Memento
Objective
Store the internal state of an object in an environment external to it, so that this state can be restored later.
Function
Return to previous system states.
Structure
As shown in figure 1
- Client: Component that affects the Originator and records the new status with the Caretaker. In other words, it is the one who makes the change on the object and records the status.
- Originator: It is the component that changes its state.
- Memento: Component that stores the state of the Originator at a given time.
- Vigilant: Component that records the changes of the Originator. This component allows us to travel between the different states of the Originator.
The structure that meets this pattern is shown in Figure 1
Figure 1: UML Diagram Memento Pattern
Applications
The use of the Memento pattern is recommended when:
- It is necessary to preserve the state of an object over time (persistence), and at the same time encapsulate the values of the attributes of that state of the object to external objects.
Design Patterns Collaborators
Scope of action
Applied at the object level.
Problem
An application wants to implement the functionality of undoing actions performed on objects, which implies saving their states; this requires the implementation of the functionality of saving and recovering the state in the object itself, saturating the memory.
Solution
The Memento design pattern suggests separating the object from the functions of saving and retrieving a particular state, while retaining the encapsulation of the object's attribute values, by restricting the access of other objects to the persistence of the object's states.
Diagram or Implementation
Figure 2: UML Diagram Memento Pattern
Figure 2 explains the behaviour of the pattern by means of a sequence diagram.
- The client class applies a change to the Originator component.
- The Originator component creates a new Memento that represents its current state.
- The client class saves the Memento in the Caretaker component so that it can later switch between the states of the Originator component.
- After some time, the client class asks the Caretaker component for the previous state of the Originator component.
- The client class restores the state of the Originator by means of the Memento obtained from the Caretaker component.
Implementations of the Memento 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();
}
}