Question

In my project, which is written entirely with Core Data and bindings, I have two text fields which contain float values. I want to make a third text field that is the first field divided by the second. What is the best way to go about this using Core Data? Should I make a custom accessor for the third field?

Answer 1

Make a method -thirdValue in your NSManagedObject subclass that does the calculation:

- (CGFloat)thirdValue
{
  return value1 / value2; // should probably do a value2 != 0 check
}

and then +keyPathsForValuesAffectingValueForKey for the class:

+ (NSSet *)keyPathsForValuesAffectingValueForKey:(NSString *)key
{   
    NSSet *superSet = [super keyPathsForValuesAffectingValueForKey:key];

    if ([key isEqualToString:@"thirdValue"])
        return [[NSMutableSet setWithObjects:@"value1", @"value2", nil] unionSet:superSet];

    return superSet;
}

this sets up thirdValue to be dependent on value1 and value2. Whenever value1 or value2 change, there will also be a KVO notification for thirdValue. So if you have a field that binds to it, the field will display the new calculated value. It’s a a handy trick when you need to display a key who’s value is dependent on other keys.