看Laravel的IoC容器文档只是介绍实例，但是没有说原理，之前用MVC框架都没有在意这个概念，无意中在phalcon的文档中看到这个详细的介绍，感觉豁然开朗，复制粘贴过来，主要是好久没有写东西了，现在确实很懒变得！
首先，我们假设，我们要开发一个组件命名为SomeComponent。这个组件中现在将要注入一个数据库连接。
在这个例子中，数据库连接在component中被创建，这种方法是不切实际的，这样做的话，我们将不能改变数据库连接参数及数据库类型等一些参数。
class SomeComponent { /** * The instantiation of the connection is hardcoded inside * the component so is difficult to replace it externally * or change its behavior */ public function someDbTask() { $connection = new Connection(array( "host" => "localhost", "username" => "root", "password" => "secret", "dbname" => "invo" )); // ... } } $some = new SomeComponent(); $some->someDbTask();
为了解决上面所说的问题，我们需要在使用前创建一个外部连接，并注入到容器中。就目前而言，这看起来是一个很好的解决方案：
class SomeComponent { protected $_connection; /** * Sets the connection externally */ public function setConnection($connection) { $this->_connection = $connection; } public function someDbTask() { $connection = $this->_connection; // ... } } $some = new SomeComponent(); //Create the connection $connection = new Connection(array( "host" => "localhost", "username" => "root", "password" => "secret", "dbname" => "invo" )); //Inject the connection in the component $some->setConnection($connection); $some->someDbTask();
现在我们来考虑一个问题，我们在应用程序中的不同地方使用此组件，将多次创建数据库连接。使用一种类似全局注册表的方式，从这获得一个数据库连接实例，而不是使用一次就创建一次。
class Registry { /** * Returns the connection */ public static function getConnection() { return new Connection(array( "host" => "localhost", "username" => "root", "password" => "secret", "dbname" => "invo" )); } } class SomeComponent { protected $_connection; /** * Sets the connection externally */ public function setConnection($connection){ $this->_connection = $connection; } public function someDbTask() { $connection = $this->_connection; // ... } } $some = new SomeComponent(); //Pass the connection defined in the registry $some->setConnection(Registry::getConnection()); $some->someDbTask();
现在，让我们来想像一下，我们必须在组件中实现两个方法，首先需要创建一个新的数据库连接，第二个总是获得一个共享连接：
class Registry { protected static $_connection; /** * Creates a connection */ protected static function _createConnection() { return new Connection(array( "host" => "localhost", "username" => "root", "password" => "secret", "dbname" => "invo" )); } /** * Creates a connection only once and returns it */ public static function getSharedConnection() { if (self::$_connection===null){ $connection = self::_createConnection(); self::$_connection = $connection; } return self::$_connection; } /** * Always returns a new connection */ public static function getNewConnection() { return self::_createConnection(); } } class SomeComponent { protected $_connection; /** * Sets the connection externally */ public function setConnection($connection){ $this->_connection = $connection; } /** * This method always needs the shared connection */ public function someDbTask() { $connection = $this->_connection; // ... } /** * This method always needs a new connection */ public function someOtherDbTask($connection) { } } $some = new SomeComponent(); //This injects the shared connection $some->setConnection(Registry::getSharedConnection()); $some->someDbTask(); //Here, we always pass a new connection as parameter $some->someOtherDbTask(Registry::getConnection());
到此为止，我们已经看到了如何使用依赖注入解决我们的问题。不是在代码内部创建依赖关系，而是让其作为一个参数传递，这使得我们的程序更容易维护，降低程序代码的耦合度，实现一种松耦合。但是从长远来看，这种形式的依赖注入也有一些缺点。
例如，如果组件中有较多的依赖关系，我们需要创建多个setter方法传递，或创建构造函数进行传递。另外，每次使用组件时，都需要创建依赖组件，使代码维护不太易，我们编写的代码可能像这样：
//Create the dependencies or retrieve them from the registry $connection = new Connection(); $session = new Session(); $fileSystem = new FileSystem(); $filter = new Filter(); $selector = new Selector(); //Pass them as constructor parameters $some = new SomeComponent($connection, $session, $fileSystem, $filter, $selector); // ... or using setters $some->setConnection($connection); $some->setSession($session); $some->setFileSystem($fileSystem); $some->setFilter($filter); $some->setSelector($selector);