这是一篇关于Symfony依赖注入(Dependency Injection)的笔记,原文地址:http://fabien.potencier.org/article/11/what-is-dependency-injection

假设需要实现下面的需求:编写一个User类,需要实现Session的相关操作 下面是一种简单的实现方式:

    class SessionStorage
    {
      function __construct($cookieName = 'PHP_SESS_ID')
      {
        session_name($cookieName);
        session_start();
      }
    
      function set($key, $value)
      {
        $_SESSION[$key] = $value;
      }
    
      function get($key)
      {
        return $_SESSION[$key];
      }
    
      // ...
    }
    
    class User
    {
      protected $storage;
    
      function __construct()
      {
        $this->storage = new SessionStorage();
      }
    
      function setLanguage($language)
      {
        $this->storage->set('language', $language);
      }
    
      function getLanguage()
      {
        return $this->storage->get('language');
      }
    
      // ...
    }

我们可以这样调用上面的代码:

    $user = new User();
    $user->setLanguage('fr');
    $user_language = $user->getLanguage();

但这里有一个问题:代码中将User的storage属性写死了,第三方调用User时无法对storage进行配置。因此,对User代码进行下面的修改:

    class User
    {
      function __construct($storage)
      {
        $this->storage = $storage;
      }
    
      // ...
    }

不在User类中创建SessionStorage对象,而是将storage对象作为参数传递给User的构造函数,这就是依赖注入。 “Dependency Injection is where components are given their dependencies through their constructors, methods, or directly into fields.” 上面就是依赖注入的定义,根据其定义,依赖注入可以通过下面三种方式进行:

构造器注入 Constructor Injection:

    class User
    {
      function __construct($storage)
      {
        $this->storage = $storage;
      }
    
      // ...
    }

设值注入 Setter Injection:

    class User
    {
      function setSessionStorage($storage)
      {
        $this->storage = $storage;
      }
    
      // ...
    }

属性 Property Injection:

    class User
    {
      public $sessionStorage;
    }
    
    $user->sessionStorage = $storage;

相关资料:

http://fabien.potencier.org/article/11/what-is-dependency-injection

http://www.martinfowler.com/articles/injection.html

http://www.cnblogs.com/xingyukun/archive/2007/10/20/931331.html