責任鏈模式

責任鏈模式在物件導向程式設計裏是一種軟件設計模式，它包含了一些命令對象和一系列的處理對象。每一個處理對象決定它能處理哪些命令對象，它也知道如何將它不能處理的命令對象傳遞給該鏈中的下一個處理對象。該模式還描述了往該處理鏈的末尾添加新的處理對象的方法。

責任鏈的UML類圖

代碼示例

Java

以下的日誌類(logging)例子演示了該模式。 每一個logging handler首先決定是否需要在該層做處理，然後將控制傳遞到下一個logging handler。程序的輸出是:

  Writting to stdout: Entering function y.
  Writting to stdout: Step1 completed.
  Sending via e-mail:      Step1 completed.
  Writting to stdout: An error has occurred.
  Sending via e-mail:      An error has occurred.
  Writing to stderr:       An error has occurred.

注意：該例子不是日誌類的推薦實現方式。

同時，需要注意的是，通常在責任鏈模式的實現中，如果在某一層已經處理了這個logger，那麼這個logger就不會傳遞下去。在我們這個例子中，消息會一直傳遞到最底層不管它是否已經被處理。

import java.util.*;

abstract class Logger 
{
    public static int ERR = 3;
    public static int NOTICE = 5;
    public static int DEBUG = 7;
    protected int mask;

    // The next element in the chain of responsibility
    protected Logger next;
    public Logger setNext( Logger l)
    {
        next = l;
        return this;
    }

    public final void message( String msg, int priority )
    {
        if ( priority <= mask ) 
        {
            writeMessage( msg );
            if ( next != null )
            {
                next.message( msg, priority );
            }
        }
    }
    
    protected abstract void writeMessage( String msg );

}

class StdoutLogger extends Logger 
{

    public StdoutLogger( int mask ) { this.mask = mask; }

    protected void writeMessage( String msg )
    {
        System.out.println( "Writting to stdout: " + msg );
    }
}


class EmailLogger extends Logger 
{

    public EmailLogger( int mask ) { this.mask = mask; }

    protected void writeMessage( String msg )
    {
        System.out.println( "Sending via email: " + msg );
    }
}

class StderrLogger extends Logger 
{

    public StderrLogger( int mask ) { this.mask = mask; }

    protected void writeMessage( String msg )
    {
        System.out.println( "Sending to stderr: " + msg );
    }
}

public class ChainOfResponsibilityExample
{
    public static void main( String[] args )
    {
        // Build the chain of responsibility
        Logger l = new StdoutLogger( Logger.DEBUG).setNext(
                            new EmailLogger( Logger.NOTICE ).setNext(
                            new StderrLogger( Logger.ERR ) ) );

        // Handled by StdoutLogger
        l.message( "Entering function y.", Logger.DEBUG );

        // Handled by StdoutLogger and EmailLogger
        l.message( "Step1 completed.", Logger.NOTICE );

        // Handled by all three loggers
        l.message( "An error has occurred.", Logger.ERR );
    }
}

PHP

<?php
abstract class Logger {
	const ERR = 3;
	const NOTICE = 5;
	const DEBUG = 7;

	protected $mask;
	protected $next; // The next element in the chain of responsibility

	public function setNext(Logger $l) {
		$this->next = $l;
		return $this;
	}

	abstract public function message($msg, $priority);
}

class DebugLogger extends Logger {
	public function __construct($mask) {
		$this->mask = $mask;
	}

	public function message($msg, $priority) {
		if ($priority <= $this->mask) {
			echo "Writting to stdout: {$msg}\n";
		}

		if (false == is_null($this->next)) {
			$this->next->message($msg, $priority);
		}
	}
}

class EmailLogger extends Logger {
	public function __construct($mask) {
		$this->mask = $mask;
	}

	public function message($msg, $priority) {
		if ($priority <= $this->mask) {
			echo "Sending via email: {$msg}\n";
		}

		if (false == is_null($this->next)) {
			$this->next->message($msg, $priority);
		}
	}
}

class StderrLogger extends Logger {
	public function __construct($mask) {
		$this->mask = $mask;
	}

	public function message($msg, $priority) {
		if ($priority <= $this->mask) {
			echo "Writing to stderr: {$msg}\n";
		}

		if (false == is_null($this->next)) {
			$this->next->message($msg, $priority);
		}
	}
}

class ChainOfResponsibilityExample {
	public function __construct() {
		// build the chain of responsibility
		$l = new DebugLogger(Logger::DEBUG);
		$e = new EmailLogger(Logger::NOTICE);
		$s = new StderrLogger(Logger::ERR);
		
		$e->setNext($s);
		$l->setNext($e);

		$l->message("Entering function y.",		Logger::DEBUG);		// handled by DebugLogger
		$l->message("Step1 completed.",			Logger::NOTICE);	// handled by DebugLogger and EmailLogger
		$l->message("An error has occurred.",	Logger::ERR);		// handled by all three Loggers
	}
}

new ChainOfResponsibilityExample();
?>

C++

《設計模式》書中例子的C++11實現^[1]：

#include <iostream>
#include <memory>

typedef int Topic;
constexpr Topic NO_HELP_TOPIC = -1;

// defines an interface for handling requests.
class HelpHandler { // Handler
public:
  HelpHandler(HelpHandler* h = nullptr, Topic t = NO_HELP_TOPIC)
    : successor(h), topic(t) {}
  virtual bool hasHelp() {
    return topic != NO_HELP_TOPIC;
  }
  virtual void setHandler(HelpHandler*, Topic) {}
  virtual void handleHelp() {
      std::cout << "HelpHandler::handleHelp\n";
      // (optional) implements the successor link.
      if (successor != nullptr) {
          successor->handleHelp();
      }
  }
  virtual ~HelpHandler() = default;
  HelpHandler(const HelpHandler&) = delete; // rule of three
  HelpHandler& operator=(const HelpHandler&) = delete;
private:
  HelpHandler* successor;
  Topic topic;
};


class Widget : public HelpHandler {
public:
  Widget(const Widget&) = delete; // rule of three
  Widget& operator=(const Widget&) = delete;
protected:
  Widget(Widget* w, Topic t = NO_HELP_TOPIC) 
    : HelpHandler(w, t), parent(nullptr) {
    parent = w;
  }
private:
  Widget* parent;
};

// handles requests it is responsible for.
class Button : public Widget { // ConcreteHandler
public:
  Button(std::shared_ptr<Widget> h, Topic t = NO_HELP_TOPIC) : Widget(h.get(), t) {}
  virtual void handleHelp() {
    // if the ConcreteHandler can handle the request, it does so; otherwise it forwards the request to its successor.
    std::cout << "Button::handleHelp\n";
    if (hasHelp()) {
      // handles requests it is responsible for.
    } else {      
      // can access its successor.
      HelpHandler::handleHelp();
    }
  }
};

class Dialog : public Widget { // ConcreteHandler
public:
  Dialog(std::shared_ptr<HelpHandler> h, Topic t = NO_HELP_TOPIC) : Widget(nullptr) {
    setHandler(h.get(), t);
  }
  virtual void handleHelp() {
    std::cout << "Dialog::handleHelp\n";
    // Widget operations that Dialog overrides...
    if(hasHelp()) {
      // offer help on the dialog
    } else {
      HelpHandler::handleHelp();
    }
  }
};

class Application : public HelpHandler {
public:
  Application(Topic t) : HelpHandler(nullptr, t) {}
  virtual void handleHelp() {
    std::cout << "Application::handleHelp\n";
    // show a list of help topics
  }
};

int main() {
  constexpr Topic PRINT_TOPIC = 1;
  constexpr Topic PAPER_ORIENTATION_TOPIC = 2;
  constexpr Topic APPLICATION_TOPIC = 3;
  // The smart pointers prevent memory leaks.
  std::shared_ptr<Application> application = std::make_shared<Application>(APPLICATION_TOPIC);
  std::shared_ptr<Dialog> dialog = std::make_shared<Dialog>(application, PRINT_TOPIC);
  std::shared_ptr<Button> button = std::make_shared<Button>(dialog, PAPER_ORIENTATION_TOPIC);

  button->handleHelp();
}