Dependency Injection Container
==============================

A dependency injection (DI) container is an object that knows how to instantiate and configure objects and
all their dependent objects. [Martin's article](http://martinfowler.com/articles/injection.html) has well
explained why DI container is useful. Here we will mainly explain the usage of the DI container provided by Yii.


<a name="dependency-injection"></a>
Dependency Injection
--------------------

Yii provides the DI container feature through the class [[yii\di\Container]]. It supports the following kinds of
dependency injection:

* Constructor injection;
* Setter and property injection;
* PHP callable injection.


<a name="constructor-injection"></a>
### Constructor Injection

The DI container supports constructor injection with the help of type hints for constructor parameters.
The type hints tell the container which classes or interfaces are dependent when it is used to create a new object.
The container will try to get the instances of the dependent classes or interfaces and then inject them
into the new object through the constructor. For example,

```php
class Foo
{
    public function __construct(Bar $bar)
    {
    }
}

$foo = $container->get('Foo');
// which is equivalent to the following:
$bar = new Bar;
$foo = new Foo($bar);
```


<a name="setter-and-property-injection"></a>
### Setter and Property Injection

Setter and property injection is supported through [configurations](concept-configurations.md).
When registering a dependency or when creating a new object, you can provide a configuration which
will be used by the container to inject the dependencies through the corresponding setters or properties.
For example,

```php
use yii\base\Object;

class Foo extends Object
{
    public $bar;

    private $_qux;

    public function getQux()
    {
        return $this->_qux;
    }

    public function setQux(Qux $qux)
    {
        $this->_qux = $qux;
    }
}

$container->get('Foo', [], [
    'bar' => $container->get('Bar'),
    'qux' => $container->get('Qux'),
]);
```


<a name="php-callable-injection"></a>
### PHP Callable Injection

In this case, the container will use a registered PHP callable to build new instances of a class.
The callable is responsible to resolve the dependencies and inject them appropriately to the newly
created objects. For example,

```php
$container->set('Foo', function () {
    return new Foo(new Bar);
});

$foo = $container->get('Foo');
```


<a name="registering-dependencies"></a>
Registering Dependencies
------------------------

You can use [[yii\di\Container::set()]] to register dependencies. The registration requires a dependency name
as well as a dependency definition. A dependency name can be a class name, an interface name, or an alias name;
and a dependency definition can be a class name, a configuration array, or a PHP callable.

```php
$container = new \yii\di\Container;

// register a class name as is. This can be skipped.
$container->set('yii\db\Connection');

// register an interface
// When a class depends on the interface, the corresponding class
// will be instantiated as the dependent object
$container->set('yii\mail\MailInterface', 'yii\swiftmailer\Mailer');

// register an alias name. You can use $container->get('foo')
// to create an instance of Connection
$container->set('foo', 'yii\db\Connection');

// register a class with configuration. The configuration
// will be applied when the class is instantiated by get()
$container->set('yii\db\Connection', [
    'dsn' => 'mysql:host=127.0.0.1;dbname=demo',
    'username' => 'root',
    'password' => '',
    'charset' => 'utf8',
]);

// register an alias name with class configuration
// In this case, a "class" element is required to specify the class
$container->set('db', [
    'class' => 'yii\db\Connection',
    'dsn' => 'mysql:host=127.0.0.1;dbname=demo',
    'username' => 'root',
    'password' => '',
    'charset' => 'utf8',
]);

// register a PHP callable
// The callable will be executed each time when $container->get('db') is called
$container->set('db', function ($container, $params, $config) {
    return new \yii\db\Connection($config);
});

// register a component instance
// $container->get('pageCache') will return the same instance each time it is called
$container->set('pageCache', new FileCache);
```

> Tip: If a dependency name is the same as the corresponding dependency definition, you do not
  need to register it with the DI container.

A dependency registered via `set()` will generate an instance each time the dependency is needed.
You can use [[yii\di\Container::setSingleton()]] to register a dependency that only generates
a single instance:

```php
$container->setSingleton('yii\db\Connection', [
    'dsn' => 'mysql:host=127.0.0.1;dbname=demo',
    'username' => 'root',
    'password' => '',
    'charset' => 'utf8',
]);
```


<a name="resolving-dependencies"></a>
Resolving Dependencies
----------------------

Once you have registered dependencies, you can use the DI container to create new objects,
and the container will automatically resolve dependencies by instantiating them and injecting
them into the newly created objects. The dependency resolution is recursive, meaning that
if a dependency has other dependencies, those dependencies will also be resolved automatically.

You can use [[yii\di\Container::get()]] to create new objects. The method takes a dependency name,
which can be a class name, an interface name or an alias name. The dependency name may or may
not be registered via `set()` or `setSingleton()`. You may optionally provide a list of class
constructor parameters and a [configuration](concept-configurations.md) to configure the newly created object.
For example,

```php
// "db" is a previously registered alias name
$db = $container->get('db');

// equivalent to: $engine = new \app\components\SearchEngine($apiKey, ['type' => 1]);
$engine = $container->get('app\components\SearchEngine', [$apiKey], ['type' => 1]);
```

Behind the scene, the DI container does much more work than just creating a new object.
The container will first inspect the class constructor to find out dependent class or interface names
and then automatically resolve those dependencies recursively.

The following code shows a more sophisticated example. The `UserLister` class depends on an object implementing
the `UserFinderInterface` interface; the `UserFinder` class implements this interface and depends on
a `Connection` object. All these dependencies are declared through type hinting of the class constructor parameters.
With property dependency registration, the DI container is able to resolve these dependencies automatically
and creates a new `UserLister` instance with a simple call of `get('userLister')`.

```php
namespace app\models;

use yii\base\Object;
use yii\db\Connection;
use yii\di\Container;

interface UserFinderInterface
{
    function findUser();
}

class UserFinder extends Object implements UserFinderInterface
{
    public $db;

    public function __construct(Connection $db, $config = [])
    {
        $this->db = $db;
        parent::__construct($config);
    }

    public function findUser()
    {
    }
}

class UserLister extends Object
{
    public $finder;

    public function __construct(UserFinderInterface $finder, $config = [])
    {
        $this->finder = $finder;
        parent::__construct($config);
    }
}

$container = new Container;
$container->set('yii\db\Connection', [
    'dsn' => '...',
]);
$container->set('app\models\UserFinderInterface', [
    'class' => 'app\models\UserFinder',
]);
$container->set('userLister', 'app\models\UserLister');

$lister = $container->get('userLister');

// which is equivalent to:

$db = new \yii\db\Connection(['dsn' => '...']);
$finder = new UserFinder($db);
$lister = new UserLister($finder);
```


<a name="practical-usages"></a>
Practical Usages
----------------

Yii creates a DI container when you include the `Yii.php` file in the [entry script](structure-entry-scripts.md)
of your application. The DI container is accessible via [[Yii::$container]]. When you call [[Yii::createObject()]],
the method will actually call the container's [[yii\di\Container::get()|get()]] method to create a new object.
As aforementioned, the DI container will automatically resolve the dependencies (if any) and inject them
into the newly created object. Because Yii uses [[Yii::createObject()]] in most of its core code to create
new objects, this means you can customize the objects globally by dealing with [[Yii::$container]].

For example, you can customize globally the default number of pagination buttons of [[yii\widgets\LinkPager]]:

```php
\Yii::$container->set('yii\widgets\LinkPager', ['maxButtonCount' => 5]);
```

Now if you use the widget in a view with the following code, the `maxButtonCount` property will be initialized
as 5 instead of the default value 10 as defined in the class.

```php
echo \yii\widgets\LinkPager::widget();
```

You can still override the value set via DI container, though:

```php
echo \yii\widgets\LinkPager::widget(['maxButtonCount' => 20]);
```

Another example is to take advantage of the automatic constructor injection of the DI container.
Assume your controller class depends on some other objects, such as a hotel booking service. You
can declare the dependency through a constructor parameter and let the DI container to resolve it for you.

```php
namespace app\controllers;

use yii\web\Controller;
use app\components\BookingInterface;

class HotelController extends Controller
{
    protected $bookingService;

    public function __construct($id, $module, BookingInterface $bookingService, $config = [])
    {
        $this->bookingService = $bookingService;
        parent::__construct($id, $module, $config);
    }
}
```

If you access this controller from browser, you will see an error complaining the `BookingInterface`
cannot be instantiated. This is because you need to tell the DI container how to deal with this dependency:

```php
\Yii::$container->set('app\components\BookingInterface', 'app\components\BookingService');
```

Now if you access the controller again, an instance of `app\components\BookingService` will be
created and injected as the 3rd parameter to the controller's constructor.


<a name="when-to-register-dependencies"></a>
When to Register Dependencies
-----------------------------

Because dependencies are needed when new objects are being created, their registration should be done
as early as possible. The followings are the recommended practices:

* If you are the developer of an application, you can register dependencies in your
  application's [entry script](structure-entry-scripts.md) or in a script that is included by the entry script.
* If you are the developer of a redistributable [extension](structure-extensions.md), you can register dependencies
  in the bootstrap class of the extension.


<a name="summary"></a>
Summary
-------

Both dependency injection and [service locator](concept-service-locator.md) are popular design patterns
that allow building software in a loosely-coupled and more testable fashion. We highly recommend you to read
[Martin's article](http://martinfowler.com/articles/injection.html) to get a deeper understanding of
dependency injection and service locator.

Yii implements its [service locator](concept-service-locator.md) on top of the dependency injection (DI) container.
When a service locator is trying to create a new object instance, it will forward the call to the DI container.
The latter will resolve the dependencies automatically as described above.

