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/*
* Ekiga -- A VoIP and Video-Conferencing application
* Copyright (C) 2000-2009 Damien Sandras <dsandras@seconix.com>
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or (at
* your option) any later version. This program is distributed in the hope
* that it will be useful, but WITHOUT ANY WARRANTY; without even the
* implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
* See the GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License along
* with this program; if not, write to the Free Software Foundation, Inc.,
* 51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA.
*
* Ekiga is licensed under the GPL license and as a special exception, you
* have permission to link or otherwise combine this program with the
* programs OPAL, OpenH323 and PWLIB, and distribute the combination, without
* applying the requirements of the GNU GPL to the OPAL, OpenH323 and PWLIB
* programs, as long as you do follow the requirements of the GNU GPL for all
* the rest of the software thus combined.
*/
/*
* chain-of-responsibility.h - description
* ------------------------------------------
* begin : written in 2007 by Julien Puydt
* copyright : (c) 2007 by Julien Puydt
* description : a chain of responsibility implementation
*
*/
#ifndef __CHAIN_OF_RESPONSIBILITY_H__
#define __CHAIN_OF_RESPONSIBILITY_H__
#include <sigc++/sigc++.h>
/* This code uses sigc++ signals to implement the "chain of responsibility"
* design pattern -- big words, but simple concept!
*
* BASIC IDEA
*
* Here is the situation you're interested in : one of your objects sometimes
* needs to get some kind of requests processed, but doesn't have the required
* competences to do it itself.
*
* The obvious solution is to have handlers register to your object, which
* will try to process requests on its behalf. Trying to use signals and slots
* to achieve that is quite natural too, but leads to the problem that a
* signal is one-to-many, so using them directly will lead to the bad situation
* where all requests get handled not once, but once by each handler!
*
* PRINCIPLE
*
* So we need to adapt the signal and slot to make sure only one handler will
* really process a request. This is done by making the slots return a boolean,
* so we know if the request was really processed or not. If it wasn't, the
* next handler is polled -- if it was, the treatment stops.
*
*
* HOW TO USE
*
* A request has type T_request, the object will declare :
* Ekiga::ChainOfResponsibility<T_request> chain;
*
* The handlers will register like this :
* chain.add_handler (sigc::mem_fun (this, &Foo::request_handler));
*
* A request handler looks like :
* bool
* Foo::request_handler (const T_request request)
* {
* if (can_handle_request (request)) {
*
* <handle request here>
* return true;
* else
* return false;
* }
*
* Trying to get a request handled looks like :
* chain.handle_request (request);
*
* or for better error reporting :
* if (!chain.handle_request (request)) {
*
* <report that you couldn't deal with a request>
* }
*
*/
namespace Ekiga
{
struct responsibility_accumulator
{
typedef bool result_type;
template <typename T_slot_iterator>
bool operator() (T_slot_iterator begin, T_slot_iterator end)
{
bool result = false;
for (T_slot_iterator iter = begin ;
iter != end && !result;
iter++)
result = *iter;
return result;
}
};
template<typename T_request>
struct ChainOfResponsibility:
public sigc::signal1<bool,
T_request,
responsibility_accumulator>
{
typedef typename sigc::signal1<bool,
T_request,
responsibility_accumulator>::iterator iterator;
typedef typename sigc::signal1<bool,
T_request,
responsibility_accumulator>::slot_type slot_type;
iterator add_handler (const slot_type& slot_)
{ return this->connect (slot_); }
bool handle_request (typename sigc::type_trait<T_request>::take request)
{ return this->emit (request); }
};
};
#endif
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