A short introduction to the Value Object pattern in PHP. Use the type system to your advantage, ensure your values are actually valid.

1. Value Objects
Make your code simpler

2. Who am I
Product Developer - Systems Architect - Freelancer - Development Manager
Past experience
Senior Developer for Cleverbug
Lecturer at the Digital Skills Academy
Director of Tercet, software development consultancy
Current position
Software Development Manager for OliveMedia
Barry O Sullivan

3. Design Patterns
When developing, there are common patterns of problems.
Design patterns are common solutions to those problems.
The ValueObject is a simple design pattern that you can use right away.

4. The Problem
public function changeName($firstname, $lastname)
{
if (!is_string($firstname) || $firstname == “”) {
throw new InvalidNameException(“Firstname ‘$firstname’ is invalid”);
}
if (!is_string($lastname) || $lastname == “”) {
throw new InvalidNameException(“Lastname ‘$lastname is invalid”);
}
$this->firstname = $firstname;
$this->lastname = $lastname;
}
Have you ever written code like this?

5. Guarding
This is called Guarding
You are guarding against bad input
The idea is, you can’t trust that other
objects/developers are actually sending you valid
data, you’re trying to protect against inconsistent
state.
Guarding is a defensive patterns and is encoraged
in the book “The Mythical Man Month”.

6. Too much guarding
The problem arises when you start adding this logic everywhere.
You end up with
● Duplicated code
● Messy Logic
● Poor legibility
Which leads to a system that’s very
difficult to modify and understand.
Leading to more bugs, not less. Developers

7. The Solution
How do we solve this problem?
With encapsulation!
You create a class that represents the value.
On creation, it gets passed the basic types and checks if they’re valid.
● If they are not, an exception is thrown.
● If they are, then the object is successfully created.
That’s the basic gist of it.

8. Example, Name as a ValueObject
class Name implements ValueObject {
private $firstname;
private $lastname;
public function __construct($firstname, $lastname) {
if (!is_string($firstname) || $firstname == “”) {
throw new InvalidNameException(“Firstname ‘$firstname’ is invalid”);
}
if (!is_string($lastname) || $lastname == “”) {
throw new InvalidNameException(“Lastname ‘$lastname is invalid”);
}
$this->firstname = $firstname;
. . . . . .

9. Using the Name ValueObject
//Controller method to change name
public function changeName()
{
$user = $this->get_user($this->user_id);
$name = new Name(Input::get(‘firstname’), Input::get(‘lastname’));
$user->changeName($name);
}
. . . . . .
//Then in our User class
public function changeName(Name $name)
{
$this->name = $name;
}

10. ValueObjects represent the value
Key Concept:
A value object represents the value,
not the data contained within it.
You shouldn’t care about the internal data when using them for business logic.
When you want to reference a value, use a value object.
The only place the internals matter is when you’re encoding/decoding them for
transmission (JSON/HTML) or storage (MySQL/REDIS), nowhere else.

11. Key features in the wild
They are immutable
They are comparable

12. Quick Recap:
ValueObjects . . .
● Encapsulate a value
● Allow you to guard against bad input
● Represent the value, not the data within
● Are immutable
● Make code and system design much easier, leading to
more structured system that’s easier to maintain.

13. More reading
Thanks for listening
If you’re interested in learning more
about this pattern and others like it, I’d
recommend
Domain Driven Design.

14. Q&A
“Judge a man by his questions rather than by his answers.”
- Voltaire
“I needed content for this slide, so I added quotes.”
- Barry O Sullivan