--- /dev/null
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+That's all there is to it!
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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