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From Craig McClanahan <craig...@apache.org>
Subject Re: [chain] how to delegate to a command?
Date Mon, 12 Jul 2004 19:43:39 GMT
Manfred Wolff wrote:

>
> Vic.
>
> Hmmm. I think the "chain of responsibility" pattern is not made to 
> configure the chains on the fly ;-). If you have a decision configure 
> all chains that are envolved and let the commands  themself decide if 
> they can fulfill the business logic or not.
>
I would agree with that sentiment in general, but there are likely to be 
use cases for dynamically configuring a chain (either one-off for a 
specific need based on (say) a bunch of query parameters or the input 
data in a web service call, or synthesized at app startup based on 
overall configuration parameters, and then stored in a catalog for reuse).

However, trying to figure out the next command to execute in a 
*currently executing* chain seems to go against the purpose of the Chain 
of Responsibility pattern.  The idea is that each command should be 
small and self contained, so that it is reusable (and unit testable) 
without knowing anything about the other commands that might be part of 
the chain.  Commands that need to pass state information along should do 
so by storing it in the context under some agreed upon attribute name.

After all, in the execute method of a specific command, you can say 
something like:

    Command nextCommand = new FooCommand();
    if (nextCommand.execute(context))
      return true;

if you really want to -- but why bother to make FooCommand into a 
command in the first place?  If it's just arbitrary business logic, you 
can also call it directly and be done with it.

Vic, it would be interesting to explore in more detail the use case you 
are thinking about, in order to determine whether Chain of 
Responsibility is the right pattern or not.  What did you have in mind?

Craig


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