1. Dependency Injection
Fabien Potencier

2. Fabien Potencier
Serial entrepreneur
Developer by passion
Founder of Sensio
Creator and lead developer of Symfony
On Twitter @fabpot
On github http://www.github.com/fabpot
Blog http://fabien.potencier.org/

3. Dependency Injection
A real world « web » example

4. In most web applications, you need to manage the user preferences
– The user language
– Whether the user is authenticated or not
– The user credentials
– …

5. This can be done with a User object
– setLanguage(), getLanguage()
– setAuthenticated(), isAuthenticated()
– addCredential(), hasCredential()
– ...

6. The User information
need to be persisted
between HTTP requests
We use the PHP session for the Storage

7. class SessionStorage
{
function __construct($cookieName = 'PHP_SESS_ID')
{
session_name($cookieName);
session_start();
}
function set($key, $value)
{
$_SESSION[$key] = $value;
}
// ...
}

8. class User
{
protected $storage;
function __construct()
{
$this->storage = new SessionStorage();
}
function setLanguage($language)
{
$this->storage->set('language', $language);
}
// ...
}
$user = new User();
Very easy
to use
Very hard to
customize

9. class User
{
protected $storage;
function __construct($storage)
{
$this->storage = $storage;
}
}
$storage = new SessionStorage();
$user = new User($storage);
Slightly more
difficult to use
Very easy to
customize

10. That’s Dependency Injection
Nothing more

11. Let’s understand why the first example
is not a good idea

12. I want to change the session cookie name

13. class User
{
protected $storage;
function __construct()
{
$this->storage = new SessionStorage('SESSION_ID');
}
function setLanguage($language)
{
$this->storage->set('language', $language);
}
// ...
}
$user = new User();
Hardcode it in the
User class

14. class User
{
protected $storage;
function __construct()
{
$this->storage = new SessionStorage(STORAGE_SESSION_NAME);
}
}
define('STORAGE_SESSION_NAME', 'SESSION_ID');
$user = new User();
Add a global
configuration?

15. class User
{
protected $storage;
function __construct($sessionName)
{
$this->storage = new SessionStorage($sessionName);
}
}
$user = new User('SESSION_ID');
Configure via
User?

16. class User
{
protected $storage;
function __construct($storageOptions)
{
$this->storage = new SessionStorage($storageOptions
['session_name']);
$user = new User(
array('session_name' => 'SESSION_ID')
);
Configure with an
array?

17. I want to change the session storage implementation
Filesystem
MySQL
Memcached
…

18. class User
{
protected $storage;
function __construct()
{
$this->storage = Registry::get('session_storage');
}
}
$storage = new SessionStorage();
Registry::set('session_storage', $storage);
$user = new User();
Use a global
registry object?

19. Now, the User depends on the Registry

20. Instead of harcoding
the Storage dependency
inside the User class constructor
Inject the Storage dependency
in the User object

21. class User
{
protected $storage;
function __construct($storage)
{
$this->storage = $storage;
}
}
$storage = new SessionStorage('SESSION_ID');
$user = new User($storage);

22. What are the advantages?

23. Use different Storage strategies

24. class User
{
protected $storage;
function __construct($storage)
{
$this->storage = $storage;
}
}
$storage = new MySQLSessionStorage('SESSION_ID');
$user = new User($storage);
Use a different
Storage engine

25. Configuration becomes natural

26. class User
{
protected $storage;
function __construct($storage)
{
$this->storage = $storage;
}
}
$storage = new MySQLSessionStorage('SESSION_ID');
$user = new User($storage);
Configuration
is natural

27. Wrap third-party classes (Interface / Adapter)

28. class User
{
protected $storage;
function __construct(SessionStorageInterface $storage)
{
$this->storage = $storage;
}
}
interface SessionStorageInterface
{
function get($key);
function set($key, $value);
}
Add an interface

29. Mock the Storage object (for testing)

30. class User
{
protected $storage;
function __construct(SessionStorageInterface $storage)
{
$this->storage = $storage;
}
}
class SessionStorageForTests implements SessionStorageInterface
{
protected $data = array();
static function set($key, $value)
{
self::$data[$key] = $value;
}
}
Mock the Session

31. Use different Storage strategies
Configuration becomes natural
Wrap third-party classes (Interface / Adapter)
Mock the Storage object (for testing)
Easy without changing the User class

32. « Dependency Injection is where
components are given their dependencies
through their constructors, methods, or
directly into fields. »
http://www.picocontainer.org/injection.html

33. $storage = new SessionStorage();
// constructor injection
$user = new User($storage);
// setter injection
$user = new User();
$user->setStorage($storage);
// property injection
$user = new User();
$user->storage = $storage;

34. A slightly more complex
web example

35. $request = new Request();
$response = new Response();
$storage = new FileSessionStorage('SESSION_ID');
$user = new User($storage);
$cache = new FileCache(
array('dir' => dirname(__FILE__).'/cache')
);
$routing = new Routing($cache);

36. class Application
{
function __construct()
{
$this->request = new WebRequest();
$this->response = new WebResponse();
$storage = new FileSessionStorage('SESSION_ID');
$this->user = new User($storage);
$cache = new FileCache(
array('dir' => dirname(__FILE__).'/cache')
);
$this->routing = new Routing($cache);
}
}
$application = new Application();

37. Back to square 1

38. class Application
{
function __construct()
{
$this->request = new WebRequest();
$this->response = new WebResponse();
$storage = new FileSessionStorage('SESSION_ID');
$this->user = new User($storage);
$cache = new FileCache(
array('dir' => dirname(__FILE__).'/cache')
);
$this->routing = new Routing($cache);
}
}
$application = new Application();

39. We need a Container
Describes objects
and their dependencies
Instantiates and configures
objects on-demand

40. A container
SHOULD be able to manage
ANY PHP object (POPO)
The objects MUST not know
that they are managed
by a container

41. • Parameters
– The SessionStorageInterface implementation we want to use (the class name)
– The session name
• Objects
– SessionStorage
– User
• Dependencies
– User depends on a SessionStorageInterface implementation

42. Let’s build a simple container with PHP 5.3

43. DI Container
Managing parameters

44. class Container
{
protected $parameters = array();
public function setParameter($key, $value)
{
$this->parameters[$key] = $value;
}
public function getParameter($key)
{
return $this->parameters[$key];
}
}

45. $container = new Container();
$container->setParameter('session_name', 'SESSION_ID');
$container->setParameter('storage_class', 'SessionStorage');
$class = $container->getParameter('storage_class');
$sessionStorage = new $class($container->getParameter('session_name'));
$user = new User($sessionStorage);
Decoupling
Customization
Objects creation

46. class Container
{
protected $parameters = array();
public function __set($key, $value)
{
$this->parameters[$key] = $value;
}
public function __get($key)
{
return $this->parameters[$key];
}
}
Using PHP
magic methods

47. $container = new Container();
$container->session_name = 'SESSION_ID';
$container->storage_class = 'SessionStorage';
$sessionStorage = new $container->storage_class($container->session_name);
$user = new User($sessionStorage);
Interface
is cleaner

48. DI Container
Managing objects

49. We need a way to describe how to create objects,
without actually instantiating anything!
Anonymous functions to the rescue!

50. Anonymous Functions / Lambdas
A lambda is a function
defined on the fly
with no name
function () { echo 'Hello world!'; };

51. Anonymous Functions / Lambdas
A lambda can be stored
in a variable
$hello = function () { echo 'Hello world!'; };

52. Anonymous Functions / Lambdas
And then it can be used
as any other PHP callable
$hello();
call_user_func($hello);

53. Anonymous Functions / Lambdas
You can also pass a lambda
as an argument to a function or method
function foo(Closure $func)
{
$func();
}
foo($hello);

54. Fonctions anonymes
$hello = function ($name) { echo 'Hello '.$name; };
$hello('Fabien');
call_user_func($hello, 'Fabien');
function foo(Closure $func, $name)
{
$func($name);
}
foo($hello, 'Fabien');

55. DI Container
Managing objects

56. class Container
{
protected $parameters = array();
protected $objects = array();
public function __set($key, $value)
{
$this->parameters[$key] = $value;
}
public function __get($key)
{
return $this->parameters[$key];
}
public function setService($key, Closure $service)
{
$this->objects[$key] = $service;
}
public function getService($key)
{
return $this->objects[$key]($this);
}
}
Store a lambda
able to create the
object on-demand
Ask the closure to create
the object and pass the
current Container

57. $container = new Container();
$container->session_name = 'SESSION_ID';
$container->storage_class = 'SessionStorage';
$container->setService('user', function ($c)
{
return new User($c->getService('storage'));
});
$container->setService('storage', function ($c)
{
return new $c->storage_class($c->session_name);
});
$user = $container->getService('user');
Creating the User
is now as easy as before
Description

58. class Container
{
protected $values = array();
function __set($id, $value)
{
$this->values[$id] = $value;
}
function __get($id)
{
if (is_callable($this->values[$id]))
{
return $this->values[$id]($this);
}
else
{
return $this->values[$id];
}
}
}
Simplify the code

59. $container = new Container();
$container->session_name = 'SESSION_ID';
$container->storage_class = 'SessionStorage';
$container->user = function ($c)
{
return new User($c->storage);
};
$container->storage = function ($c)
{
return new $c->storage_class($c->session_name);
};
$user = $container->user;
Unified interface

60. DI Container
Scope

61. For some objects, like the user,
the container must always
return the same instance

62. spl_object_hash($container->user)
!==
spl_object_hash($container->user)

63. $container->user = function ($c)
{
static $user;
if (is_null($user))
{
$user = new User($c->storage);
}
return $user;
};

64. spl_object_hash($container->user)
===
spl_object_hash($container->user)

65. $container->user = $container->asShared(function ($c)
{
return new User($c->storage);
});

66. A closure is a lambda
that remembers the context
of its creation…

67. class Article
{
public function __construct($title)
{
$this->title = $title;
}
public function getTitle()
{
return $this->title;
}
}
$articles = array(
new Article('Title 1'),
new Article('Title 2'),
);

68. $mapper = function ($article)
{
return $article->getTitle();
};
$titles = array_map($mapper, $articles);

69. $method = 'getTitle';
$mapper = function ($article) use($method)
{
return $article->$method();
};
$method = 'getAuthor';
$titles = array_map($mapper, $articles);

70. $mapper = function ($method)
{
return function ($article) use($method)
{
return $article->$method();
};
};

71. $titles = array_map($mapper('getTitle'), $articles);
$authors = array_map($mapper('getAuthor'), $articles);

72. $container->user = $container->asShared(function ($c)
{
return new User($c->storage);
});

73. function asShared(Closure $lambda)
{
return function ($container) use ($lambda)
{
static $object;
if (is_null($object))
{
$object = $lambda($container);
}
return $object;
};
}

74. class Container
{
protected $values = array();
function __set($id, $value)
{
$this->values[$id] = $value;
}
function __get($id)
{
if (!isset($this->values[$id]))
{
throw new InvalidArgumentException(sprintf('Value "%s" is not defined.', $id));
}
if (is_callable($this->values[$id]))
{
return $this->values[$id]($this);
}
else
{
return $this->values[$id];
}
}
}
Error management

75. class Container
{
protected $values = array();
function __set($id, $value)
{
$this->values[$id] = $value;
}
function __get($id)
{
if (!isset($this->values[$id]))
{
throw new InvalidArgumentException(sprintf('Value "%s" is not defined.', $id));
}
if (is_callable($this->values[$id]))
{
return $this->values[$id]($this);
}
else
{
return $this->values[$id];
}
}
function asShared($callable)
{
return function ($c) use ($callable)
{
static $object;
if (is_null($object))
{
$object = $callable($c);
}
return $object;
};
}
}
40 LOC for a fully-
featured container

76. I’m NOT advocating
the usage of lambdas everywhere
This presentation is about
showing how they work
on practical examples

77. A DI Container
does NOT manage
ALL your objects

78. Good rule of thumb:
It manages “Global” objects
Objects with only one instance (!= Singletons)

79. LIKE
a User, a Request,
a database Connection, a Logger, …

80. UNLIKE
Model objects (a Product, a blog Post, …)

81. Symfony Components
Dependency Injection

82. Rock-solid implementation of a DIC in PHP 5.3

83. At the core of the Symfony 2.0 framework
… which is one of the fastest framework

84. Very flexible
Configuration in PHP, XML, YAML, or INI

85. $container = new Builder();
$container->register('output', 'FancyOutput');
$container->
register('message', 'Message')->
setArguments(array(
new sfServiceReference('output'),
array('with_newline' => true)
))
;
$container->message->say('Hello World!');

86. services:
output: { class: FancyOutput }
message:
class: Message
arguments:
- @output
- { with_newline: true }

87. <container xmlns="http://symfony-project.org/schema/dic/services">
<services>
<service id="output" class="FancyOutput" />
<service id="message" class="Message">
<argument type="service" id="output" />
<argument type="collection">
<argument key="with_newline">true</argument>
</argument>
</service>
</services>
</container>

88. <container xmlns="http://symfony-project.org/schema/dic/services">
<import resource="parameters.yml" />
<import resource="parameters.ini" />
<import resource="services.xml" />
</container>
imports:
- { resource: parameters.yml }
- { resource: parameters.ini }
- { resource: services.xml }

89. As fast as it can be
The container can be
“compiled” down
to plain PHP code

90. use SymfonyComponentsDependencyInjectionContainer;
use SymfonyComponentsDependencyInjectionReference;
use SymfonyComponentsDependencyInjectionParameter;
class ProjectServiceContainer extends Container
{
protected $shared = array();
protected function getOutputService()
{
if (isset($this->shared['output'])) return $this->shared['output'];
$instance = new FancyOutput();
return $this->shared['output'] = $instance;
}
protected function getMessageService()
{
if (isset($this->shared['message'])) return $this->shared['message'];
$instance = new Message($this->getOutputService(), array('with_newline' => true));
return $this->shared['message'] = $instance;
}
}

91. Semantic configuration
Thanks to an extension mechanism

92. <container xmlns="http://www.symfony-project.org/schema/dic/services">
<zend:logger
priority="debug"
path="%kernel.root_dir%/logs/%kernel.environment%.log" />
<doctrine:dbal dbname="dbname" username="root" password="" />
<swift:mailer transport="gmail">
<swift:username>fabien.potencier</swift:username>
<swift:password>xxxxxx</swift:password>
</swift:mailer>
</container>

93. <container xmlns="http://www.symfony-project.org/schema/dic/services"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:swift="http://www.symfony-project.org/schema/dic/swiftmailer"
xmlns:doctrine="http://www.symfony-project.org/schema/dic/doctrine"
xmlns:zend="http://www.symfony-project.org/schema/dic/zend"
xsi:schemaLocation="http://www.symfony-project.org/schema/dic/services http://www.symfony-
project.org/schema/dic/services/services-1.0.xsd
http://www.symfony-project.org/schema/dic/doctrine http://www.symfony-
project.org/schema/dic/doctrine/doctrine-1.0.xsd
http://www.symfony-project.org/schema/dic/swiftmailer http://
www.symfony-project.org/schema/dic/swiftmailer/swiftmailer-1.0.xsd">
<zend:logger priority="debug" path="%kernel.root_dir%/logs/%kernel.environment%.log" />
<doctrine:dbal dbname="dbname" username="root" password="" />
<swift:mailer transport="gmail">
<swift:username>fabien.potencier</swift:username>
<swift:password>xxxxxx</swift:password>
</swift:mailer>
</container>
auto-completion
and validation
with XSD

94. zend.logger:
level: debug
path: %kernel.root_dir%/logs/%kernel.environment%.log
doctrine.dbal:
dbname: dbname
username: root
password: ~
swift.mailer:
transport: gmail
username: fabien.potencier
password: xxxxxxx

95. Everything is converted by the extension
to plain services and parameters
no overhead

96. Loader::registerExtension(new SwiftMailerExtension());
Loader::registerExtension(new DoctrineExtension());
Loader::registerExtension(new ZendExtension());
$loader = new XmlFileLoader(__DIR__);
$config = $loader->load('services.xml');
$container = new Builder();
$container->merge($config);
$container->mailer->...
$dumper = new PhpDumper($container);
echo $dumper->dump();

97. More about Dependency Injection
http://fabien.potencier.org/article/17/on-php-5-3-lambda-functions-and-closures
http://components.symfony-project.org/dependency-injection/ (5.2)
http://github.com/fabpot/symfony/tree/master/src/Symfony/Components/DependencyInjection/(5.3)
http://github.com/fabpot/pimple
http://twittee.org/

98. Remember, most of the time,
you don’t need a Container
to use Dependency Injection

99. You can start to use and benefit from
Dependency Injection today

100. by implementing it
in your projects
by using externals libraries
that already use DI
without the need of a container

101. Symfony
Zend Framework
ezComponents
Doctrine
Swift Mailer
…

102. Questions?
My slides will be available on
slideshare.com/fabpot

103. Sensio S.A.
92-98, boulevard Victor Hugo
92 115 Clichy Cedex
FRANCE
Tél. : +33 1 40 99 80 80
Contact
Fabien Potencier
fabien.potencier at sensio.com
http://www.sensiolabs.com/
http://www.symfony-project.org/
http://fabien.potencier.org/