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554 lines
20 KiB
Markdown
554 lines
20 KiB
Markdown
Dependency Injection Container
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==============================
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A dependency injection (DI) container is an object that knows how to instantiate and configure objects and
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all their dependent objects. [Martin Fowler's article](https://martinfowler.com/articles/injection.html) has well
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explained why DI container is useful. Here we will mainly explain the usage of the DI container provided by Yii.
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Dependency Injection <span id="dependency-injection"></span>
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--------------------
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Yii provides the DI container feature through the class [[yii\di\Container]]. It supports the following kinds of
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dependency injection:
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* Constructor injection;
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* Method injection;
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* Setter and property injection;
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* PHP callable injection;
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### Constructor Injection <span id="constructor-injection"></span>
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The DI container supports constructor injection with the help of type hints for constructor parameters.
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The type hints tell the container which classes or interfaces are dependent when it is used to create a new object.
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The container will try to get the instances of the dependent classes or interfaces and then inject them
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into the new object through the constructor. For example,
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```php
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class Foo
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{
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public function __construct(Bar $bar)
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{
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}
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}
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$foo = $container->get('Foo');
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// which is equivalent to the following:
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$bar = new Bar;
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$foo = new Foo($bar);
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```
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### Method Injection <span id="method-injection"></span>
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Usually the dependencies of a class are passed to the constructor and are available inside the class during the whole lifecycle.
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With Method Injection it is possible to provide a dependency that is only needed by a single method of the class
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and passing it to the constructor may not be possible or may cause too much overhead in the majority of use cases.
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A class method can be defined like the `doSomething()` method in the following example:
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```php
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class MyClass extends \yii\base\Component
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{
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public function __construct(/*Some lightweight dependencies here*/, $config = [])
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{
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// ...
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}
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public function doSomething($param1, \my\heavy\Dependency $something)
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{
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// do something with $something
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}
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}
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```
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You may call that method either by passing an instance of `\my\heavy\Dependency` yourself or using [[yii\di\Container::invoke()]] like the following:
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```php
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$obj = new MyClass(/*...*/);
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Yii::$container->invoke([$obj, 'doSomething'], ['param1' => 42]); // $something will be provided by the DI container
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```
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### Setter and Property Injection <span id="setter-and-property-injection"></span>
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Setter and property injection is supported through [configurations](concept-configurations.md).
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When registering a dependency or when creating a new object, you can provide a configuration which
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will be used by the container to inject the dependencies through the corresponding setters or properties.
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For example,
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```php
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use yii\base\BaseObject;
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class Foo extends BaseObject
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{
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public $bar;
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private $_qux;
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public function getQux()
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{
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return $this->_qux;
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}
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public function setQux(Qux $qux)
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{
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$this->_qux = $qux;
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}
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}
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$container->get('Foo', [], [
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'bar' => $container->get('Bar'),
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'qux' => $container->get('Qux'),
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]);
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```
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> Info: The [[yii\di\Container::get()]] method takes its third parameter as a configuration array that should
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be applied to the object being created. If the class implements the [[yii\base\Configurable]] interface (e.g.
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[[yii\base\BaseObject]]), the configuration array will be passed as the last parameter to the class constructor;
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otherwise, the configuration will be applied *after* the object is created.
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### PHP Callable Injection <span id="php-callable-injection"></span>
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In this case, the container will use a registered PHP callable to build new instances of a class.
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Each time when [[yii\di\Container::get()]] is called, the corresponding callable will be invoked.
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The callable is responsible to resolve the dependencies and inject them appropriately to the newly
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created objects. For example,
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```php
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$container->set('Foo', function ($container, $params, $config) {
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$foo = new Foo(new Bar);
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// ... other initializations ...
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return $foo;
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});
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$foo = $container->get('Foo');
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```
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To hide the complex logic for building a new object, you may use a static class method as callable. For example,
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```php
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class FooBuilder
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{
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public static function build($container, $params, $config)
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{
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$foo = new Foo(new Bar);
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// ... other initializations ...
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return $foo;
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}
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}
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$container->set('Foo', ['app\helper\FooBuilder', 'build']);
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$foo = $container->get('Foo');
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```
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By doing so, the person who wants to configure the `Foo` class no longer needs to be aware of how it is built.
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Registering Dependencies <span id="registering-dependencies"></span>
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------------------------
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You can use [[yii\di\Container::set()]] to register dependencies. The registration requires a dependency name
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as well as a dependency definition. A dependency name can be a class name, an interface name, or an alias name;
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and a dependency definition can be a class name, a configuration array, or a PHP callable.
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```php
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$container = new \yii\di\Container;
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// register a class name as is. This can be skipped.
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$container->set('yii\db\Connection');
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// register an interface
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// When a class depends on the interface, the corresponding class
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// will be instantiated as the dependent object
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$container->set('yii\mail\MailInterface', 'yii\symfonymailer\Mailer');
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// register an alias name. You can use $container->get('foo')
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// to create an instance of Connection
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$container->set('foo', 'yii\db\Connection');
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// register an alias with `Instance::of`
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$container->set('bar', Instance::of('foo'));
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// register a class with configuration. The configuration
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// will be applied when the class is instantiated by get()
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$container->set('yii\db\Connection', [
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'dsn' => 'mysql:host=127.0.0.1;dbname=demo',
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'username' => 'root',
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'password' => '',
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'charset' => 'utf8',
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]);
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// register an alias name with class configuration
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// In this case, a "class" or "__class" element is required to specify the class
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$container->set('db', [
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'class' => 'yii\db\Connection',
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'dsn' => 'mysql:host=127.0.0.1;dbname=demo',
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'username' => 'root',
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'password' => '',
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'charset' => 'utf8',
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]);
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// register callable closure or array
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// The callable will be executed each time when $container->get('db') is called
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$container->set('db', function ($container, $params, $config) {
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return new \yii\db\Connection($config);
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});
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$container->set('db', ['app\db\DbFactory', 'create']);
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// register a component instance
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// $container->get('pageCache') will return the same instance each time it is called
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$container->set('pageCache', new FileCache);
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```
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> Tip: If a dependency name is the same as the corresponding dependency definition, you do not
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need to register it with the DI container.
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A dependency registered via `set()` will generate an instance each time the dependency is needed.
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You can use [[yii\di\Container::setSingleton()]] to register a dependency that only generates
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a single instance:
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```php
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$container->setSingleton('yii\db\Connection', [
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'dsn' => 'mysql:host=127.0.0.1;dbname=demo',
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'username' => 'root',
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'password' => '',
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'charset' => 'utf8',
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]);
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```
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Resolving Dependencies <span id="resolving-dependencies"></span>
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----------------------
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Once you have registered dependencies, you can use the DI container to create new objects,
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and the container will automatically resolve dependencies by instantiating them and injecting
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them into the newly created objects. The dependency resolution is recursive, meaning that
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if a dependency has other dependencies, those dependencies will also be resolved automatically.
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You can use [[yii\di\Container::get()|get()]] to either create or get object instance.
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The method takes a dependency name, which can be a class name, an interface name or an alias name.
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The dependency name may be registered via [[yii\di\Container::set()|set()]]
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or [[yii\di\Container::setSingleton()|setSingleton()]]. You may optionally provide a list of class
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constructor parameters and a [configuration](concept-configurations.md) to configure the newly created object.
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For example:
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```php
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// "db" is a previously registered alias name
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$db = $container->get('db');
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// equivalent to: $engine = new \app\components\SearchEngine($apiKey, $apiSecret, ['type' => 1]);
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$engine = $container->get('app\components\SearchEngine', [$apiKey, $apiSecret], ['type' => 1]);
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// equivalent to: $api = new \app\components\Api($host, $apiKey);
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$api = $container->get('app\components\Api', ['host' => $host, 'apiKey' => $apiKey]);
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```
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Behind the scene, the DI container does much more work than just creating a new object.
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The container will first inspect the class constructor to find out dependent class or interface names
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and then automatically resolve those dependencies recursively.
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The following code shows a more sophisticated example. The `UserLister` class depends on an object implementing
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the `UserFinderInterface` interface; the `UserFinder` class implements this interface and depends on
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a `Connection` object. All these dependencies are declared through type hinting of the class constructor parameters.
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With proper dependency registration, the DI container is able to resolve these dependencies automatically
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and creates a new `UserLister` instance with a simple call of `get('userLister')`.
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```php
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namespace app\models;
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use yii\base\BaseObject;
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use yii\db\Connection;
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use yii\di\Container;
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interface UserFinderInterface
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{
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function findUser();
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}
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class UserFinder extends BaseObject implements UserFinderInterface
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{
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public $db;
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public function __construct(Connection $db, $config = [])
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{
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$this->db = $db;
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parent::__construct($config);
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}
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public function findUser()
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{
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}
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}
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class UserLister extends BaseObject
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{
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public $finder;
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public function __construct(UserFinderInterface $finder, $config = [])
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{
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$this->finder = $finder;
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parent::__construct($config);
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}
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}
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$container = new Container;
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$container->set('yii\db\Connection', [
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'dsn' => '...',
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]);
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$container->set('app\models\UserFinderInterface', [
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'class' => 'app\models\UserFinder',
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]);
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$container->set('userLister', 'app\models\UserLister');
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$lister = $container->get('userLister');
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// which is equivalent to:
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$db = new \yii\db\Connection(['dsn' => '...']);
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$finder = new UserFinder($db);
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$lister = new UserLister($finder);
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```
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Practical Usage <span id="practical-usage"></span>
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---------------
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Yii creates a DI container when you include the `Yii.php` file in the [entry script](structure-entry-scripts.md)
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of your application. The DI container is accessible via [[Yii::$container]]. When you call [[Yii::createObject()]],
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the method will actually call the container's [[yii\di\Container::get()|get()]] method to create a new object.
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As aforementioned, the DI container will automatically resolve the dependencies (if any) and inject them
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into obtained object. Because Yii uses [[Yii::createObject()]] in most of its core code to create
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new objects, this means you can customize the objects globally by dealing with [[Yii::$container]].
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For example, let's customize globally the default number of pagination buttons of [[yii\widgets\LinkPager]].
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```php
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\Yii::$container->set('yii\widgets\LinkPager', ['maxButtonCount' => 5]);
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```
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Now if you use the widget in a view with the following code, the `maxButtonCount` property will be initialized
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as 5 instead of the default value 10 as defined in the class.
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```php
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echo \yii\widgets\LinkPager::widget();
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```
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You can still override the value set via DI container, though:
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```php
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echo \yii\widgets\LinkPager::widget(['maxButtonCount' => 20]);
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```
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> Note: Properties given in the widget call will always override the definition in the DI container.
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> Even if you specify an array, e.g. `'options' => ['id' => 'mypager']` these will not be merged
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> with other options but replace them.
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Another example is to take advantage of the automatic constructor injection of the DI container.
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Assume your controller class depends on some other objects, such as a hotel booking service. You
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can declare the dependency through a constructor parameter and let the DI container to resolve it for you.
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```php
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namespace app\controllers;
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use yii\web\Controller;
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use app\components\BookingInterface;
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class HotelController extends Controller
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{
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protected $bookingService;
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public function __construct($id, $module, BookingInterface $bookingService, $config = [])
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{
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$this->bookingService = $bookingService;
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parent::__construct($id, $module, $config);
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}
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}
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```
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If you access this controller from browser, you will see an error complaining the `BookingInterface`
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cannot be instantiated. This is because you need to tell the DI container how to deal with this dependency:
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```php
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\Yii::$container->set('app\components\BookingInterface', 'app\components\BookingService');
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```
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Now if you access the controller again, an instance of `app\components\BookingService` will be
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created and injected as the 3rd parameter to the controller's constructor.
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Since Yii 2.0.36 when using PHP 7 action injection is available for both web and console controllers:
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```php
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namespace app\controllers;
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use yii\web\Controller;
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use app\components\BookingInterface;
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class HotelController extends Controller
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{
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public function actionBook($id, BookingInterface $bookingService)
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{
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$result = $bookingService->book($id);
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// ...
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}
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}
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```
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Advanced Practical Usage <span id="advanced-practical-usage"></span>
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---------------
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Say we work on API application and have:
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- `app\components\Request` class that extends `yii\web\Request` and provides additional functionality
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- `app\components\Response` class that extends `yii\web\Response` and should have `format` property
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set to `json` on creation
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- `app\storage\FileStorage` and `app\storage\DocumentsReader` classes that implement some logic on
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working with documents that are located in some file storage:
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```php
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class FileStorage
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{
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public function __construct($root) {
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// whatever
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}
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}
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class DocumentsReader
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{
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public function __construct(FileStorage $fs) {
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// whatever
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}
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}
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```
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It is possible to configure multiple definitions at once, passing configuration array to
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[[yii\di\Container::setDefinitions()|setDefinitions()]] or [[yii\di\Container::setSingletons()|setSingletons()]] method.
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Iterating over the configuration array, the methods will call [[yii\di\Container::set()|set()]]
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or [[yii\di\Container::setSingleton()|setSingleton()]] respectively for each item.
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The configuration array format is:
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- `key`: class name, interface name or alias name. The key will be passed to the
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[[yii\di\Container::set()|set()]] method as a first argument `$class`.
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- `value`: the definition associated with `$class`. Possible values are described in [[yii\di\Container::set()|set()]]
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documentation for the `$definition` parameter. Will be passed to the [[set()]] method as
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the second argument `$definition`.
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For example, let's configure our container to follow the aforementioned requirements:
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```php
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$container->setDefinitions([
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'yii\web\Request' => 'app\components\Request',
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'yii\web\Response' => [
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'class' => 'app\components\Response',
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'format' => 'json'
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],
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'app\storage\DocumentsReader' => function ($container, $params, $config) {
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$fs = new app\storage\FileStorage('/var/tempfiles');
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return new app\storage\DocumentsReader($fs);
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}
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]);
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$reader = $container->get('app\storage\DocumentsReader');
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// Will create DocumentReader object with its dependencies as described in the config
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```
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> Tip: Container may be configured in declarative style using application configuration since version 2.0.11.
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Check out the [Application Configurations](concept-configurations.md#application-configurations) subsection of
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the [Configurations](concept-configurations.md) guide article.
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Everything works, but in case we need to create `DocumentWriter` class,
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we shall copy-paste the line that creates `FileStorage` object, that is not the smartest way, obviously.
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As described in the [Resolving Dependencies](#resolving-dependencies) subsection, [[yii\di\Container::set()|set()]]
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and [[yii\di\Container::setSingleton()|setSingleton()]] can optionally take dependency's constructor parameters as
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a third argument. To set the constructor parameters, you may use the `__construct()` option:
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Let's modify our example:
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```php
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$container->setDefinitions([
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'tempFileStorage' => [ // we've created an alias for convenience
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'class' => 'app\storage\FileStorage',
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'__construct()' => ['/var/tempfiles'], // could be extracted from some config files
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],
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'app\storage\DocumentsReader' => [
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'class' => 'app\storage\DocumentsReader',
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'__construct()' => [Instance::of('tempFileStorage')],
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],
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'app\storage\DocumentsWriter' => [
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'class' => 'app\storage\DocumentsWriter',
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'__construct()' => [Instance::of('tempFileStorage')]
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]
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]);
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$reader = $container->get('app\storage\DocumentsReader');
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// Will behave exactly the same as in the previous example.
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```
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You might notice `Instance::of('tempFileStorage')` notation. It means, that the [[yii\di\Container|Container]]
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will implicitly provide a dependency registered with the name of `tempFileStorage` and pass it as the first argument
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of `app\storage\DocumentsWriter` constructor.
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> Note: [[yii\di\Container::setDefinitions()|setDefinitions()]] and [[yii\di\Container::setSingletons()|setSingletons()]]
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methods are available since version 2.0.11.
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Another step on configuration optimization is to register some dependencies as singletons.
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A dependency registered via [[yii\di\Container::set()|set()]] will be instantiated each time it is needed.
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Some classes do not change the state during runtime, therefore they may be registered as singletons
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in order to increase the application performance.
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A good example could be `app\storage\FileStorage` class, that executes some operations on file system with a simple
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API (e.g. `$fs->read()`, `$fs->write()`). These operations do not change the internal class state, so we can
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create its instance once and use it multiple times.
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```php
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$container->setSingletons([
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'tempFileStorage' => [
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|
'class' => 'app\storage\FileStorage',
|
|
'__construct()' => ['/var/tempfiles']
|
|
],
|
|
]);
|
|
|
|
$container->setDefinitions([
|
|
'app\storage\DocumentsReader' => [
|
|
'class' => 'app\storage\DocumentsReader',
|
|
'__construct()' => [Instance::of('tempFileStorage')],
|
|
],
|
|
'app\storage\DocumentsWriter' => [
|
|
'class' => 'app\storage\DocumentsWriter',
|
|
'__construct()' => [Instance::of('tempFileStorage')],
|
|
]
|
|
]);
|
|
|
|
$reader = $container->get('app\storage\DocumentsReader');
|
|
```
|
|
|
|
When to Register Dependencies <span id="when-to-register-dependencies"></span>
|
|
-----------------------------
|
|
|
|
Because dependencies are needed when new objects are being created, their registration should be done
|
|
as early as possible. The following are the recommended practices:
|
|
|
|
* If you are the developer of an application, you can register your dependencies using application configuration.
|
|
Please, read the [Application Configurations](concept-configurations.md#application-configurations) subsection of
|
|
the [Configurations](concept-configurations.md) guide article.
|
|
* If you are the developer of a redistributable [extension](structure-extensions.md), you can register dependencies
|
|
in the bootstrapping class of the extension.
|
|
|
|
|
|
Summary <span id="summary"></span>
|
|
-------
|
|
|
|
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](https://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.
|
|
|