If you are starting to learn about Drupal 8, you must have come across a term called “Dependency Injection”. If you are wondering what it means, then this is the post you should read. Through examples, you will learn why dependency injection is useful for decoupling the code as well as unit testing effectively.
In the post on PHP Inheritance, we created HondaAccord and ChevyRam classes, both of which extend the Vehicle class. Vehicle has getWarrantyLeft(), drive() and getMilesDriven() methods. getWarrantyLeft() method is being overridden by ChevyRam while HondaAccord has a trunk and implements putInTrunk() and takeFromTrunk() methods. Vehicle also has color, warranty and milesDriven properties.
Let’s also assume that there is a Person class and each person owns a vehicle. We are initializing this vehicle in the constructor of the Person class. Person has methods travel() and getDistanceTraveled(). travel() method uses Vehicle‘s drive() method and getDistanceTraveled() method uses the Vehicle‘s getMilesDriven() method. Here is how Vehicle.php and Person.php files look:
Vehicle.php
<?php
/**
* Vehicle class.
*/
class Vehicle {
// Color of the vehicle.
public $color;
// Miles driven.
private $milesDriven;
// Warranty available.
public $warranty;
/**
* Default constructor.
*
* @param string $color
* Color of the vehicle.
* @param int $milesDriven
* Number of miles driven already.
* @param int $warrantyProvided
* Number of miles of warranty provided with the vehicle.
*/
function __construct($color, $milesDriven = 0, $warranty = 100000) {
$this->color = $color;
$this->milesDriven = $milesDriven;
$this->warranty = $warranty;
}
/**
* Returns miles of warranty left on the vehicle.
*
* @return int
* Miles of warrnty left on the vehicle.
*/
public function getWarrantyLeft() {
if ($this->warranty - $this->getMilesDriven() > 0) {
return ($this->warranty - $this->getMilesDriven());
}
return 0;
}
/**
* Drive the vehicle. This will add to miles driven.
*
* @param int $miles
* Number of miles driven in the current trip.
*/
public function drive($miles) {
if ($miles > 0) {
$this->milesDriven += $miles;
}
}
/**
* Returns the number of miles driven in total.
*
* @param int
* Total number of miles driven.
*/
public function getMilesDriven() {
return $this->milesDriven;
}
}
/**
* HondaAccord class.
*/
class HondaAccord extends Vehicle {
// Stuff in the trunk.
public $stuff;
/**
* Put stuff in trunk.
*
* @param mixed $stuff
* Stuff to be put in the trunk.
*/
public function putInTrunk($stuff) {
$this->stuff = $stuff;
}
/**
* Take stuff out from the trunk.
*
* @return mixed
* Stuff returned from the trunk.
*/
public function TakeFromTrunk() {
$stuff = $this->stuff;
unset($this->stuff);
return $stuff;
}
}
/**
* ChevyRam class.
*/
class ChevyRam extends Vehicle {
/**
* Returns miles of warranty left on the vehicle.
*
* @return int
* Miles of warrnty left on the vehicle.
*/
public function getWarrantyLeft() {
if ($this->warranty - $this->getMilesDriven() > 0) {
return (2 * ($this->warranty - $this->getMilesDriven()));
}
return 0;
}
}
?>
Person.php
<?php
require_once 'Vehicle.php';
/**
* Person class.
*/
class Person {
// Vehicle
public $vehicle;
/**
* Default constructor.
*/
function __construct() {
$this->vehicle = new HondaAccord('red');
}
/**
* Let the person travel in his vehicle.
*
* param int $miles
* Number of miles that the person travels.
*/
public function travel($miles) {
$this->vehicle->drive($miles);
}
/**
* Returns the number of miles that the person has traveled.
*
* @return int
* Number of miles that the person has traveled.
*/
public function getDistanceTraveled() {
return $this->vehicle->getMilesDriven();
}
}
$you = new Person();
$you->travel(2000);
echo 'Distance traveled: ' . $you->getDistanceTraveled() . "\n";
?>Following is the output when executing Person.php file:
$ php Person.php
Distance traveled: 2000
You will notice that in the constructor of Person class, we are initializing the vehicle he owns to be a red Honda Accord. So this code works fine as long as we expect all the people to own a red Honda Accord. But obviously this is not the case in real life
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