Reputation:

Is there a design pattern that deals with callback mechanism?

Upvotes: 38

Answers (10)

Anoynmous

Reputation: 1

Page 235 of Design Patterns

"Commands (Command Pattern) are an object-oriented replacement for callbacks."

Upvotes: 0

Paulo Merson

Reputation: 14487

A good pattern description is the Service Callback design pattern. It's part of a catalog of SOA patterns, but the pattern as described can be employed with generic components that are not SOA services.

Another related pattern is the Return Address pattern described in the classic book "Enterprise Integration Patterns" by Hohpe and Woolf.

Josuttis also talks about callback in his book "SOA in Practice". He calls it the Request/Callback message exchange pattern.

Upvotes: 2

acromm

Reputation: 880

Several. Check out http://people.bu.edu/azs/teaching/cs108/2006fall/callback_pattern.pdf, and the following:

Visitor Pattern: http://en.wikipedia.org/wiki/Visitor_pattern
Observer Pattern: http://en.wikipedia.org/wiki/Observer_pattern
Strategy Pattern: http://en.wikipedia.org/wiki/Strategy_pattern

The most appropriate will depend on the situation. What programming language do you want to use and what do you want to do exactly?

Upvotes: 12

Ranjith

Reputation: 53

I agree with the other posters about the Observer pattern as well. It's specifically designed for this purpose.

Upvotes: 0

Chap

Reputation: 2796

External polymorphism - An object has a reference to a another object and a function to call on that object. It can be viewed as a single type, thus you can mix and match objects and functions to call for the event. Delegates are an example of this pattern. This is more of a C# style approach.

Observer pattern - You use an interface/base class that an object can implement and register this interface to an event. More of a Java style approach.

Check the answer I posted here for a C++ solution for delegates/external polymorphism: raw function pointer from a bound method

Upvotes: 4

Tobias

Reputation: 6507

Your question is very general, and the most general answer I can think of is to use polymorphism when you have a problem requiring a callback.

Polymorphism allows you to specify a software contract in form of an interface (or an abstract class) about how your callback is to be used. Then clients are free to choose any implementation of the interface they see fit for their purpose.

Whether it is advisable to use the state, strategy, observer pattern or something completely different really depends on the circumstances.

Upvotes: 1

Scott Stanchfield

Reputation: 30642

It depends on how the callback is used.

Design patterns are all about communicating your intent.

If you intended to allow one or more callbacks to be registered and they can be called as notification "at some point in the future", you're talking Observer. Also -- the actual invocation of the callback in this case is usually "optional" or triggered based on some stimulus. (The callbacks may or may not ever be called)

If you intended to pass in "something to do", and that gets done in the method (or is used to "do something" during a later process) you're talking Strategy. Also -- the actual invocation usually happens.

Note that the exact same code could be either -- it's really about how you're thinking about the problem and how you want others to think about it.

Upvotes: 23

Jose Basilio

Reputation: 51498

That would be the Observer Pattern - From Wikipedia

The observer pattern (a subset of the asynchronous publish/subscribe pattern) is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods. It is mainly used to implement distributed event handling systems.

Upvotes: 28