Chain of Responsibility Design Pattern - Behavioural

1.1     Introduction

The classic CoR pattern defined by GoF in Design Patterns:

"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."

Figure illustrates the class diagram.

Typical object structure might look like Below Figure.

               

               

               

1.1     Intent

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.

1.2     Motivation

·         Consider a context-sensitive help system for a GUI

·         The object that ultimately provides the help isn't known explicitly to the object (e.g., a button) that initiates the help request

·         So use a chain of objects to decouple the senders from the receivers. The request gets passed along the chain until one of the objects handles it.

·         Each object on the chain shares a common interface for handling requests and for accessing its successor on the chain

1.3     Applicability

Use Chain of Responsibility:

·         When more than one object may handle a request and the actual handler is not know in advance

·         When requests follow a "handle or forward" model - that is, some requests can be handled where they are generated while others must be forwarded to another object to be handled

·         When a request has to be issued to several objects without specifying the receiver explicitly

·         The set of objects that can handle a request should be specified dynamically

1.4     Consequences

·         Reduced coupling between the sender of a request and the receiver – the sender and receiver have no explicit knowledge of each other

·         Receipt is not guaranteed - a request could fall off the end of the chain without being handled

·         The chain of handlers can be modified dynamically

·         Each C# object in the chain is self-contained. It knows nothing of the others and only need decide whether it can satisfy the request. This makes both writing each one and constructing the chain very easy.

·         You can decide whether the final object in the chain handles all requests it receives in some default fashion or just discards them. However, you do have to know which object will be last in the chain for this to be effective.

·         Finally, since C# cannot provide multiple inheritance, the basic Chain class sometimes needs to be an interface rather than an abstract class so the individual objects can inherit from another useful hierarchy, as we did here by deriving them all from Control. This disadvantage of this approach is that you often have to implement the linking, sending, and forwarding code in each module separately or, as we did here, by subclassing a concrete class that implements the Chain interface.

1.5     Structure

Class Diagram

Object Diagram

1.6     Example1

Scenario:

We are designing the software for a system that approves purchasing requests. The approval authority depends on the dollar amount of the purchase. The approval authority for a given dollar amount could change at any time and the system should be flexible enough to handle this situation.

Solution:

Use the Chain of Responsibility pattern. PurchaseRequest objects forward the approval request to a PurchaseApproval object. Depending on the dollar amount, the PurchaseApproval object may approve the request or forward it on to the next approving authority in the chain. The approval authority at any level in the chain can be easily modified without affecting the original PurchaseRequest object.

1.7     Example2

This code demonstrates the Chain of Responsibility pattern in which several linked objects (the Chain) are offered the opportunity to respond to a request or hand it off to the object next in line.

static void Main() {        // Setup Chain of Responsibility        Handler h1 = new ConcreteHandler1();        Handler h2 = new ConcreteHandler2();        Handler h3 = new ConcreteHandler3();        h1.SetSuccessor(h2);        h2.SetSuccessor(h3);        // Generate and process request        int[] requests = { 2, 5, 14, 22, 18, 3, 27, 20 };        foreach (int request in requests)        {               h1.HandleRequest(request);        }        // Wait for user        Console.Read(); }	abstract class Handler {        protected Handler successor;        public void SetSuccessor(Handler successor)        {               this.successor = successor;        }        public abstract void HandleRequest(int request); }
class ConcreteHandler1 : Handler     {         public override void HandleRequest(int request)         {             if (request >= 0 && request < 10)             {                 Console.WriteLine("{0} handled request {1}",                   this.GetType().Name, request);             }             else if (successor != null)             {                 successor.HandleRequest(request);             }         }     }	// "ConcreteHandler2"     class ConcreteHandler2 : Handler     {         public override void HandleRequest(int request)         {             if (request >= 10 && request < 20)             {                 Console.WriteLine("{0} handled request {1}",                   this.GetType().Name, request);             }             else if (successor != null)             {                 successor.HandleRequest(request);             }         }     }
// "ConcreteHandler3"     class ConcreteHandler3 : Handler     {         public override void HandleRequest(int request)         {             if (request >= 20 && request < 30)             {                 Console.WriteLine("{0} handled request {1}",                   this.GetType().Name, request);             }             else if (successor != null)             {                 successor.HandleRequest(request);             }         }     } }

1.8     Example3

This real-world code demonstrates the Chain of Responsibility pattern in which several linked managers and executives can respond to a purchase request or hand it off to a superior. Each position has can have its own set of rules which orders they can approve.

static void Main() {        // Setup Chain of Responsibility        Director Larry = new Director();        VicePresident Sam = new VicePresident();        President Tammy = new President();        Larry.SetSuccessor(Sam);        Sam.SetSuccessor(Tammy);        // Generate and process purchase requests        Purchase p = new Purchase(2034, 350.00, "Supplies");        Larry.ProcessRequest(p);        p = new Purchase(2035, 32590.10, "Project X");        Larry.ProcessRequest(p);        p = new Purchase(2036, 122100.00, "Project Y");        Larry.ProcessRequest(p);        // Wait for user        Console.Read(); }	abstract class Approver {   protected Approver successor;   public void SetSuccessor(Approver successor)   {    this.successor = successor;   }   public abstract void ProcessRequest(Purchase purchase); }
class Director : Approver {   public override void ProcessRequest(Purchase purchase)   {    if (purchase.Amount < 10000.0)    {        Console.WriteLine("{0} approved request# {1}",          this.GetType().Name, purchase.Number);    }    else if (successor != null)    {        successor.ProcessRequest(purchase);    }  } }	class VicePresident : Approver {    public override void ProcessRequest(Purchase purchase)    {       if (purchase.Amount < 25000.0)       {         Console.WriteLine("{0} approved request# {1}",             this.GetType().Name, purchase.Number);       }      else if (successor != null)       {         successor.ProcessRequest(purchase);       }     }  }
class President : Approver {  public override void ProcessRequest(Purchase purchase)  {   if (purchase.Amount < 100000.0)   {        Console.WriteLine("{0} approved request# {1}",          this.GetType().Name, purchase.Number);   }  else  {    Console.WriteLine("Request# {0} requires an                      executive meeting!",purchase.Number);  } } }

    // Request details

    class Purchase

    {

        private int number;

        private double amount;

        private string purpose;

        // Constructor

        public Purchase(int number, double amount, string purpose)

        {

            this.number = number;

            this.amount = amount;

            this.purpose = purpose;

        }

        // Properties

        public double Amount

        {

            get { return amount; }

            set { amount = value; }

        }

        public string Purpose

        {

            get { return purpose; }

            set { purpose = value; }

        }

        public int Number

        {

            get { return number; }

            set { number = value; }

        }

    }

}

1.9     Examples in C#

Under the covers, C# form windows receive various events, such as MouseMove, and then forward them to the controls the form contains. However, only the final control ever receives the message in C# whereas in some other languages, each containing control does as well. This is a clear implementation of Chain of Responsibility pattern.

We could also argue that, in general, the C# class inheritance structure itself exemplifies this pattern. If you call for a method to be executed in a deeply derived class, that method is passed up the inheritance chain until the first parent class containing that method is found. The fact that further parents contain other implementations of that method does not come into play.

We will also see that the Chain of Responsibility is ideal for implementing Interpreters and use one in the Interpreter pattern we discuss later.

Hope this helps.

Thanks & Regards,

Arun Manglick