Is there a design pattern that deals with callback mechanism?

Question

Is there a design pattern that deals with callback mechanism?

Answer 1

The Observer Pattern comes to mind.

One object (suscriber) can suscribe to another object (publisher). When the publisher changes or is updated, he can notify all his suscribers.

Depending on what language you are using, you can specify a function that should be called on notifies.

http://en.wikipedia.org/wiki/Observer_pattern

It is well described in Design Patterns: Elements of Reusable Object-Oriented Software [Gang of Four]

Answer 2

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

Answer 3

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.

Answer 4

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.

Answer 5

callback is a form strategy design pattern

Answer 6

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?