Chain of responsibility
Objective
Avoid coupling the class requesting an action with the class executing it to allow more than one object to handle the request. Chain the receiving objects and pass the request along the whole chain until one object handles it.
Function
Establish the line that must carry the messages for the objects to perform the indicated task.
Structure
As shown in figure 1The Handler class declares the interface, common to all ConcreteHandlers classes. It usually contains only one method for handling requests, but sometimes it can also have another method for setting up the next handler in the chain.
The structure that meets this pattern is shown in Figure 1
Figure 1: UML Diagram Chain of responsibility Pattern
Applications
The use of the Chain of Responsibility pattern is recommended when:
- Several objects must handle a request, and determining who does it is not a priority since the object that handles it is determined automatically.
- You want to make a request to one of several objects without explicitly specifying who should receive the request.
- The set of objects capable of handling a request must be defined dynamically.
Design Patterns Collaborators
- The chain of responsibility pattern is usually implemented along with the composite pattern, as a parent component can be a successor.
Scope of action
Applied at the object level.
Problem
An application needs to execute an action independently of its state, conventionally it is possible to cover this requirement, but to achieve it, each object must instantiate the respective method that is in charge of executing the request and also know the objects capable of executing the same action and invoke them in a certain order, which generates a high coupling between objects.
Solution
The Chain of Responsibility design pattern presents a more optimal solution to cover this requirement, since it proposes to prepare each object to be able to handle a request on its own or to pass the request to another dynamically defined object at execution time, which generates a chain of objects that are potential executors of a request. If the request cannot be executed, the object passes it to the next link in the chain.
Diagram or Implementation
Figure 2: UML Diagram Chain of responsibility Pattern
Figure 2 explains the behaviour of the pattern by means of a sequence diagram.
- The client class requests the processing of a request to a chain of responsibility.
- The first Handler component tries to process the message, however, it is not able to process it for some reason and sends the message to the next handler component in the chain.
- The second Handler component tries to process the message without success, so it sends the message to the next Handler component in the chain.
- The third Handler component also tries to process the message without success and sends the message to the next Handler in the chain.
- The HandlerN component (Some sequence handler) is finally able to process the message successfully and returns a response (optional) so that the response is replicated by all the past Handler components until it reaches the client class.
Implementations of the Chain of responsibility pattern:
from __future__ import annotations
from abc import ABC, abstractmethod, abstractproperty
from typing import Any
class Builder(ABC):
"""
The Builder interface specifies methods for creating the different parts of
the Product objects.
"""
@abstractproperty
def product(self) -> None:
pass
@abstractmethod
def produce_part_a(self) -> None:
pass
@abstractmethod
def produce_part_b(self) -> None:
pass
@abstractmethod
def produce_part_c(self) -> None:
pass
class ConcreteBuilder1(Builder):
"""
The Concrete Builder classes follow the Builder interface and provide
specific implementations of the building steps. Your program may have
several variations of Builders, implemented differently.
"""
def __init__(self) -> None:
"""
A fresh builder instance should contain a blank product object, which is
used in further assembly.
"""
self.reset()
def reset(self) -> None:
self._product = Product1()
@property
def product(self) -> Product1:
"""
Concrete Builders are supposed to provide their own methods for
retrieving results. That's because various types of builders may create
entirely different products that don't follow the same interface.
Therefore, such methods cannot be declared in the base Builder interface
(at least in a statically typed programming language).
Usually, after returning the end result to the client, a builder
instance is expected to be ready to start producing another product.
That's why it's a usual practice to call the reset method at the end of
the `getProduct` method body. However, this behavior is not mandatory,
and you can make your builders wait for an explicit reset call from the
client code before disposing of the previous result.
"""
product = self._product
self.reset()
return product
def produce_part_a(self) -> None:
self._product.add("PartA1")
def produce_part_b(self) -> None:
self._product.add("PartB1")
def produce_part_c(self) -> None:
self._product.add("PartC1")
class Product1():
"""
It makes sense to use the Builder pattern only when your products are quite
complex and require extensive configuration.
Unlike in other creational patterns, different concrete builders can produce
unrelated products. In other words, results of various builders may not
always follow the same interface.
"""
def __init__(self) -> None:
self.parts = []
def add(self, part: Any) -> None:
self.parts.append(part)
def list_parts(self) -> None:
print(f"Product parts: {', '.join(self.parts)}", end="")
class Director:
"""
The Director is only responsible for executing the building steps in a
particular sequence. It is helpful when producing products according to a
specific order or configuration. Strictly speaking, the Director class is
optional, since the client can control builders directly.
"""
def __init__(self) -> None:
self._builder = None
@property
def builder(self) -> Builder:
return self._builder
@builder.setter
def builder(self, builder: Builder) -> None:
"""
The Director works with any builder instance that the client code passes
to it. This way, the client code may alter the final type of the newly
assembled product.
"""
self._builder = builder
"""
The Director can construct several product variations using the same
building steps.
"""
def build_minimal_viable_product(self) -> None:
self.builder.produce_part_a()
def build_full_featured_product(self) -> None:
self.builder.produce_part_a()
self.builder.produce_part_b()
self.builder.produce_part_c()
if __name__ == "__main__":
"""
The client code creates a builder object, passes it to the director and then
initiates the construction process. The end result is retrieved from the
builder object.
"""
director = Director()
builder = ConcreteBuilder1()
director.builder = builder
print("Standard basic product: ")
director.build_minimal_viable_product()
builder.product.list_parts()
print("\n")
print("Standard full featured product: ")
director.build_full_featured_product()
builder.product.list_parts()
print("\n")
# Remember, the Builder pattern can be used without a Director class.
print("Custom product: ")
builder.produce_part_a()
builder.produce_part_b()
builder.product.list_parts()
namespace RefactoringGuru\Builder\Conceptual;
/**
* The Builder interface specifies methods for creating the different parts of
* the Product objects.
*/
interface Builder
{
public function producePartA(): void;
public function producePartB(): void;
public function producePartC(): void;
}
/**
* The Concrete Builder classes follow the Builder interface and provide
* specific implementations of the building steps. Your program may have several
* variations of Builders, implemented differently.
*/
class ConcreteBuilder1 implements Builder
{
private $product;
/**
* A fresh builder instance should contain a blank product object, which is
* used in further assembly.
*/
public function __construct()
{
$this->reset();
}
public function reset(): void
{
$this->product = new Product1();
}
/**
* All production steps work with the same product instance.
*/
public function producePartA(): void
{
$this->product->parts[] = "PartA1";
}
public function producePartB(): void
{
$this->product->parts[] = "PartB1";
}
public function producePartC(): void
{
$this->product->parts[] = "PartC1";
}
/**
* Concrete Builders are supposed to provide their own methods for
* retrieving results. That's because various types of builders may create
* entirely different products that don't follow the same interface.
* Therefore, such methods cannot be declared in the base Builder interface
* (at least in a statically typed programming language). Note that PHP is a
* dynamically typed language and this method CAN be in the base interface.
* However, we won't declare it there for the sake of clarity.
*
* Usually, after returning the end result to the client, a builder instance
* is expected to be ready to start producing another product. That's why
* it's a usual practice to call the reset method at the end of the
* `getProduct` method body. However, this behavior is not mandatory, and
* you can make your builders wait for an explicit reset call from the
* client code before disposing of the previous result.
*/
public function getProduct(): Product1
{
$result = $this->product;
$this->reset();
return $result;
}
}
/**
* It makes sense to use the Builder pattern only when your products are quite
* complex and require extensive configuration.
*
* Unlike in other creational patterns, different concrete builders can produce
* unrelated products. In other words, results of various builders may not
* always follow the same interface.
*/
class Product1
{
public $parts = [];
public function listParts(): void
{
echo "Product parts: " . implode(', ', $this->parts) . "\n\n";
}
}
/**
* The Director is only responsible for executing the building steps in a
* particular sequence. It is helpful when producing products according to a
* specific order or configuration. Strictly speaking, the Director class is
* optional, since the client can control builders directly.
*/
class Director
{
/**
* @var Builder
*/
private $builder;
/**
* The Director works with any builder instance that the client code passes
* to it. This way, the client code may alter the final type of the newly
* assembled product.
*/
public function setBuilder(Builder $builder): void
{
$this->builder = $builder;
}
/**
* The Director can construct several product variations using the same
* building steps.
*/
public function buildMinimalViableProduct(): void
{
$this->builder->producePartA();
}
public function buildFullFeaturedProduct(): void
{
$this->builder->producePartA();
$this->builder->producePartB();
$this->builder->producePartC();
}
}
/**
* The client code creates a builder object, passes it to the director and then
* initiates the construction process. The end result is retrieved from the
* builder object.
*/
function clientCode(Director $director)
{
$builder = new ConcreteBuilder1();
$director->setBuilder($builder);
echo "Standard basic product:\n";
$director->buildMinimalViableProduct();
$builder->getProduct()->listParts();
echo "Standard full featured product:\n";
$director->buildFullFeaturedProduct();
$builder->getProduct()->listParts();
// Remember, the Builder pattern can be used without a Director class.
echo "Custom product:\n";
$builder->producePartA();
$builder->producePartC();
$builder->getProduct()->listParts();
}
$director = new Director();
clientCode($director);
/* "Product" */
class Pizza {
private String dough = "";
private String sauce = "";
private String topping = "";
public void setDough(String dough) {
this.dough = dough;
}
public void setSauce(String sauce) {
this.sauce = sauce;
}
public void setTopping(String topping) {
this.topping = topping;
}
}
/* "Abstract Builder" */
abstract class PizzaBuilder {
protected Pizza pizza;
public Pizza getPizza() {
return pizza;
}
public void createNewPizzaProduct() {
pizza = new Pizza();
}
public abstract void buildDough();
public abstract void buildSauce();
public abstract void buildTopping();
}
/* "ConcreteBuilder" */
class HawaiianPizzaBuilder extends PizzaBuilder {
public void buildDough() {
pizza.setDough("cross");
}
public void buildSauce() {
pizza.setSauce("mild");
}
public void buildTopping() {
pizza.setTopping("ham+pineapple");
}
}
/* "ConcreteBuilder" */
class SpicyPizzaBuilder extends PizzaBuilder {
public void buildDough() {
pizza.setDough("pan baked");
}
public void buildSauce() {
pizza.setSauce("hot");
}
public void buildTopping() {
pizza.setTopping("pepperoni+salami");
}
}
/* "Director" */
class Waiter {
private PizzaBuilder pizzaBuilder;
public void setPizzaBuilder(PizzaBuilder pb) {
pizzaBuilder = pb;
}
public Pizza getPizza() {
return pizzaBuilder.getPizza();
}
public void constructPizza() {
pizzaBuilder.createNewPizzaProduct();
pizzaBuilder.buildDough();
pizzaBuilder.buildSauce();
pizzaBuilder.buildTopping();
}
}
/* A customer ordering a pizza. */
public class PizzaBuilderDemo {
public static void main(String[] args) {
Waiter waiter = new Waiter();
PizzaBuilder hawaiianPizzabuilder = new HawaiianPizzaBuilder();
PizzaBuilder spicyPizzaBuilder = new SpicyPizzaBuilder();
waiter.setPizzaBuilder( hawaiianPizzabuilder );
waiter.constructPizza();
Pizza pizza = waiter.getPizza();
}
}