Value Objects

While entities (like ICModel) are objects that have an important sense of identity (such as an objectID), value objects also live in the model layer, but they don’t have that same sense of identity. The only important thing about a value object is what value it contains; it’s otherwise interchangeable.

A very practical way of looking at it is that value objects don’t get their own row in a database, they usually get represented in a single field of some other object’s row.

Protocol

Instant Cocoa provides a protocol for value objects, called ICValueObject.

The backing object is normally a string or a number (since those are the primary value types in JSON), but it can be backed with anything. The only required methods in the ICValueObject protocol are:

- (instancetype)initWithBackingObject:(id)backingObject;

@property (nonatomic, readonly) id backingObject;

Optional Methods

The ICValueObject protocol also includes two optional methods:

- (instancetype)initWithString:(NSString *)string;
- (instancetype)initWithNumber:(NSNumber *)number;

These provide a little extra type information when reading, and Instant Cocoa will reflect on the backingObject’s type, and use the appropriate one if it is available.

Concrete Implementation

Instant Cocoa also includes a concrete implmentation of the ICValueObject protocol, in a class also named ICValueObject.

The ICValueObject class conforms to the ICValueObject protocol, and can easily be subclassed to make quick value objects.

It implements -copyWithZone:, -hash, -compare, -isEqual, and -description.