Why would I ever use a Chain of Responsibility over a Decorator?

The fact that you can break the chain at any point differentiates the Chain of Responsibility pattern from the Decorator pattern. Decorators can be thought of as executing all at once without any interaction with the other decorators. Links in a chain can be thought of as executing one at a time, because they each depend on the previous link.

Use the Chain of Responsibility pattern when you can conceptualize your program as a chain made up of links, where each link can either handle a request or pass it up the chain.

When I used to work with the Win32 API, I would sometimes need to use the hooking functionality it provides. Hooking a Windows message roughly follows the Chain of Responsibility pattern. When you hooked a message such as WM_MOUSEMOVE, your callback function would be called. Think of the callback function as the last link in the chain. Each link in the chain can decide whether to throw away the WM_MOUSEMOVE message or pass it up the chain to the next link.

If the Decorator pattern had been used in that example, you would have been notified of the WM_MOUSEMOVE message, but you would be powerless to prevent other hooks from handling it as well.

Another place the Chain of Command pattern is used is in game engines. Again, you can hook engine functions, events, and other things. In the case of a game engine, you don't want to simply add functionality. You want to add functionality and prevent the game engine from performing its default action.

I'd say that a Chain of Responsibility is a particular form of Decorator.

Chain

Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.

vs

Decorator

Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

I'd say its around the order in which things will happen. If you chain them, the will be called along the chain. With a decorator you're not guaranteed this order, only that additional responsibilities can be attached.

Decorator is used when you want to add functionality to an object.

COR is used when one of many actors might take action on an object.

A particular Decorator is called to take an action, based on the type; while COR passes the object along a defined chain until one of the actors decides the action is complete.

COR might be used when there are multiple levels of escalation to different handlers -- for instance, a call center where the customer's value to the company determines if the call goes to a particular level of support.

Well I can think of 2 situations:

• You don't have a core object, i.e. you don't know what to do with the request after it passed all the layers/filters. (something like an aspect like interceptor chains that don't really care where the request ends).
• You need to selectively apply some pre or post processing to the request. Not in a general enhancement form as the decorator does. i.e. Filters may or maynot handle a specific request but adding a decorator always enhances your object with some functionality.

Can't think of any more right now, would love to hear more in this topic.

I agree that from structural standpoint this two patterns are very similar. My thought is about the final behavior:

In the classic interpretation of CoR element which handles the request breaks the chain.

If any element in decorator breaks the chain then it will be wrong implementation of decorator, because base part of behavior will be lost. And the idea of decorator is transparent addition of new behavior when the base behavior remains untouched.