Cocoa (API) - Late Binding

Late Binding

In most object-oriented languages, calls to methods are represented physically by a pointer to the code in memory. This restricts the design of an application since specific "command handling" classes are required, usually organized according to the chain-of-responsibility design pattern. While Cocoa retains this approach for the most part, Objective-C's late binding opens up more flexibility.

Under Objective-C, methods are represented by a selector, a string describing the method to be called. When a message is sent, the selector is sent into the ObjC runtime, matched against a list of available methods, and the method's implementation is called. Since the selector is text data, this allows it to be saved to a file, transmitted over a network or between processes, or manipulated in other ways. The implementation of the method is looked up at runtime, not compile time. There is a small performance penalty for this, but late binding allows the same selector to reference different implementations.

By a similar token, Cocoa provides a pervasive data manipulation method called key-value coding (KVC). This permits a piece of data or property of an object to be looked up or changed at runtime by name — the property name acts as a key to the value itself. In traditional languages, this late binding is not possible. KVC leads to great design flexibility — an object's type does not need to be known, yet any property of that object can be discovered using KVC. In addition, by extending this system using something Cocoa calls key-value observing (KVO), automatic support for Undo/Redo is provided.

Late static binding is a variant of binding somewhere between static and dynamic binding. The binding of names before the program is run is called static ("early"); bindings performed as the program runs are dynamic ("late" or "virtual").