Chain-of-responsibility pattern

In object-oriented design, the chain-of-responsibility pattern is a design pattern consisting of a source of command objects and a series of processing objects.[1] Each processing object contains logic that defines the types of command objects that it can handle; the rest are passed to the next processing object in the chain. A mechanism also exists for adding new processing objects to the end of this chain. Thus, the chain of responsibility is an object oriented version of the if ... else if ... else if ....... else ... endif idiom, with the benefit that the condition–action blocks can be dynamically rearranged and reconfigured at runtime.

In a variation of the standard chain-of-responsibility model, some handlers may act as dispatchers, capable of sending commands out in a variety of directions, forming a tree of responsibility. In some cases, this can occur recursively, with processing objects calling higher-up processing objects with commands that attempt to solve some smaller part of the problem; in this case recursion continues until the command is processed, or the entire tree has been explored. An XML interpreter might work in this manner.

This pattern promotes the idea of loose coupling.

The chain-of-responsibility pattern is structurally nearly identical to the decorator pattern, the difference being that for the decorator, all classes handle the request, while for the chain of responsibility, exactly one of the classes in the chain handles the request.

Overview

The Chain of Responsibility [2] design pattern is one of the twenty-three well-known GoF design patterns that describe common solutions to recurring design problems when designing flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse.

What problems can the Chain of Responsibility design pattern solve? [3]

  • Coupling the sender of a request to its receiver should be avoided.
  • It should be possible that more than one receiver can handle a request.

Implementing a request directly within the class that sends the request is inflexible because it couples the class to a particular receiver and makes it impossible to support multiple receivers.

What solution does the Chain of Responsibility design pattern describe?

  • Define a chain of receiver objects having the responsibility, depending on run-time conditions, to either handle a request or forward it to the next receiver on the chain (if any).

This enables to send a request to a chain of receivers without having to know which one handles the request. The request gets passed along the chain until a receiver handles the request. The sender of a request is no longer coupled to a particular receiver.

See also the UML class and sequence diagram below.

Structure

UML class and sequence diagram

In the above UML class diagram, the Sender class doesn't refer to a particular receiver class directly. Instead, Sender refers to the Handler interface for handling a request (handler.handleRequest()), which makes the Sender independent of which receiver handles the request. The Receiver1, Receiver2, and Receiver3 classes implement the Handler interface by either handling or forwarding a request (depending on run-time conditions).
The UML sequence diagram shows the run-time interactions: In this example, the Sender object calls handleRequest() on the receiver1 object (of type Handler). The receiver1 forwards the request to receiver2, which in turn forwards the request to receiver3, which handles (performs) the request.

Example

Java example

Below is an example of this pattern in Java. In this example we have different roles, each having a fixed purchasing limit and a successor. Every time a user in a role receives a purchase request that exceeds his or her limit, the request is passed to his or her successor.

The PurchasePower abstract class with a concrete processRequest method:

abstract class PurchasePower {
    protected static final double BASE = 500;
    protected PurchasePower successor;

    abstract protected double getAllowable();
    abstract protected String getRole();

    public void setSuccessor(PurchasePower successor) {
        this.successor = successor;
    }

    public void processRequest(PurchaseRequest request) {
        if (request.getAmount() < this.getAllowable()) {
            System.out.println(this.getRole() + " will approve $" + request.getAmount());
        } else if (successor != null) {
            successor.processRequest(request);
        }
    }
}

Four implementations of the abstract class above: Manager, Director, Vice President, President

class ManagerPPower extends PurchasePower {
    
    protected double getAllowable() {
        return BASE * 10;
    }

    protected String getRole() {
        return "Manager";
    }
}

class DirectorPPower extends PurchasePower {
    
    protected double getAllowable() {
        return BASE * 20;
    }

    protected String getRole() {
        return "Director";
    }
}

class VicePresidentPPower extends PurchasePower {
    
    protected double getAllowable() {
        return BASE * 40;
    }

    protected String getRole() {
        return "Vice President";
    }
}

class PresidentPPower extends PurchasePower {

    protected double getAllowable() {
        return BASE * 60;
    }

    protected String getRole() {
        return "President";
    }
}

The following code defines the PurchaseRequest class that keeps the request data in this example.

class PurchaseRequest {

    private double amount;
    private String purpose;

    public PurchaseRequest(double amount, String purpose) {
        this.amount = amount;
        this.purpose = purpose;
    }

    public double getAmount() {
        return this.amount;
    }

    public void setAmount(double amount)  {
        this.amount = amount;
    }

    public String getPurpose() {
        return this.purpose;
    }
    public void setPurpose(String purpose) {
        this.purpose = purpose;
    }
}

In the following usage example, the successors are set as follows: Manager -> Director -> Vice President -> President

class CheckAuthority {

    public static void main(String[] args) {
        ManagerPPower manager = new ManagerPPower();
        DirectorPPower director = new DirectorPPower();
        VicePresidentPPower vp = new VicePresidentPPower();
        PresidentPPower president = new PresidentPPower();
        manager.setSuccessor(director);
        director.setSuccessor(vp);
        vp.setSuccessor(president);

        // Press Ctrl+C to end.
        try {
            while (true) {
                System.out.println("Enter the amount to check who should approve your expenditure.");
                System.out.print(">");
                double d = Double.parseDouble(new BufferedReader(new InputStreamReader(System.in)).readLine());
                manager.processRequest(new PurchaseRequest(d, "General"));
            }
        }
        catch (Exception exc) {
            System.exit(1);
        }
    }
}

C# example

This C# examples uses the logger application to select different sources based on the log level;

namespace ChainOfResponsibility
{
    [Flags]
    public enum LogLevel
    {
        None = 0,                 //        0
        Info = 1,                 //        1
        Debug = 2,                //       10
        Warning = 4,              //      100
        Error = 8,                //     1000
        FunctionalMessage = 16,   //    10000
        FunctionalError = 32,     //   100000
        All = 63                  //   111111
    }
 
    /// <summary>
    /// Abstract Handler in chain of responsibility pattern.
    /// </summary>
    public abstract class Logger
    {
        protected LogLevel logMask;
 
        // The next Handler in the chain
        protected Logger next;
 
        public Logger(LogLevel mask)
        {
            this.logMask = mask;
        }
 
        /// <summary>
        /// Sets the Next logger to make a list/chain of Handlers.
        /// </summary>
        public Logger SetNext(Logger nextlogger)
        {
            next = nextlogger;
            return nextlogger;
        }
 
        public void Message(string msg, LogLevel severity)
        {
            if ((severity & logMask) != 0) //True only if all logMask bits are set in severity
            {
                WriteMessage(msg);
            }
            if (next != null) 
            {
                next.Message(msg, severity); 
            }
        }
 
        abstract protected void WriteMessage(string msg);
    }
 
    public class ConsoleLogger : Logger
    {
        public ConsoleLogger(LogLevel mask)
            : base(mask)
        { }
 
        protected override void WriteMessage(string msg)
        {
            Console.WriteLine("Writing to console: " + msg);
        }
    }
 
    public class EmailLogger : Logger
    {
        public EmailLogger(LogLevel mask)
            : base(mask)
        { }
 
        protected override void WriteMessage(string msg)
        {
            // Placeholder for mail send logic, usually the email configurations are saved in config file.
            Console.WriteLine("Sending via email: " + msg);
        }
    }
 
    class FileLogger : Logger
    {
        public FileLogger(LogLevel mask)
            : base(mask)
        { }
 
        protected override void WriteMessage(string msg)
        {
            // Placeholder for File writing logic
            Console.WriteLine("Writing to Log File: " + msg);
        }
    }
 
    public class Program
    {
        public static void Main(string[] args)
        {
            // Build the chain of responsibility
            Logger logger, logger1, logger2;
            logger = new ConsoleLogger(LogLevel.All);
            logger1 = logger.SetNext(new EmailLogger(LogLevel.FunctionalMessage | LogLevel.FunctionalError));
            logger2 = logger1.SetNext(new FileLogger(LogLevel.Warning | LogLevel.Error));
 
            // Handled by ConsoleLogger since the console has a loglevel of all
            logger.Message("Entering function ProcessOrder().", LogLevel.Debug);
            logger.Message("Order record retrieved.", LogLevel.Info);
 
            // Handled by ConsoleLogger and FileLogger since filelogger implements Warning & Error
            logger.Message("Customer Address details missing in Branch DataBase.", LogLevel.Warning);
            logger.Message("Customer Address details missing in Organization DataBase.", LogLevel.Error);
 
            // Handled by ConsoleLogger and EmailLogger as it implements functional error
            logger.Message("Unable to Process Order ORD1 Dated D1 For Customer C1.", LogLevel.FunctionalError);
 
            // Handled by ConsoleLogger and EmailLogger
            logger.Message("Order Dispatched.", LogLevel.FunctionalMessage);
        }
    }
}
 
/* Output
Writing to console: Entering function ProcessOrder().
Writing to console: Order record retrieved.
Writing to console: Customer Address details missing in Branch DataBase.
Writing to Log File: Customer Address details missing in Branch DataBase.
Writing to console: Customer Address details missing in Organization DataBase.
Writing to Log File: Customer Address details missing in Organization DataBase.
Writing to console: Unable to Process Order ORD1 Dated D1 For Customer C1.
Sending via email: Unable to Process Order ORD1 Dated D1 For Customer C1.
Writing to console: Order Dispatched.
Sending via email: Order Dispatched.
*/

Crystal example

enum LogLevel
  None,
  Info,
  Debug,
  Warning,
  Error,
  FunctionalMessage,
  FunctionalError,
  All
end

abstract class Logger
  protected getter log_levels
  protected getter next : Logger | Nil

  def initialize(*levels)
    @log_levels = [] of LogLevel

    levels.each do |level|
      @log_levels << level
    end
  end

  def set_next(next_logger : Logger)
    @next = next_logger
  end

  def message(msg : String, severity : LogLevel)
    write_message(msg) if @log_levels.includes?(LogLevel::All) || @log_levels.includes?(severity)
    @next.try(&.message(msg, severity)) if @next
  end

  abstract def write_message(msg : String)
end

class ConsoleLogger < Logger
  def write_message(msg : String)
    puts "Writing to console: #{msg}"
  end
end

class EmailLogger < Logger
  def write_message(msg : String)
    puts "Sending via email: #{msg}"
  end
end

class FileLogger < Logger
  def write_message(msg : String)
    puts "Writing to Log File: #{msg}"
  end
end

# Program
# Build the chain of responsibility
logger = ConsoleLogger.new(LogLevel::All)
logger1 = logger.set_next(EmailLogger.new(LogLevel::FunctionalMessage, LogLevel::FunctionalError))
logger2 = logger1.set_next(FileLogger.new(LogLevel::Warning, LogLevel::Error))

# Handled by ConsoleLogger since the console has a loglevel of all
logger.message("Entering function ProcessOrder().", LogLevel::Debug)
logger.message("Order record retrieved.", LogLevel::Info)

# Handled by ConsoleLogger and FileLogger since filelogger implements Warning & Error
logger.message("Customer Address details missing in Branch DataBase.", LogLevel::Warning)
logger.message("Customer Address details missing in Organization DataBase.", LogLevel::Error)

# Handled by ConsoleLogger and EmailLogger as it implements functional error
logger.message("Unable to Process Order ORD1 Dated D1 For Customer C1.", LogLevel::FunctionalError)

# Handled by ConsoleLogger and EmailLogger
logger.message("Order Dispatched.", LogLevel::FunctionalMessage)

Output

Writing to console: Entering function ProcessOrder().
Writing to console: Order record retrieved.
Writing to console: Customer Address details missing in Branch DataBase.
Writing to Log File: Customer Address details missing in Branch DataBase.
Writing to console: Customer Address details missing in Organization DataBase.
Writing to Log File: Customer Address details missing in Organization DataBase.
Writing to console: Unable to Process Order ORD1 Dated D1 For Customer C1.
Sending via email: Unable to Process Order ORD1 Dated D1 For Customer C1.
Writing to console: Order Dispatched.
Sending via email: Order Dispatched.

Python Example

"""
Chain of responsibility pattern example.
"""
from abc import ABCMeta, abstractmethod
from enum import Enum, auto


class LogLevel(Enum):
    """
    Log Levels Enum.
    """
    NONE = auto()
    INFO = auto()
    DEBUG = auto()
    WARNING = auto()
    ERROR = auto()
    FUNCTIONAL_MESSAGE = auto()
    FUNCTIONAL_ERROR = auto()
    ALL = auto()


class Logger:
    """
    Abstract handler in chain of responsibility pattern.
    """
    __metaclass__ = ABCMeta

    next = None

    def __init__(self, levels):
        """
        Initialize new logger

        Args:
            levels (list[str]): List of log levels.
        """
        self.log_levels = []

        for level in levels:
            self.log_levels.append(level)

    def set_next(self, next_logger):
        """
        Set next responsible logger in the chain.

        Args:
            next_logger (Logger): Next responsible logger.

        Returns:
            Logger: Next responsible logger.
        """
        self.next = next_logger
        return self.next

    def message(self, msg, severity):
        """
        Message writer handler.

        Args:
            msg (str): Message string.
            severity (LogLevel): Severity of message as log level enum.
        """
        if LogLevel.ALL in self.log_levels or severity in self.log_levels:
            self.write_message(msg)

        if self.next is not None:
            self.next.message(msg, severity)

    @abstractmethod
    def write_message(self, msg):
        """
        Abstract method to write a message.

        Args:
            msg (str): Message string.

        Raises:
            NotImplementedError
        """
        raise NotImplementedError("You should implement this method.")


class ConsoleLogger(Logger):
    def write_message(self, msg):
        """
        Overrides parent's abstract method to write to console.

        Args:
            msg (str): Message string.
        """
        print("Writing to console:", msg)


class EmailLogger(Logger):
    """
    Overrides parent's abstract method to send an email.

    Args:
        msg (str): Message string.
    """
    def write_message(self, msg):
        print("Sending via email:", msg)


class FileLogger(Logger):
    """
    Overrides parent's abstract method to write a file.

    Args:
        msg (str): Message string.
    """
    def write_message(self, msg):
        print("Writing to log file:", msg)


def main():
    """
    Building the chain of responsibility.
    """
    logger = ConsoleLogger([LogLevel.ALL])
    email_logger = logger.set_next(
        EmailLogger([LogLevel.FUNCTIONAL_MESSAGE, LogLevel.FUNCTIONAL_ERROR])
    )
    # As we don't need to use file logger instance anywhere later
    # We will not set any value for it.
    email_logger.set_next(
        FileLogger([LogLevel.WARNING, LogLevel.ERROR])
    )

    # ConsoleLogger will handle this part of code since the message
    # has a log level of all
    logger.message("Entering function ProcessOrder().", LogLevel.DEBUG)
    logger.message("Order record retrieved.", LogLevel.INFO)

    # ConsoleLogger and FileLogger will handle this part since file logger
    # implements WARNING and ERROR
    logger.message(
        "Customer Address details missing in Branch DataBase.",
        LogLevel.WARNING
    )
    logger.message(
        "Customer Address details missing in Organization DataBase.",
        LogLevel.ERROR
    )

    # ConsoleLogger and EmailLogger will handle this part as they implement
    # functional error
    logger.message(
        "Unable to Process Order ORD1 Dated D1 for customer C1.",
        LogLevel.FUNCTIONAL_ERROR
    )
    logger.message("OrderDispatched.", LogLevel.FUNCTIONAL_MESSAGE)


if __name__ == "__main__":
    main()

Implementations

Full Implementation of the Chain Of Responsibility pattern in Java

Cocoa and Cocoa Touch

The Cocoa and Cocoa Touch frameworks, used for OS X and iOS applications respectively, actively use the chain-of-responsibility pattern for handling events. Objects that participate in the chain are called responder objects, inheriting from the NSResponder (OS X)/UIResponder (iOS) class. All view objects (NSView/UIView), view controller objects (NSViewController/UIViewController), window objects (NSWindow/UIWindow), and the application object (NSApplication/UIApplication) are responder objects.

Typically, when a view receives an event which it can't handle, it dispatches it to its superview until it reaches the view controller or window object. If the window can't handle the event, the event is dispatched to the application object, which is the last object in the chain. For example:

  • On OS X, moving a textured window with the mouse can be done from any location (not just the title bar), unless on that location there's a view which handles dragging events, like slider controls. If no such view (or superview) is there, dragging events are sent up the chain to the window which does handle the dragging event.
  • On iOS, it's typical to handle view events in the view controller which manages the view hierarchy, instead of subclassing the view itself. Since a view controller lies in the responder chain after all of its managed subviews, it can intercept any view events and handle them.

See also

References

  1. ↑ http://www.blackwasp.co.uk/ChainOfResponsibility.aspx
  2. ↑ Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides (1994). Design Patterns: Elements of Reusable Object-Oriented Software. Addison Wesley. pp. 223ff. ISBN 0-201-63361-2.
  3. ↑ "The Chain of Responsibility design pattern - Problem, Solution, and Applicability". w3sDesign.com. Retrieved 2017-08-12.
  4. ↑ "The Chain of Responsibility design pattern - Structure and Collaboration". w3sDesign.com. Retrieved 2017-08-12.
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