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DDD?

Domain Driven Design

● Domains, Subdomains, and Bounded Context

● Architecture● Entities● Value Objects● Services● Domain Events● Modules● Aggregates● Factories● Repositories● Integrating Bounded Context● Application

“Value Object: An object that contains attributes but has no conceptual identity. They should be treated as

immutable.”

Wikipedia

“A Value Object is a simple object that is defined by its properties, and reflects a concept from the business domain - e.g. Money, Email Address,

Temperature, Location etc. The main characteristic of a Value Object is that it is

immutable. This means that it has ...”

http://www.phpvalueobjects.info

Numbers

● 42● 3,14● 82● …

Strings

● “Demo”● “@sensorario”● “Domain-Driven Design”● …

Dates

● 10/09/1982● 17/05/2015● …

Currency

● EUR● USD● …

Roadmap

● Characteristics of a domain concepts● Use of Standard types● Storing Value Objects● Testing Value Objects● Implementations● Resources

Characteristics

● Measures, quantifies, or describe● Immutability● Conceptual whole● Replaceability● Equality● Side-Effect-Free behavior with collaborators

Measures, quantifies, or describe

● It is not a thing in your domain● Is a concept that measures, quantifies, or

describe a thing in your domain

Person

● A person has an age● The age measure numbers of year of that

person

Person

● A person has a name● The name is not a thing, but describes how

we can call (the value) the person (the thing)

Characteristics

● Measures, quantifies, or describe● Immutability● Conceptual whole● Replaceability● Equality● Side-Effect-Free behavior with collaborators

Value Object Immutability

Its methods, whether public or private, will not cause its state to change

class FullName {

public static function withNameMiddleSurname(...) {

...

}

public static function withNameAndSurname(...) {

...

}

}

public function testImmutability()

{

$demo = FullName::withNameMiddleSurname(

new StringLiteral("Simone"),

new StringLiteral("Demo"),

new StringLiteral("Gentili")

);

$sensorario = $demo->withMiddleName(

new StringLiteral("Sensorario")

);

$this->assertTrue($sensorario != $demo);

}

class FullName {

private function __construct(...) {

// ...

}

public function withMiddleName(StringLiteral $middleName) {

return new self(

$this->name,

$middleName,

$this->surname

);

}

}

class FullName {

public function withMiddleName(...) {

$that = clone $this;

$that->name = “Davide”;

$that->middleName = “Sbiellone”;

$that->surname = “Bellettini”;

return $that;

}

}

Immutability with references

Value Objects can hold references to Entities, but when referenced Entity change state, Value Object changes too, which violate the quality of immutability

Characteristics

● Measures, quantifies, or describe● Immutability● Conceptual whole● Replaceability● Comparability● Side-Effect-Free behavior with collaborators

Conceptual Whole of attributes

A Value Object may possess just one, a few, or a number of individual attributes, each of which is related to the others

Conceptual Whole meaning

Only together do all the attributes form the others, each of the attributes form the complete intended measure or description

Conceptual Whole

● {42 euro} has two attributes● The attribute 42 and the attribute euro● Separately these attributes describe something

else or nothing special

Characteristics

● Measures, quantifies, or describe● Immutability● Conceptual whole● Replaceability● Comparability● Side-Effect-Free behavior with collaborators

Replace variable “Value”

$total = 42;

// ...

$total = 4;

Replace variable “Value Object”

Is exactly what replacement even when a given Value Object Type is more complex than an integer

Replaceability

$speaker = Speaker::withFullName(

“Simone”,

“Gentili”

);

$speaker = Speaker::withFullNameAndNickName(

“Simone”,

“Gentili”,

“Demo”

);

Characteristics

● Measures, quantifies, or describe● Immutability● Conceptual whole● Replaceability● Equality● Side-Effect-Free behavior with collaborators

Value Objects Equality

● When tho value objects have same values, they are equals

● Values has no identity

== in PHP

“When using the comparison operator (==), object variables are compared in a simple manner, namely: Two object instances are equal if they have the same attributes and values, and are instances of the same class.”

Equality that could differ from ==

public function sameValueAs(ValueObjectInterface $date)

{

if (false === Util::classEquals($this, $date)) {

return false;

}

return $this->getYear()->sameValueAs($date->getYear()) &&

$this->getMonth()->sameValueAs($date->getMonth()) &&

$this->getDay()->sameValueAs($date->getDay());

}

Equality

$simone = new Speaker(

new StringLiteral("Simone"),

new StringLiteral("Gentili"),

new IntegerValue("32")

);

$twin = new Speaker(

new StringLiteral("Simone"),

new StringLiteral("Gentili"),

new IntegerValue("32")

);

$this->assertTrue($simone == $twin);

Characteristics

● Measures, quantifies, or describe● Immutability● Conceptual whole● Replaceability● Equality● Side-Effect-Free behavior with collaborators

Side-Effect-Free Behavior

Since no modification occurs when executing a specific operation, that operation is said to be side-effect-free

Side-Effect-Free Behavior

The methods of an immutable Value Object must all be Side-Effect-Free because they must not violate its immutability quality

Side-Effect-Free Behavior

When a Value's method takes an Entity as parameter, it may be best for it to answer a result that the Entity could use to modify itself on its own terms

Roadmap

● Characteristics of a domain concepts● Use of Standard types● Storing Value Objects● Testing Value Objects● Implementations● Resources

Standard Types Expressed as Values

● Currency (Value)● “EUR, USD, ...” (Standard Types)

Standard Types Expressed as Values

● HTTP Status Code (Value)● “201, 302, 404, ...” (Standard Types)

Standard Types Expressed as Values

Using a Standard Type here helps you avoid bogus currencies (or HTTP Status Codes)

Roadmap

● Characteristics of a domain concepts● Use of Standard types● Storing Value Objects● Testing Value Objects● Implementations● Resources

Persisting Value Objects

When persisted, an instance of a Value Object will occupy its own row in a relational database table that exists specifically for its type, and have its own database primary key column: Collection of Value Objects

Persisting Value Objects

In a general sense, it involves serializing the object to some text or binary format and saving it to disk

Roadmap

● Characteristics of a domain concepts● Use of Standard types● Storing Value Objects● Testing Value Objects● Implementations● Resources

Testing Value Objects

If Values are final classes, cannot be mocked

Roadmap

● Characteristics of a domain concepts● Use of Standard types● Storing Value Objects● Testing Value Objects● Implementations● Resources

Implementations

“If you have achieved the definitive functionality fit your class or method, and you feel that overriding it can only damage the ultimate perfection of your work, you may need the final keyword”

Matt Zandstra

Roadmap

● Characteristics of a domain concepts● Use of Standard types● Storing Value Objects● Testing Value Objects● Implementations● Resources

Github Repositories

● sensorario/kataphp● nicolopignatelli/valueobjects● gws/php-valueobjects● sebastianbergmann/money

Resources

“URIs are values, with identity defined by the value, and thus should be modeled as value

objects.”

https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-7-http-

message-meta.md#why-are-uris-represented-as-objects

Resources

“A value object should always override .equals() in Java (or = in Smalltalk). (Remember to override .hashCode() as well.)”

http://c2.com/cgi/wiki?ValueObject

Contacts

sensorario@gmail.com

twitter.com/sensorario

instagram.com/sensorario

linkedin.com/in/sensorario

github.com/sensorario

facebook.com/sensorario

Q/A