+++ /dev/null
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+++ /dev/null
-Introduction
-============
-
-Dependency injection
-(http://en.wikipedia.org/wiki/Dependency_injection) is a useful design
-pattern that can help to avoid using global variables or state. If a
-class depends on a connection to a database then this connection is
-often pulled into the class using a singleton pattern or by using a
-global variable.
-
-Instead of providing the class with knowlegde about the global state
-it is often preferable to "inject" the dependency into the class from
-the outside. This usually happens within the class constructor. Thus
-the dependencies of a class are immediately visible without searching
-the code for references to the global scope. Another major benefit of
-dependency injection is the fact that it facilitates unit testing of
-complex systems.
-
-Horde_Provider
-==============
-
-For PHP there exist several "Dependency Injection" frameworks
-(e.g. http://stubbles.net,
-http://components.symfony-project.org/dependency-injection) with
-extensive feature lists. So there is hardly any need for another
-framework with similar capabilities.
-
-The essential part of dependency injection is the structure of classes
-with dependencies. They need to be amenable for an external management
-of their dependencies. If that is the case for a given class then most
-frameworks should have no problem handling this class within the
-framework. The choice of the actual framework should not matter
-anymore.
-
-In consequence Horde_Provider provides only a minimal version of
-dependency injection that is mainly inspired by Twittee
-(http://twittee.org/). The goal is to drive refactoring of classes
-with complex dependencies so that they can be used with a dependency
-injection framework.
-
-Making classes amenable to dependency injection
-===============================================
-
-As trivial as it may sound: a class can be managed by a dependency
-injection framework if the class allows the framework to inject its
-dependencies from the outside. That means that the class may *NOT*
-
- - pull in an dependency using global state via the singleton
- pattern:
-
- External_Class::singleton())
-
- - create new objects with dependencies:
-
- $db = new DB();
- $b = new User($db);
-
- - use global variables:
-
- if ($conf['a']) echo 'yes';
-
-In most cases the class should receive dependencies and required
-parameters within the constructor.
-
-Using Horde_Provider
-====================
-
-The Horde_Provider_Base class is a simple container that allows you to
-fill it with a number of elements that can be retrieved later:
-
- $a = new Horde_Provider_Base();
- $a->a = 'a';
- echo $a->a;
-
- a
-
-Classes with dependencies are represented by registering a wrapper
-object that implements the Horde_Provider_Injection interface:
-
- function factory() { return 'constructed'; }
- $a = new Horde_Provider_Base();
- $b = new Horde_Provider_Injection_Factory('factory');
- $a->inject = $b;
- echo $a->inject;
-
- constructed
-
-The example above demonstrates this approach by using the
-Horde_Provider_Injection_Factory which implements
-Horde_Provider_Injection. The class allows to register a callable
-function. It will use this function to construct the required
-object. Here the factory function is simple though and does not return
-an object but a string.
-
-Preparing a class for Horde_Provider
-====================================
-
-Assume you have the following simple class that represents a common
-structure found in many of the Horde packages:
-
- class Horde_X
- {
- /**
- * Instance object.
- *
- * @var Horde_X
- */
- static protected $_instance;
-
- /**
- * Pointer to a DB instance.
- *
- * @var DB
- */
- protected $_db;
-
- /**
- * Attempts to return a reference to a concrete Horde_X instance.
- *
- * @return Horde_X The concrete Horde_X reference.
- * @throws Horde_Exception
- */
- static public function singleton()
- {
- if (!isset(self::$_instance)) {
- self::$_instance = new Horde_X();
- }
-
- return self::$_instance;
- }
-
- /**
- * Constructor.
- */
- public function __construct()
- {
- global $conf;
-
- $this->_db = DB::connect($conf['sql']);
- }
- }
-
-The class obviously depends on a database connection. The constructor
-above does not allow for dependency injection as it constructs the
-database connection itself. It uses the global variable $conf in order
-to get the settings for this connection. A constructor allowing
-dependency injection would look like this:
-
- /**
- * Constructor.
- *
- * @param DB $db A database connection.
- */
- public function __construct(DB $db)
- {
- $this->_db = $db;
- }
-
-Of course this connection must be provided from somewhere. The
-application using Horde_X might simply provide it when creating the
-Horde_X instance. If the application is however using a dependency
-injection framework then this framework would be required to provide
-the required database connection.
-
-If the application uses Horde_Provider it would be necessary to
-provide a factory method as Horde_Provider currently only allows to
-call factory methods via Horde_Provider_Injection_Factory:
-
- /**
- * Return a reference to a new Horde_X instance.
- *
- * @return Horde_X The new Horde_X reference.
- */
- static public function factory($provider)
- {
- $x = new Horde_X($provider->db)
- return $x;
- }
-
-The fact that we need this factory method is a specific shortcoming of
-the current capabilites of the Horde_Provider system. A more complex
-dependency injection framework would have the ability to look at the
-Horde_X constructor. It would then fetch the required dependencies
-based on the constructor arguments. Type hinting is usually required
-to inform the framework of the required dependency. In case of Horde_X
-we used type hinting to indicate that we need an object of type "DB"
-as parameter "$db".
-
-Getting rid of singletons?
-==========================
-
-From the viewpoint of dependency injection Horde_X can be used now as
-it allows external injection of its dependencies. We could throw away
-the singleton now. There might be some reasons however why we would
-like to keep the singleton() method. One of the reasons might be
-backward compatibility as some other classes or applications are bound
-to use the method. Another reason might be that we want to clarify how
-to get a functional instance of the class to somebody just looking at
-the Horde_X class. The factory method requires that there is a
-$provider object "magically" providing the db connection.
-
-We could keep the following singleton method:
-
- static public function singleton()
- {
- if (!isset(self::$_instance)) {
- global $conf;
-
- $provider = new stdClass;
- $provider->db = DB::connect($conf['sql']);
- self::$_instance = Horde_X::factory($provider);
- }
-
- return self::$_instance;
- }
-
-In this specific case it would probably be easier to directly use the
-Horde_X constructor rather than the factory method but there might be
-situations where the factory method has a higher complexity.
-
-We do not gain much here: The magic $provider->db parameter is
-exchanged against the magic $conf['sql'] parameter. However the
-singleton demonstrates how the DB class is instantiated via the
-DB::connect() method. That is something that was not visible in the
-factory method.
-
-A decent singleton method would require the database configuration as
-a parameter for the method and refer to the DB class for details
-concerning the possible configuration settings. That would however
-break backward compatibility for the method.
-
-Result
-======
-
-The final result that can be used with a dependency injection
-framework and still provides a backward compatible singleton method:
-
- class Horde_X
- {
- /**
- * Instance object.
- *
- * @var Horde_X
- */
- static protected $_instance;
-
- /**
- * Pointer to a DB instance.
- *
- * @var DB
- */
- protected $_db;
-
- /**
- * Attempts to return a reference to a concrete Horde_X instance.
- *
- * @return Horde_X The concrete Horde_X reference.
- */
- static public function singleton()
- {
- if (!isset(self::$_instance)) {
- global $conf;
-
- $provider = new stdClass;
- $provider->db = DB::connect($conf['sql']);
- self::$_instance = Horde_X::factory($provider);
- }
-
- return self::$_instance;
- }
-
- /**
- * Return a reference to a new Horde_X instance.
- *
- * @return Horde_X The new Horde_X reference.
- */
- static public function factory($provider)
- {
- $x = new Horde_X($provider->db)
- return $x;
- }
-
- /**
- * Constructor.
- *
- * @param DB $db A database connection.
- */
- public function __construct(DB $db)
- {
- $this->_db = $db;
- }
- }
projects / horde.git / commitdiff