How to update an NSManagedObject whenever a specific attribute is changed?

Question

Imagine I have a Core Data object, Product. Each Product has a quantity, price, and total attribute. Total is there for efficiency when re

Answer 1

You should override the setter method for the quantity attribute of your entity:

- (void)setQuantity:(NSNumber *)quantity
{
    [self willChangeValueForKey:@"quantity"];
    [self setPrimitiveValue:quantity forKey:@"quantity"];
    [self didChangeValueForKey:@"quantity"];

    NSNumber *price = ... // compute new price
    self.price = price;
}

You can add that code to a category of the Product class if you don't want to change the Xcode generated files.