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依赖注入容器

依赖注入（Dependency Injection，DI）容器就是一个对象，它知道怎样初始化并配置对象及其依赖的所有对象。 Martin 的文章已经解释了 DI 容器为什么很有用。这里我们主要讲解 Yii 提供的 DI 容器的使用方法。

依赖注入

Yii 通过 yii\di\Container 类提供 DI 容器特性。它支持如下几种类型的依赖注入：

构造方法注入;
Method injection;
Setter 和属性注入;
PHP 回调注入.

构造方法注入

在参数类型提示的帮助下，DI 容器实现了构造方法注入。当容器被用于创建一个新对象时，类型提示会告诉它要依赖什么类或接口。容器会尝试获取它所依赖的类或接口的实例，然后通过构造器将其注入新的对象。例如：

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

$foo = $container->get('Foo');
// 上面的代码等价于：
$bar = new Bar;
$foo = new Foo($bar);

Method Injection

Usually the dependencies of a class are passed to the constructor and are available inside of the class during the whole lifecycle. With Method Injection it is possible to provide a dependency that is only needed by a single method of the class and passing it to the constructor may not be possible or may cause too much overhead in the majority of use cases.

A class method can be defined like the doSomething() method in the following example:

class MyClass extends \yii\base\Component
{
    public function __construct(/*Some lightweight dependencies here*/, $config = [])
    {
        // ...
    }

    public function doSomething($param1, \my\heavy\Dependency $something)
    {
        // do something with $something
    }
}

You may call that method either by passing an instance of \my\heavy\Dependency yourself or using yii\di\Container::invoke() like the following:

$obj = new MyClass(/*...*/);
Yii::$container->invoke([$obj, 'doSomething'], ['param1' => 42]); // $something will be provided by the DI container

Setter 和属性注入

Setter 和属性注入是通过配置提供支持的。当注册一个依赖或创建一个新对象时，你可以提供一个配置，该配置会提供给容器用于通过相应的 Setter 或属性注入依赖。例如：

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'),
]);

信息：The yii\di\Container::get() method takes its third parameter as a configuration array that should be applied to the object being created. If the class implements the yii\base\Configurable interface (e.g. yii\base\BaseObject), the configuration array will be passed as the last parameter to the class constructor; otherwise, the configuration will be applied after the object is created.

PHP 回调注入

In this case, the container will use a registered PHP callable to build new instances of a class. Each time when yii\di\Container::get() is called, the corresponding callable will be invoked. The callable is responsible to resolve the dependencies and inject them appropriately to the newly created objects. For example,

$container->set('Foo', function () {
    $foo = new Foo(new Bar);
    // ... other initializations ...
    return $foo;
});

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

To hide the complex logic for building a new object, you may use a static class method as callable. For example,

class FooBuilder
{
    public static function build()
    {
        $foo = new Foo(new Bar);
        // ... other initializations ...
        return $foo;
    }
}

$container->set('Foo', ['app\helper\FooBuilder', 'build']);

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

By doing so, the person who wants to configure the Foo class no longer needs to be aware of how it is built.

注册依赖关系

可以用 yii\di\Container::set() 注册依赖关系。注册会用到一个依赖关系名称和一个依赖关系的定义。依赖关系名称可以是一个类名，一个接口名或一个别名。依赖关系的定义可以是一个类名，一个配置数组，或者一个 PHP 回调。

$container = new \yii\di\Container;

// 注册一个同类名一样的依赖关系，这个可以省略。
$container->set('yii\db\Connection');

// 注册一个接口
// 当一个类依赖这个接口时，
//相应的类会被初始化作为依赖对象。
$container->set('yii\mail\MailInterface', 'yii\swiftmailer\Mailer');

// 注册一个别名。
// 你可以使用 $container->get('foo') 创建一个 Connection 实例
$container->set('foo', 'yii\db\Connection');

// 通过配置注册一个类
// 通过 get() 初始化时，配置将会被使用。
$container->set('yii\db\Connection', [
    'dsn' => 'mysql:host=127.0.0.1;dbname=demo',
    'username' => 'root',
    'password' => '',
    'charset' => 'utf8',
]);

// 通过类的配置注册一个别名
// 这种情况下，需要通过一个 “class” 元素指定这个类
$container->set('db', [
    'class' => 'yii\db\Connection',
    'dsn' => 'mysql:host=127.0.0.1;dbname=demo',
    'username' => 'root',
    'password' => '',
    'charset' => 'utf8',
]);

// 注册一个 PHP 回调
// 每次调用 $container->get('db') 时，回调函数都会被执行。
$container->set('db', function ($container, $params, $config) {
    return new \yii\db\Connection($config);
});

// 注册一个组件实例
// $container->get('pageCache') 每次被调用时都会返回同一个实例。
$container->set('pageCache', new FileCache);

提示：如果依赖关系名称和依赖关系的定义相同，则不需要通过 DI 容器注册该依赖关系。

通过 set() 注册的依赖关系，在每次使用时都会产生一个新实例。可以使用 yii\di\Container::setSingleton() 注册一个单例的依赖关系：

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

解决依赖关系

注册依赖关系后，就可以使用 DI 容器创建新对象了。容器会自动解决依赖关系，将依赖实例化并注入新创建的对象。依赖关系的解决是递归的，如果一个依赖关系中还有其他依赖关系，则这些依赖关系都会被自动解决。

可以使用 yii\di\Container::get() 创建新的对象。该方法接收一个依赖关系名称，它可以是一个类名，一个接口名或一个别名。依赖关系名或许是通过 set() 或 setSingleton() 注册的。你可以随意地提供一个类的构造器参数列表和一个 configuration 用于配置新创建的对象。

例如：

// "db" 是前面定义过的一个别名
$db = $container->get('db');

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

代码背后，DI 容器做了比创建对象多的多的工作。容器首先将检查类的构造方法，找出依赖的类或接口名，然后自动递归解决这些依赖关系。

如下代码展示了一个更复杂的示例。UserLister 类依赖一个实现了 UserFinderInterface 接口的对象； UserFinder 类实现了这个接口，并依赖于一个 Connection 对象。所有这些依赖关系都是通过类构造器参数的类型提示定义的。通过属性依赖关系的注册，DI 容器可以自动解决这些依赖关系并能通过一个简单的 get('userLister') 调用创建一个新的 UserLister 实例。

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');

// 等价于:

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

实践中的运用

当在应用程序的入口脚本中引入 Yii.php 文件时， Yii 就创建了一个 DI 容器。这个 DI 容器可以通过 Yii::$container 访问。当调用 Yii::createObject() 时，此方法实际上会调用这个容器的 yii\di\Container::get() 方法创建新对象。如上所述，DI 容器会自动解决依赖关系（如果有）并将其注入新创建的对象中。因为 Yii 在其多数核心代码中都使用了Yii::createObject() 创建新对象，所以你可以通过 Yii::$container 全局性地自定义这些对象。

例如，你可以全局性自定义 yii\widgets\LinkPager 中分页按钮的默认数量:

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

这样如果你通过如下代码在一个视图里使用这个挂件，它的 maxButtonCount 属性就会被初始化为 5 而不是类中定义的默认值 10。

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

然而你依然可以覆盖通过 DI 容器设置的值：

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

注意：Properties given in the widget call will always override the definition in the DI container. Even if you specify an array, e.g. 'options' => ['id' => 'mypager'] these will not be merged with other options but replace them.

另一个例子是借用 DI 容器中自动构造方法注入带来的好处。假设你的控制器类依赖一些其他对象，例如一个旅馆预订服务。你可以通过一个构造器参数声明依赖关系，然后让 DI 容器帮你自动解决这个依赖关系。

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);
    }
}

如果你从浏览器中访问这个控制器，你将看到一个报错信息，提醒你 BookingInterface 无法被实例化。这是因为你需要告诉 DI 容器怎样处理这个依赖关系。

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

现在如果你再次访问这个控制器，一个 app\components\BookingService 的实例就会被创建并被作为第三个参数注入到控制器的构造器中。

Advanced Practical Usage

Say we work on API application and have:

app\components\Request class that extends yii\web\Request and provides additional functionality
app\components\Response class that extends yii\web\Response and should have format property set to json on creation

app\storage\FileStorage and app\storage\DocumentsReader classes that implement some logic on working with documents that are located in some file storage:

class FileStorage
{
    public function __construct($root) {
        // whatever
    }
}

class DocumentsReader
{
    public function __construct(FileStorage $fs) {
        // whatever
    }
}

It is possible to configure multiple definitions at once, passing configuration array to yii\di\Container::setDefinitions() or yii\di\Container::setSingletons() method. Iterating over the configuration array, the methods will call yii\di\Container::set() or yii\di\Container::setSingleton() respectively for each item.

The configuration array format is:

key: class name, interface name or alias name. The key will be passed to the yii\di\Container::set() method as a first argument $class.
value: the definition associated with $class. Possible values are described in yii\di\Container::set() documentation for the $definition parameter. Will be passed to the set() method as the second argument $definition.

For example, let's configure our container to follow the aforementioned requirements:

$container->setDefinitions([
    'yii\web\Request' => 'app\components\Request',
    'yii\web\Response' => [
        'class' => 'app\components\Response',
        'format' => 'json'
    ],
    'app\storage\DocumentsReader' => function () {
        $fs = new app\storage\FileStorage('/var/tempfiles');
        return new app\storage\DocumentsReader($fs);
    }
]);

$reader = $container->get('app\storage\DocumentsReader); 
// Will create DocumentReader object with its dependencies as described in the config

提示：Container may be configured in declarative style using application configuration since version 2.0.11. Check out the Application Configurations subsection of the Configurations guide article.

Everything works, but in case we need to create DocumentWriter class, we shall copy-paste the line that creates FileStorage object, that is not the smartest way, obviously.

As described in the Resolving Dependencies subsection, yii\di\Container::set() and yii\di\Container::setSingleton() can optionally take dependency's constructor parameters as a third argument. To set the constructor parameters, you may use the following configuration array format:

key: class name, interface name or alias name. The key will be passed to the yii\di\Container::set() method as a first argument $class.
value: array of two elements. The first element will be passed to the yii\di\Container::set() method as the second argument $definition, the second one — as $params.

Let's modify our example:

$container->setDefinitions([
    'tempFileStorage' => [ // we've created an alias for convenience
        ['class' => 'app\storage\FileStorage'],
        ['/var/tempfiles'] // could be extracted from some config files
    ],
    'app\storage\DocumentsReader' => [
        ['class' => 'app\storage\DocumentsReader'],
        [Instance::of('tempFileStorage')]
    ],
    'app\storage\DocumentsWriter' => [
        ['class' => 'app\storage\DocumentsWriter'],
        [Instance::of('tempFileStorage')]
    ]
]);

$reader = $container->get('app\storage\DocumentsReader); 
// Will behave exactly the same as in the previous example.

You might notice Instance::of('tempFileStorage') notation. It means, that the yii\di\Container will implicitly provide a dependency registered with the name of tempFileStorage and pass it as the first argument of app\storage\DocumentsWriter constructor.

注意：yii\di\Container::setDefinitions() and yii\di\Container::setSingletons() methods are available since version 2.0.11.

Another step on configuration optimization is to register some dependencies as singletons. A dependency registered via yii\di\Container::set() will be instantiated each time it is needed. Some classes do not change the state during runtime, therefore they may be registered as singletons in order to increase the application performance.

A good example could be app\storage\FileStorage class, that executes some operations on file system with a simple API (e.g. $fs->read(), $fs->write()). These operations do not change the internal class state, so we can create its instance once and use it multiple times.

$container->setSingletons([
    'tempFileStorage' => [
        ['class' => 'app\storage\FileStorage'],
        ['/var/tempfiles']
    ],
]);

$container->setDefinitions([
    'app\storage\DocumentsReader' => [
        ['class' => 'app\storage\DocumentsReader'],
        [Instance::of('tempFileStorage')]
    ],
    'app\storage\DocumentsWriter' => [
        ['class' => 'app\storage\DocumentsWriter'],
        [Instance::of('tempFileStorage')]
    ]
]);

$reader = $container->get('app\storage\DocumentsReader');


什么时候注册依赖关系 <span id="when-to-register-dependencies"></span>
-----------------------------

由于依赖关系在创建新对象时需要解决，因此它们的注册应该尽早完成。
如下是推荐的实践：

* 如果你是一个应用程序的开发者，
  你可以在应用程序的[入口脚本](structure-entry-scripts.md)
  或者被入口脚本引入的脚本中注册依赖关系。
* 如果你是一个可再分发[扩展](structure-extensions.md)的开发者，
  你可以将依赖关系注册到扩展的引导类中。


总结 <span id="summary"></span>
-------

依赖注入和[服务定位器](concept-service-locator.md)都是流行的设计模式，
它们使你可以用充分解耦且更利于测试的风格构建软件。
强烈推荐你阅读 [Martin 的文章](http://martinfowler.com/articles/injection.html) ，
对依赖注入和服务定位器有个更深入的理解。

Yii 在依赖住入（DI）容器之上实现了它的[服务定位器](concept-service-locator.md)。
当一个服务定位器尝试创建一个新的对象实例时，它会把调用转发到 DI 容器。
后者将会像前文所述那样自动解决依赖关系。

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