#implementation Company
#dynamic company_name;
#dynamic has_changed;
#dynamic person;
#end
And
//Person.h
#class Company;
#interface Person : NSManagedObject
#property (nonatomic, retain) NSString * first_name;
#property (nonatomic, retain) NSString * last_name;
#property (nonatomic, retain) Company *company;
#end
//Person.m
#import "Person.h"
#import "Company.h"
#implementation Person
#dynamic first_name;
#dynamic last_name;
#dynamic company;
#end
Now, suppose I want to set the boolean (implemented as NSNumber in Core Data) attribute has_changed to TRUE every time one of the following occurs:
a attribute of the company entity is changed, except the has_changed attribute itself of course (since this would cause a loop)
a attribute of the person entity is changed
What would be the best (easy to implement + fast to process) way to implement something like this? For what I was able to find out, is that there are two options:
Key value observing (KVO)
custom method accessors
However, everything I find related to this topic seems to be outdated because of the changes in objective-c 2.0, core-data / cocoa or iOS. For example, automatically generating the accessor methods by using the core-data modelling editor dosn't seem to work when using xcode 4.6 with ARC enabled, since everything I get pasted are the #dynamic lines which core-data generates anyways (see code example above).
Also I find it a bit confusing what the documentation says. What would be the best way to implement this? KVO? Custom accessors? Bit of both or even something compleatly different? And how would a possible implementation look like for the given example?

Answer 1

You could do this as such:
#implementation Company
// ...
- (void) didChangeValueForKey: (NSString *) key
{
[super didChangeValueForKey: key];
if (! [key isEqualToString: #"has_changed"])
self.has_changed = YES;
}
// ...
#end
and similar for Person though the company property. You'd also want to implement didChangeValueForKey:withSetMutation:usingObjects:
Note that this suggestion does not address the common need of having a controller know about a change. There are other ways to do that.