Non-Hierarchical Similarities
If more than one class implements a set of methods, those classes are often grouped under an abstract class that declares the methods they have in common. Each subclass may re-implement the methods in its own way, but the inheritance hierarchy and the common declaration in the abstract class captures the essential similarity between the subclasses.
However, sometimes it’s not possible to group common methods in an abstract class. Classes that are unrelated in most respects might nevertheless need to implement some similar methods. This limited similarity may not justify a hierarchical relationship. For example, you might want to add support for creating XML representations of objects in your application and for initializing objects from an XML representation:
- (NSXMLElement *)XMLRepresentation; |
- initFromXMLRepresentation:(NSXMLElement *)xmlString; |
These methods could be grouped into a protocol and the similarity between implementing classes accounted for by noting that they all conform to the same protocol.
Objects can be typed by this similarity (the protocols they conform to), rather than by their class. For example, an NSMatrix instance must communicate with the objects that represent its cells. The matrix could require each of these objects to be a kind of NSCell (a type based on class) and rely on the fact that all objects that inherit from the NSCell class have the methods needed to respond to NSMatrix messages. Alternatively, the NSMatrix object could require objects representing cells to have methods that can respond to a particular set of messages (a type based on protocol). In this case, the NSMatrix object wouldn’t care what class a cell object belonged to, just that it implemented the methods.
Last updated: 2008-02-05
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