Question

I’m trying to update a custom property in an NSManagedObject whenever an item that it has a to-many relation to changes. I’ve overridden the following methods as prescribed by apple:

- (void)addDevicesObject:(Device *)value;
- (void)removeDevicesObject:(Device *)value;
- (void)addDevices:(NSSet *)value;
- (void)removeDevices:(NSSet *)value;

and inside the implementation I add or remove observers on the changed objects. The problem is my override methods are not called when my bindings based UI makes changes to the data. How should I go about doing this?

Answer 1

If the custom property is calculated when it is asked for, use +keyPathsForValuesAffectingValueForKey: to trigger update notifications when devices is changed.

+ (NSSet *)keyPathsForValuesAffectingValueForKey:(NSString *)key {
    if([key isEqualToString:@"MyCustomProperty"]) return [NSSet setWithObject:@"devices"];
    return [super keyPathsForValuesAffectingValueForKey:key];
}

If you want to perform calculations when devices is changed only, then use KVO to get notified when it is changed.

//Put this in the various awake... methods
[self addObserver:self forKeyPath:@"devices" options:0 context:nil];
//Put this in the didTurnIntoFault method
[self removeObserver:self forKeyPath:@"devices"];

- (void)observeValueForKeyPath:(NSString *)path ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    if(object == self && [path isEqualToString:@"devices"]) {
        //Update custom property here
    } else [super observeValueForKeyPath:path ofObject:object change:change context:context];
}