Factory pattern in PHP - Step-by-Step Execution

Concept Flow - Factory pattern

Client calls Factory

↓

Factory decides which class to create

↓

Factory creates object

↓

Client uses object

↓

End

The client asks the factory to create an object. The factory decides which class to instantiate and returns the object. The client then uses the object.

Execution Sample

PHP

<?php
interface Animal {
  public function speak();
}

class Dog implements Animal {
  public function speak() { return "Woof!"; }
}

class Cat implements Animal {
  public function speak() { return "Meow!"; }
}

class AnimalFactory {
  public static function create($type): Animal {
    return match($type) {
      'dog' => new Dog(),
      'cat' => new Cat(),
      default => throw new Exception("Unknown animal type")
    };
  }
}

$animal = AnimalFactory::create('dog');
echo $animal->speak();
?>

This code uses a factory to create either a Dog or Cat object based on input, then calls speak() on it.

Execution Table

Step	Action	Input	Factory Decision	Object Created	Output
1	Client calls AnimalFactory::create	'dog'	Matches 'dog'	Dog instance
2	Dog->speak() called				Woof!
3	Program ends

💡 Factory returns Dog object; speak() outputs 'Woof!'; program ends.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
$type	undefined	'dog'	'dog'	'dog'
$animal	undefined	Dog instance	Dog instance	Dog instance
Output	none	none	Woof!	Woof!

Key Moments - 2 Insights

Why does the client not create Dog or Cat directly?

The client calls the factory (see Step 1 in execution_table) to avoid knowing the exact class. This makes code flexible and easier to change.

What happens if an unknown type is passed to the factory?

The factory throws an Exception (not shown in this trace). This prevents creating invalid objects and keeps code safe.

Visual Quiz - 3 Questions

Look at the execution_table, what object is created at Step 1?

A Dog instance

B Cat instance

C Animal interface

D Exception

💡 Hint

Check the 'Object Created' column in Step 1 of execution_table.

At which step does the speak() method output 'Woof!'?

A Step 1

B Step 2

C Step 3

D Never

💡 Hint

Look at the 'Output' column in execution_table for each step.

If the client called AnimalFactory::create('cat'), what would change in the execution_table?

A Output would be 'Woof!'

B An Exception would be thrown

C Factory Decision would be 'Matches cat' and Object Created would be Cat instance

D No change

💡 Hint

Refer to the 'Factory Decision' and 'Object Created' columns in execution_table Step 1.

Concept Snapshot

Factory pattern creates objects without exposing creation logic.
Client calls factory with a type.
Factory returns an object of the correct class.
Helps keep code flexible and easy to extend.
Use static methods or classes to implement factory.

Full Transcript

The factory pattern lets a client ask a factory to create an object. The factory decides which class to instantiate based on input. In the example, the client calls AnimalFactory::create('dog'). The factory matches 'dog' and creates a Dog object. Then the client calls speak() on the Dog object, which outputs 'Woof!'. This way, the client does not need to know about Dog or Cat classes directly. If the client asked for 'cat', the factory would create a Cat object instead. This pattern helps keep code flexible and easier to maintain.

Practice

1. What is the main purpose of the Factory Pattern in PHP?

easy

A. To create objects without exposing the creation logic to the client

B. To store data in a database

C. To handle user input validation

D. To perform mathematical calculations

Solution

1. Step 1: Understand the role of the Factory Pattern

   The Factory Pattern is designed to create objects while hiding the details of how they are created.

2. Step 2: Compare options with the pattern's purpose

   Only To create objects without exposing the creation logic to the client describes creating objects without exposing creation logic, which matches the Factory Pattern.

3. Final Answer:

   To create objects without exposing the creation logic to the client -> Option A

4. Quick Check:

   Factory Pattern = Object creation hiding [OK]

Hint: Factory pattern hides object creation details [OK]

Common Mistakes:

• Confusing factory pattern with database operations
• Thinking it handles input validation
• Assuming it performs calculations

2. Which of the following is the correct way to define a factory method in PHP?

easy

A. public static function create() {}

B. function create() {}

C. public function create() {}

D. private function create() {}

Solution

1. Step 1: Identify typical factory method signature

   Factory methods are often static so they can be called without creating an instance.

2. Step 2: Check options for static keyword

   Only public static function create() {} uses public static function create(), which is the common factory method style.

3. Final Answer:

   public static function create() {} -> Option A

4. Quick Check:

   Factory method is usually static [OK]

Hint: Factory methods are usually static for easy access [OK]

Common Mistakes:

• Omitting static keyword for factory methods
• Using private visibility which blocks access
• Not specifying visibility (defaults to public but less clear)

3. What will be the output of the following PHP code?

class Book { }
class ProductFactory {
    public static function create($type) {
        if ($type === 'Book') {
            return new Book();
        }
        return null;
    }
}
$product = ProductFactory::create('Book');
echo get_class($product);

medium

A. null

B. ProductFactory

C. Book

D. Error: Class not found

Solution

1. Step 1: Analyze the factory method call

   The call ProductFactory::create('Book') checks if the type is 'Book' and returns a new Book object.

2. Step 2: Check the output of get_class

   Since the returned object is an instance of Book, get_class($product) returns "Book".

3. Final Answer:

   Book -> Option C

4. Quick Check:

   Factory returns Book object, get_class = Book [OK]

Hint: Factory returns object; get_class shows its class name [OK]

Common Mistakes:

• Expecting factory class name instead of product class
• Assuming null is returned
• Thinking code throws error due to missing class

4. Identify the error in this factory pattern implementation:

class Car {}
class VehicleFactory {
    public static function create($type) {
        if ($type = 'Car') {
            return new Car();
        }
        return null;
    }
}

medium

A. Missing return type declaration

B. Using assignment (=) instead of comparison (==) in the if condition

C. Class Car is not defined

D. Factory method should not be static

Solution

1. Step 1: Check the if condition syntax

   The condition uses = which assigns 'Car' to $type instead of comparing it.

2. Step 2: Understand impact of assignment in condition

   Assignment always returns true, so the if block always runs regardless of input, which is a logic error.

3. Final Answer:

   Using assignment (=) instead of comparison (==) in the if condition -> Option B

4. Quick Check:

   Use == for comparison, not = [OK]

Hint: Use == for comparison, not = assignment in conditions [OK]

Common Mistakes:

• Confusing = and == in if statements
• Thinking static method is wrong here
• Assuming missing return type causes error

5. You want to extend a factory to create different types of vehicles: Car, Bike, and Truck. Which approach best follows the Factory Pattern principles in PHP?

hard

A. Create separate factory classes for CarFactory, BikeFactory, and TruckFactory each with their own create method

B. Use global functions to create Car, Bike, and Truck objects directly

C. Create one class Vehicle with properties to switch between Car, Bike, and Truck behaviors

D. Create a VehicleFactory with a static create method that returns new Car, Bike, or Truck objects based on input

Solution

1. Step 1: Understand the Factory Pattern goal

   The pattern centralizes object creation in one place, hiding details from the client.

2. Step 2: Evaluate options for best factory design

   Create a VehicleFactory with a static create method that returns new Car, Bike, or Truck objects based on input uses one factory class with a method that creates different vehicle objects based on input, which is the classic factory pattern.

3. Step 3: Compare other options

   Create separate factory classes for CarFactory, BikeFactory, and TruckFactory each with their own create method splits factories, which is more like Abstract Factory or multiple factories, not simple Factory Pattern. Create one class Vehicle with properties to switch between Car, Bike, and Truck behaviors mixes behaviors in one class, breaking single responsibility. Use global functions to create Car, Bike, and Truck objects directly uses global functions, which is not object-oriented.

4. Final Answer:

   Create a VehicleFactory with a static create method that returns new Car, Bike, or Truck objects based on input -> Option D

5. Quick Check:

   Single factory method creates varied objects [OK]

Hint: One factory method returns different objects by input [OK]

Common Mistakes:

• Splitting factories unnecessarily
• Mixing multiple behaviors in one class
• Using global functions instead of classes