## Chain of Responsibility pattern

### Introduction

Avoid coupling the sender of a request to its receiver by giving more than object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.

### Participants:

#### Handler

- defines an interface for handling requests.
- implements the successor link.
- this participant can not be read-only.
- this participant can not be an interface.

#### Concrete handler

- handles requests it is responsible for.
- can access its successor.
- if the Concrete Handler can handle the request, it does so; otherwise it forwards the requests to its successor.

In the "Attribute" field you can specify the name of an attribute where Concrete Handler will keep a reference to its successor.

The "Copy documentation" option controls whether to copy JavaDoc comments from methods in interfaces participating in the pattern to the stubs of these methods made by the pattern in classes implementing such interfaces.

The "Create pattern links" option controls whether to generate additional links which can be used by this pattern later to determine classes and interfaces participating in the pattern. That means that if you selected this option and used the pattern to create a set of classes and interfaces, the pattern invoked for some participant later (via the popup menu item "Choose Pattern...") will automatically find (if it is possible) all other participants and fill in participant fields with their names.

Also, if you applied the pattern with this option on and invoked the pattern via the popup menu item "Choose Pattern..." for some participant later, the pattern will have additional field called "Use selected class as", containing possible roles for the selected element.

This option is very useful when you are planning to change something in the classes/interfaces participating in the pattern. For example, if this option is on and after creation of the classes and interfaces you added several methods to certain interface-participant (and this change must be reflected somehow in other participants), all you have to do is to select this changed interface, invoke the "Choose Pattern..." dialog for this element and select the original pattern. After that the pattern will determine other participants and you will only have to press "Finish". The pattern will modify all other classes and interfaces accordingly to your changes.

### Applicability

Use Chain of Responsibility when

- more than one object may handle a request, and the handler isn't known a priory. The handler should be ascertained automatically.
- you want to issue a request to one of several objects without specifying the receiver explicitly.
- the set of objects that can handle a request should be specified dynamically.