An experiment: using traits for dependency injection

trait-deps.php

<?php
// An experiment: using traits for dependency injection

// A trait provides a way to inject a Logger and a way to use it
trait aLogger {
  protected $logger = null;
  public function setLogger(Logger $logger){
    $this->logger = $logger;
  }
  public function log($msg){
    if (is_null($this->logger)) return false;
    $this->logger->log($msg);
  }
}

// The logger that will be injected
class Logger {
  public function log($msg){
    echo $msg.PHP_EOL;
  }
}

// Some other class defines that it needs to use "a Logger"
class Widget {
  use aLogger;

  public function test(){
    echo "A test!\n";

    // aLogger::log does nothing if $this->logger is null
    $this->log("A log!");
  }
}


// The factory for creating objects
class Factory {
  // A way to get a logger; nothing special
  public function newLogger(){
    return new Logger();  
  }

  // A way to get a new Widget
  public function newWidget(){
    $w = new Widget();

    // Look for and inject dependencies
    return $this->injectDeps($w);
  }

  protected function injectDeps($obj){
    // Look up the traits the class is using
    $traits = class_uses($obj);

    // If the class is *use*ing aLogger, then we know to inject one
    if ($traits['aLogger']){
      $obj->setLogger($this->newLogger());
    }

    return $obj;
  }
}


// Create a factory
$f = new Factory();

// Ask the factory for a new Widget; the factory will 
// detect that the Widget wants "a logger" and inject one.
$w = $f->newWidget();

// Outputs "A test!\nA log!\n"
$w->test(); 


// We can create an object without using the factory (e.g. for testing)
// and it still "work"; it's just nothing will be logged.
$testWidget = new Widget();

// Outputs "A test!\n"
$testWidget->test();