Dependency injection in PHP traits

Question

Do traits actually work with dependency injection? Consider the following code:

Trait Class

namespace Frame\Slick\Block;
use Frame\Slider\Slick\Block\Data as Helper

trait Slick
{
   protected $_slickHelper;
   public function __construct(Helper $slickHelper) 
   {
     $this->_slickHelper = $slickHelper;
   }
}

Class using the trait

namespace Frame\Slick\Block;

class Product ListProduct implements BlockInterface 
{
   use Slick;
   public function testTrait()
   {
      return $this->_slickHelper->getHelloWorld();
   }
}

This seems to always return null, am very sure everything is being included properly. Can trait really support dependency injection?

Answer 1

Yes they work the code from trait is "pasted" on compliation level. Consider the following code. It works as expected and echos proper value. Your problem is elsewhere.

<?php
class Dependency
{
    public function foo()
    {
        return 'test';
    }
}

trait Slick
{
   protected $dep;

   public function __construct(Dependency $dep) 
   {
       $this->dep = $dep;
   }
}

class Product 
{
   use Slick;

   public function testTrait()
   {
      return $this->dep->foo();
   }
}

echo (new Product(new Dependency()))->testTrait();

The code will echo 'test'. Working fiddle

Answer 2

A properly implemented framework should be able to do dependency injection on trait constructors. Typically parameters which are eligible for injection are determined using reflection. Consider the following example:

<?php 

class D {}

trait T {

    public function __construct(D $d) { }


}

class A {

use T;

}


$cls = new ReflectionClass("A");

$ctor = $cls->getConstructor();

print_r($ctor->getParameters()[0]->getClass());

Prints:

ReflectionClass Object
(
    [name] => D
)

This indicates that the framework can use reflection to determine whether or not to inject the dependency, the fact that the constructor is in the trait does not matter.

If this is not happening in magento then I suggest you move this to their suggestion thread (if any).

I would also recommend you read up on how dependency injection works in magento via their dependency injection documentation