In object-oriented design, the chain-of-responsibility pattern is a behavioral design pattern consisting of a source of command objects and a series of processing objects. 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. 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. This is a strict definition of the Responsibility concept in the GoF book. However, many implementations (such as loggers below, or UI event handling, or servlet filters in Java, etc.) allow several elements in the chain to take responsibility.

Overview

The Chain of Responsibility 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?

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?

This enables us 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 class doesn't refer to a particular receiver class directly. Instead, refers to the interface for handling a request, which makes the independent of which receiver handles the request. The, , and classes implement the 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 object calls on the object (of type ). The forwards the request to , which in turn forwards the request to , which handles (performs) the request.

Example

This C++11 implementation is based on the pre C++98 implementation in the book.

Implementations

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 (OS X)/ (iOS) class. All view objects ( / ), view controller objects ( / ), window objects ( / ), and the application object ( / ) 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: