Derived Dependency

Intro

Adds dependency key paths for to-many relationships.

As you know Apple's support for dependency paths in KVO does not include to-many relationships. The following example for calculating the total price of an invoice will not work out of the box.

- (NSDecimalNumber *)totalPrice {
	return [items valueForKeyPath: @"@sum.totalPrice"];
}

+ (NSSet *)keyPathsForValuesAffectingTotalPrice {
	return [NSSet setWithObject: @"items.totalPrice"];
}

This will not work because items is a to-many relation. Adding the classes beginning with INDerived* from this project will add the missing support for to-many relationships.

How to use

The changes you need to make in your code to suport dependencies for to-many relations are minimal. This library does not override or extend Apple's KVO. It just uses the wonderful world of KVO. The derived property must be a declared property.

@property (nonatomic, readonly) NSDecimalNumber *totalPrice;

The library recognizes class methods starting with keyPathsForValuesAffectingDerived* and registers observers to be notified about the changes.

- (NSDecimalNumber *)totalPrice {
	return [items valueForKeyPath: @"@sum.totalPrice"];
}

+ (NSSet *)keyPathsForValuesAffectingDerivedTotalPrice {
	return [NSSet setWithObject: @"items.totalPrice"];
}

Calculated Properties

Most of the time you want to cache values to avoid unnecessary recalculations. It is simply done by appending updateDerived* to the getter method you want to cache its value.

@dynamic totalPrice;

- (NSDecimalNumber *)updateDerivedTotalPrice {
	return [items valueForKeyPath: @"@sum.totalPrice"];
}

Here you don't need the method totalPrice anymore. It will be added dynamically to the class.

Setup

There are 3 steps to setup the environment for derived properties.

1. Register the Class.
2. Start/Stop observing for each Instance.
3. Forward observeValueForKeyPath:ofObject:change:context:

The skeleton for a class that supports derived property dependencies could look like:

@implementation BaseObject

+ (void)initialize {
  [self registerDynamicDerivedProperties];
}

- (id)init {
  self = [super init];
  if (self) {
    [self startObservingForDerivedProperties];
  }
  return self;
}

- (void)dealloc {
  [self stopObservingForDerivedProperties];
  [super dealloc];
}

- (void)observeValueForKeyPath:(NSString *)keyPath 
	ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
  [self observeDerivedValueForKeyPath: keyPath ofObject: object 
  	change: change context: context];
}

@end

Caching calcuated properties is optional. If you want to use it you must call setupDynamicDerivedProperties and clearDynamicDerivedProperties.

- (id)init {
  self = [super init];
  if (self) {
    [self setupDynamicDerivedProperties];
    [self startObservingForDerivedProperties];
  }
  return self;
}

- (void)dealloc {
  [self stopObservingForDerivedProperties];
  [self clearDynamicDerivedProperties];
  [super dealloc];
}

Bonus - Core Data

It works also with NSManagedObject instances. The ExampleCoreData project covers what you need to do when you work with CoreData.

License

MIT-License