dependency-injection.md

➲ Dependency Injection:

Dependency injection is a technique that promotes loose coupling between different components. It involves passing dependencies to a class constructor or methods via arguments.

☴ Overview:

1. Need for Dependency Injection
2. Constructor Injection
3. Setter Injection
4. Method Injection

✦ Need for Dependency Injection:

• It allows easy testing by injecting mock or stub objects.
• It makes code more flexible and adaptable to changes.
• It allow to reuse components in different contexts with different needs.

✦ Constructor Injection:

Dependencies are injected through the constructor arguments.

Example:

Class Logger {
   public function log($message)
   {
       print $message;
   }
}

class ActualClass {
  private $logger;

  public function __construct(Logger $logger) {
      $this->logger = $logger;
  }

  public function execute() {
      $this->logger->log('executing');
  }
}

$logger = new Logger;
$obj = new ActualClass($logger);
$obj->execute();

✦ Setter Injection:

Dependencies are injected through separate setter methods.

Example:

Class Logger {
    public function log($message)
    {
        print $message;
    }
}

class ActualClass {
   private $logger;

    public function setLogger(Logger $logger) {
         $this->logger = $logger;
     }

   public function execute() {         
       $this->logger->log('executing');
   }
}


$logger = new Logger;
$obj = new ActualClass;
$obj->setLogger($logger);
$obj->execute();

✦ Method Injection:

Dependencies are injected as arguments to methods.

Example:

Class Logger {
    public function log($message)
    {
        print $message;
    }
}

class ActualClass {
   private $logger;

   public function execute(Logger $logger) {
       $this->logger = $logger;
       $this->logger->log('executing');
   }
}


$logger = new Logger;
$obj = new ActualClass;
$obj->execute($logger);

Real-World Example:

class Product {
    public function __construct(public string $name, public float $price) {}
}

function calculateTotalPrice(array $products): float {
    $total = 0;
    foreach ($products as $product) {
        // validating whether it is a class object of Product Class
        if ($product instanceof Product) { 
            $total += $product->price;
        } else {
            // Handle invalid product
            throw new InvalidArgumentException('Invalid product class object');
        }
    }
    return $total;
}

// Usage:
$products = [
    new Product('Product 1', 10.35),
    new Product('Product 2', 50.75),
];
// Inject product class objects
$totalPrice = calculateTotalPrice($products);
echo $totalPrice;

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