sudo apt-get update\r\nsudo apt-get install apache2 php5 libapache2-mod-php5 php5-sqlite\r\nsudo service apache2 restart<\/pre>\nTo install Composer, we need to download the PHP archive from its website, and move to it to binary files directory to execute it globally:<\/p>\n
curl -sS https:\/\/getcomposer.org\/installer | php\r\nsudo mv composer.phar \/usr\/local\/bin\/composer<\/pre>\n1.2. PHP Configuration<\/h3>\n
In \/etc\/php5\/apache2\/php.ini file, we need to include the SQLite extension, in order to use it:<\/p>\n
\/etc\/php5\/apache2\/php.ini<\/em><\/span><\/p>\nextension=sqlite.so<\/pre>\nDon’t forget to restart Apache after doing any change.<\/p>\n
2 Introduction to DI<\/h2>\n2.1. What is Dependency Injection (DI)?<\/h3>\n
Dependency Injection is a design pattern where we supply a class (a client) the object instances it needs (the dependency), instead of instantiating itself<\/strong>. This pattern is based in a principle known as Dependency Inversion Principle<\/strong> (DIP). This principle says, basically, that high level modules should not depend directly on low level modules<\/strong>, where low level modules are those that implement basic operations; and high level modules, which use those modules for a specific purpose (what we call business logic).<\/p>\nThe aim of DIP, conceptually, is to change from:<\/p>\n
A<\/em> needs to create, or use, B<\/em>, in order to do C<\/em><\/p><\/blockquote>\nTo:<\/p>\n
A<\/em> needs something<\/em><\/strong> that allows doing C<\/em><\/p><\/blockquote>\nWhat A<\/em> wants is to do C<\/em>, no matter how. The following diagram illustrates how Dependency Injection does this:<\/p>\n![](\"http:\/\/www.webcodegeeks.com\/wp-content\/uploads\/2016\/05\/dependeny-injection.jpg\")
<\/p>\n
Let’s describe each component:<\/p>\n
\n- The class
Client<\/code> uses an interface, where some methods are described. This aggregation relation describes what we defined above as needs something that allows doing something<\/em>. That interface allows to do an operation. The client doesn’t know how will be it done, but it can do it.<\/li>\n- As said before, our interface
DependencyInterface<\/code> defines a set of methods. Definition means only method name, receiving parameters and returning type (this last actually not important in languages like PHP). This interface will be implemented by the dependencies that are going to be injected.<\/li>\n- We have, in this case,
DependencyA<\/code> and DependencyB<\/code> specific implementations of DependencyInterface<\/code> definitions. Each dependency will implement the operation()<\/code> method according to its nature.<\/li>\n- The
Injector<\/code> is the component which will instantiate the required dependency, to inject it into the Client<\/code>. The Client<\/code> won’t know what is receiving; it will use the implementation decided by the Injector<\/code>.<\/li>\n<\/ul>\nRemember that DI and DIP are not equivalent, but that DI follows the DIP.<\/p>\n
2.2. How does DI help us?<\/h3>\n\n- Responsibility division: each dependency can be developed individually; the only restriction is the interface it must implement<\/strong>, which has to be agreed before. This allows to divide the work between developers, avoiding problems at integration time<\/strong>.<\/li>\n
- Maintainable code: does one of the features need changes or fixes? With DI, we target immediately the piece of code susceptible for changes<\/strong>, and we will be sure that the changes will have to be done only in that piece of code<\/strong>.<\/li>\n
- Reusable code: Dependency Injection makes us to design independent pieces of code (the dependencies), which can be reused as many times as we want, without repeating the code<\/strong>.<\/li>\n
- Testeable code: without DI, testing is much more complicated<\/strong>. Why? Because, without DI, if we testing a component that is using another compontent that is not being injected, it will actually use that component in the test. In most of the cases, this is not interesting, because we would be testing several times the same thing. With Dependency Injection, we can easily mock the dependencies to test better and faster<\/strong>.<\/li>\n<\/ul>\n
To put all this in context, let’s see an example.<\/p>\n
3. A non-DI example<\/h2>\n
Let’s consider the following scenario: we are going to develop a system that inserts and reads data.<\/p>\n
The very first version, will work with a simple text file. So, we would have the following class to deal with the text file:<\/p>\n
Textfile.php<\/em><\/span><\/p>\n<?php\r\n\r\n\/**\r\n\u00a0* Methods for text file handling.\r\n\u00a0*\/\r\nclass Textfile {\r\n\r\n\u00a0\u00a0\u00a0 const TEXTFILE = 'data.txt';\r\n\r\n\u00a0\u00a0\u00a0 \/**\r\n\u00a0\u00a0\u00a0\u00a0 * Writes the data into the textfile.\r\n\u00a0\u00a0\u00a0\u00a0 *\r\n\u00a0\u00a0\u00a0\u00a0 * @param $data The data to write into the textfile.\r\n\u00a0\u00a0\u00a0\u00a0 *\/\r\n\u00a0\u00a0\u00a0 public function writeData($data) {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 file_put_contents(self::TEXTFILE, $data . '<br>', FILE_APPEND);\r\n\u00a0\u00a0\u00a0 }\r\n\r\n\u00a0\u00a0\u00a0 \/**\r\n\u00a0\u00a0\u00a0\u00a0 * Reads the data from the textfile.\r\n\u00a0\u00a0\u00a0\u00a0 *\r\n\u00a0\u00a0\u00a0\u00a0 * @return Textfile data.\r\n\u00a0\u00a0\u00a0\u00a0 *\/\r\n\u00a0\u00a0\u00a0 public function readData() {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 $data = file_get_contents(self::TEXTFILE);\r\n\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 return $data;\r\n\u00a0\u00a0\u00a0 }\r\n}<\/pre>\nAnd, then, a script to use this class:<\/p>\n
Controller.php<\/em><\/span><\/p>\n<?php\r\n\r\nrequire_once('Textfile.php');\r\n\r\ndefine('ACTION_READ', 'read');\r\ndefine('ACTION_WRITE', 'write');\r\ndefine('WRITE_DATA', 'data');\r\n\r\n\/**\r\n\u00a0* Checks if the given parameters are set. If one of the specified parameters is not set,\r\n\u00a0* die() is called.\r\n\u00a0*\r\n\u00a0* @param $parameters The parameters to check.\r\n\u00a0*\/\r\nfunction checkGETParametersOrDie($parameters) {\r\n\u00a0\u00a0 \u00a0foreach ($parameters as $parameter) {\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0isset($_GET[$parameter]) || die(\"Please, provide '$parameter' parameter.\");\r\n\u00a0\u00a0 \u00a0}\r\n}\r\n\r\n\/\/ Flow starts here.\r\n\r\ncheckGETParametersOrDie(['action']);\r\n\r\n$action = $_GET['action'];\r\n\r\n$textfile = new Textfile();\r\n\r\nswitch ($action) {\r\n\u00a0\u00a0 \u00a0case ACTION_READ:\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0$response = $textfile->readData();\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0break;\r\n\r\n\u00a0\u00a0 \u00a0case ACTION_WRITE:\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0checkGETParametersOrDie([WRITE_DATA]);\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0$data = $_GET[WRITE_DATA];\r\n\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0$textfile->writeData($data);\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0$response = 'Data written.';\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0break;\r\n\r\n\u00a0\u00a0 \u00a0default:\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0$response = 'Please, provide a valid \"' . WRITE_DATA . '\" parameter.';\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0break;\r\n}\r\n\r\necho $response;<\/pre>\nNothing really special: we include the required Textfile<\/code> class in line 3, we instantiate it in line 27, and we call its methods in lines 31 and 38, depending on what we are going to do.<\/p>\nAs time goes by, we realize that a text file is no longer a suitable option because we need something more sophisticated; a relational database. So, we would develop another class to deal with the database:<\/p>\n
DB.php<\/em><\/span><\/p>\n<?php\r\n\r\n\/**\r\n\u00a0* Methods for database handling.\r\n\u00a0*\/\r\nclass DB extends SQLite3 {\r\n\r\n\u00a0\u00a0 \u00a0const DATABASE = 'data.db';\r\n\r\n\u00a0\u00a0 \u00a0\/**\r\n\u00a0\u00a0 \u00a0 * DB class constructor. Initialize method is called, which will create data table if\r\n\u00a0\u00a0 \u00a0 * it does not exist already.\r\n\u00a0\u00a0 \u00a0 *\/\r\n\u00a0\u00a0 \u00a0public function __construct() {\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0$this->open(self::DATABASE);\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0$this->initialize();\r\n\u00a0\u00a0 \u00a0}\r\n\r\n\u00a0\u00a0 \u00a0\/**\r\n\u00a0\u00a0 \u00a0 * Craetes the table if it does not exist already.\r\n\u00a0\u00a0 \u00a0 *\/\r\n\u00a0\u00a0 \u00a0protected function initialize() {\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0$sql = 'CREATE TABLE IF NOT EXISTS data (\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0data STRING\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0)';\r\n\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0$this->exec($sql);\r\n\u00a0\u00a0 \u00a0}\r\n\r\n\u00a0\u00a0 \u00a0\/**\r\n\u00a0\u00a0 \u00a0 * Inserts the data into the databse.\r\n\u00a0\u00a0 \u00a0 *\r\n\u00a0\u00a0 \u00a0 * @param $data Data to insert.\r\n\u00a0\u00a0 \u00a0 *\/\r\n\u00a0\u00a0 \u00a0public function insertData($data) {\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0$sql = 'INSERT INTO data\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0VALUES (:data)';\r\n\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0$statement = $this->prepare($sql);\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0$statement->bindValue(':data', $data);\r\n\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0$statement->execute();\r\n\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0$statement->close();\r\n\u00a0\u00a0 \u00a0}\r\n\r\n\u00a0\u00a0 \u00a0\/**\r\n\u00a0\u00a0 \u00a0 * Selects data from database.\r\n\u00a0\u00a0 \u00a0 *\r\n\u00a0\u00a0 \u00a0 * @return Database data.\r\n\u00a0\u00a0 \u00a0 *\/\r\n\u00a0\u00a0 \u00a0public function selectData() {\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0$sql = 'SELECT data\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0FROM\u00a0\u00a0 data';\r\n\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0$results = $this->query($sql);\r\n\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0$data = '';\r\n\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0while ($row = $results->fetchArray()) {\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0$data .= $row['data'] . '<br>';\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0}\r\n\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0return $data;\r\n\u00a0\u00a0 \u00a0}\r\n}<\/pre>\nAnd now, we have to update our Controller.php<\/code> file, in order to use our brand-new DB<\/code> class:<\/p>\nController.php<\/em><\/span><\/p>\n<?php\r\n\r\nrequire_once('DB.php');\r\n\r\ndefine('ACTION_READ', 'read');\r\ndefine('ACTION_WRITE', 'write');\r\ndefine('WRITE_DATA', 'data');\r\n\r\n\/**\r\n\u00a0* Checks if the given parameters are set. If one of the specified parameters is not set,\r\n\u00a0* die() is called.\r\n\u00a0*\r\n\u00a0* @param $parameters The parameters to check.\r\n\u00a0*\/\r\nfunction checkGETParametersOrDie($parameters) {\r\n\u00a0\u00a0 \u00a0foreach ($parameters as $parameter) {\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0isset($_GET[$parameter]) || die(\"Please, provide '$parameter' parameter.\");\r\n\u00a0\u00a0 \u00a0}\r\n}\r\n\r\n\/\/ Flow starts here.\r\ncheckGETParametersOrDie(['action']);\r\n\r\n$action = $_GET['action'];\r\n\r\n$db = new DB();\r\n\r\nswitch ($action) {\r\n\u00a0\u00a0 \u00a0case ACTION_READ:\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0$response = $db->selectData();\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0break;\r\n\r\n\u00a0\u00a0 \u00a0case ACTION_WRITE:\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0checkGETParametersOrDie([WRITE_DATA]);\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0$data = $_GET[WRITE_DATA];\r\n\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0$db->insertData($data);\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0$response = 'Data written.';\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0break;\r\n\r\n\u00a0\u00a0 \u00a0default:\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0$response = 'Please, provide a valid \"action\" parameter.';\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0break;\r\n}\r\n\r\necho $response;<\/pre>\nLet’s see carefully what has happened:<\/p>\n
\n- The file requirement and the instance have changed. Well, this is something that must be done somewhere.<\/li>\n
- Here is the important part: every object method call has changed<\/strong>. Apart from the object name itself, the methods have changed. Why? Because we have not forced to the
Textfile<\/code> and DB<\/code> classes to share the same interface<\/strong>. And, in this case, we are lucky that the only thing that has changed is the name, and not the parameters, or the return type.<\/li>\n- There is another thing to take into account: how shall we test
Controller.php<\/code>? Currently, is using the DB<\/code> class. If we test it, we are using the real database. For example, the database could be that big that could make the tests quite slow.<\/li>\n<\/ul>\nThis have been affordable changes, we just changed a few lines. But imagine this problem in a real project, with several dependencies and developers. This can be a real problem not only for maintaining, but at development also.<\/p>\n
4. First approach to DI: manual injection<\/h2>\n
Let’s resolve this adding an abstraction layer between our components<\/strong>.<\/p>\nThe first step must be to ask ourselves:<\/p>\n
What<\/strong> do we want to offer?<\/p><\/blockquote>\nIn this case is pretty clear. We want to offer methods for reading and writing. It doesn’t matter how, from where, or any other implementation details<\/strong>. We only focus in what.<\/p>\n4.1. Defining the interface<\/h3>\n
For that, as in other Object Oriented Programming languages, we have the Interfaces<\/strong>. Object interfaces allow to specify which methods have to be implemented by a class, without defining how are going to be implemented.<\/p>\nSo, lets define an interface for our read and write purposes:<\/p>\n
StorageInterface.php<\/em><\/span><\/p>\n<?php\r\n\r\n\/**\r\n\u00a0* Interface for method definitions for dealing with storage.\r\n\u00a0*\/\r\ninterface StorageInterface {\r\n\u00a0\u00a0 \u00a0\r\n\u00a0\u00a0\u00a0 \/**\r\n\u00a0\u00a0\u00a0\u00a0 * Writes the given data into the storage.\r\n\u00a0\u00a0\u00a0\u00a0 *\r\n\u00a0\u00a0\u00a0\u00a0 * @param $data The data to write.\r\n\u00a0\u00a0\u00a0\u00a0 *\/\r\n\u00a0\u00a0\u00a0 public function writeData($data);\r\n\r\n\u00a0\u00a0\u00a0 \/**\r\n\u00a0\u00a0\u00a0\u00a0 * Reads the data from the storage.\r\n\u00a0\u00a0\u00a0\u00a0 *\r\n\u00a0\u00a0\u00a0\u00a0 * @return The read data from storage.\r\n\u00a0\u00a0\u00a0\u00a0 *\/\r\n\u00a0\u00a0\u00a0 public function readData();\r\n}<\/pre>\nSo simple. We just define the methods inside an interface<\/code> piece of code. Now, we will implement it in our classes.<\/p>\n4.2. Implementing the interface<\/h3>\n
Textfile.php<\/em><\/span><\/p>\n<?php\r\n\r\nrequire_once('StorageInterface.php');\r\n\r\n\/**\r\n\u00a0* Methods for text file handling.\r\n\u00a0*\/\r\nclass Textfile implements StorageInterface {\r\n\r\n\u00a0\u00a0\u00a0 const TEXTFILE = 'data.txt';\r\n\r\n\u00a0\u00a0\u00a0 \/**\r\n\u00a0\u00a0\u00a0\u00a0 * Writes the data into the textfile.\r\n\u00a0\u00a0\u00a0\u00a0 *\r\n\u00a0\u00a0\u00a0\u00a0 * @param $data The data to write into the textfile.\r\n\u00a0\u00a0\u00a0\u00a0 *\/\r\n\u00a0\u00a0\u00a0 public function writeData($data) {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 file_put_contents(self::TEXTFILE, $data . '<br>', FILE_APPEND);\r\n\u00a0\u00a0\u00a0 }\r\n\r\n\u00a0\u00a0\u00a0 \/**\r\n\u00a0\u00a0\u00a0\u00a0 * Reads the data from the textfile.\r\n\u00a0\u00a0\u00a0\u00a0 *\r\n\u00a0\u00a0\u00a0\u00a0 * @return Textfile data.\r\n\u00a0\u00a0\u00a0\u00a0 *\/\r\n\u00a0\u00a0\u00a0 public function readData() {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 $data = file_get_contents(self::TEXTFILE);\r\n\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 return $data;\r\n\u00a0\u00a0\u00a0 }\r\n}<\/pre>\nThe interfaces are implemented with implements keyword. There are some things that we must know about interfaces:<\/p>\n
\n- A class that implements an interface, needs to implement every method defined in that interface. Otherwise, a PHP fatal error will be thrown.<\/li>\n
- The implemented methods must have the same name and parameters as the function defined in the interface.<\/li>\n<\/ul>\n
Let’s do the same with DB class:<\/p>\n
DB.php<\/em><\/span><\/p>\n<?php\r\n\r\nrequire_once('StorageInterface.php');\r\n\r\n\/**\r\n\u00a0* Methods for database handling.\r\n\u00a0*\/\r\nclass DB extends SQLite3 implements StorageInterface{\r\n\r\n\u00a0\u00a0\u00a0 const DATABASE = 'data.db';\r\n\r\n\u00a0\u00a0\u00a0 \/**\r\n\u00a0\u00a0\u00a0\u00a0 * DB class constructor. Initialize method is called, which will create data table if\r\n\u00a0\u00a0\u00a0\u00a0 * it does not exist already.\r\n\u00a0\u00a0\u00a0\u00a0 *\/\r\n\u00a0\u00a0\u00a0 public function __construct() {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 $this->open(self::DATABASE);\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 $this->initialize();\r\n\u00a0\u00a0\u00a0 }\r\n\r\n\u00a0\u00a0\u00a0 \/**\r\n\u00a0\u00a0\u00a0\u00a0 * Craetes the table if it does not exist already.\r\n\u00a0\u00a0\u00a0\u00a0 *\/\r\n\u00a0\u00a0\u00a0 protected function initialize() {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 $sql = 'CREATE TABLE IF NOT EXISTS data (\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 data STRING\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 )';\r\n\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 $this->exec($sql);\r\n\u00a0\u00a0\u00a0 }\r\n\r\n\u00a0\u00a0\u00a0 \/**\r\n\u00a0\u00a0\u00a0\u00a0 * Inserts the data into the databse.\r\n\u00a0\u00a0\u00a0\u00a0 *\r\n\u00a0\u00a0\u00a0\u00a0 * @param $data Data to insert.\r\n\u00a0\u00a0\u00a0\u00a0 *\/\r\n\u00a0\u00a0\u00a0 public function writeData($data) {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 $sql = 'INSERT INTO data\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 VALUES (:data)';\r\n\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 $statement = $this->prepare($sql);\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 $statement->bindValue(':data', $data);\r\n\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 $statement->execute();\r\n\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 $statement->close();\r\n\u00a0\u00a0\u00a0 }\r\n\r\n\u00a0\u00a0\u00a0 \/**\r\n\u00a0\u00a0\u00a0\u00a0 * Selects data from database.\r\n\u00a0\u00a0\u00a0\u00a0 *\r\n\u00a0\u00a0\u00a0\u00a0 * @return Database data.\r\n\u00a0\u00a0\u00a0\u00a0 *\/\r\n\u00a0\u00a0\u00a0 public function readData() {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 $sql = 'SELECT data\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 FROM\u00a0\u00a0 data';\r\n\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 $results = $this->query($sql);\r\n\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 $data = '';\r\n\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 while ($row = $results->fetchArray()) {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 $data .= $row['data'] . '<br>';\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 }\r\n\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 return $data;\r\n\u00a0\u00a0\u00a0 }\r\n}<\/pre>\nNote that we have changed the method names (lines 37 and 54) to coincide with those defined in the interface.<\/p>\n
Nothing really exciting for the moment, right? Actually, some can think that we have done anything new.<\/p>\n
4.3. Accessing the interface<\/h3>\n
Instead of accessing the implementations directly in Controller.php<\/code>, as in the non-DI example, we are going to create a wrapper for it, where the dependencies will be injected:<\/p>\nStorageHandler.php<\/em><\/span><\/p>\n<?php\r\n\r\nrequire_once('StorageInterface.php');\r\n\r\n\/**\r\n\u00a0* StorageInterface wrap.\r\n\u00a0*\/\r\nclass StorageHandler {\r\n\r\n\u00a0\u00a0\u00a0 protected $storage;\r\n\r\n\u00a0\u00a0\u00a0 \/**\r\n\u00a0\u00a0\u00a0\u00a0 * StorageHandler constructor, with an implementation of StorageInterface.\r\n\u00a0\u00a0\u00a0\u00a0 *\r\n\u00a0\u00a0\u00a0\u00a0 * @param $storage StorageInterface implementation.\r\n\u00a0\u00a0\u00a0\u00a0 *\/\r\n\u00a0\u00a0\u00a0 public function __construct(StorageInterface $storage) {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 $this->storage = $storage;\r\n\u00a0\u00a0\u00a0 }\r\n\r\n\u00a0\u00a0\u00a0 \/**\r\n\u00a0\u00a0\u00a0\u00a0 * Writes the data using the storage implementation.\r\n\u00a0\u00a0\u00a0\u00a0 *\r\n\u00a0\u00a0\u00a0\u00a0 * @param $data Data to write.\r\n\u00a0\u00a0\u00a0\u00a0 *\/\r\n\u00a0\u00a0\u00a0 public function writeData($data) {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 $this->storage->writeData($data);\r\n\u00a0\u00a0\u00a0 }\r\n\r\n\u00a0\u00a0\u00a0 \/**\r\n\u00a0\u00a0\u00a0\u00a0 * Reads the data using the storage implementation.\r\n\u00a0\u00a0\u00a0\u00a0 *\r\n\u00a0\u00a0\u00a0\u00a0 * @return Read data.\r\n\u00a0\u00a0\u00a0\u00a0 *\/\r\n\u00a0\u00a0\u00a0 public function readData() {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 $data = $this->storage->readData();\r\n\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 return $data;\r\n\u00a0\u00a0\u00a0 }\r\n}<\/pre>\nThis is the class that deals with data reading and writing. But it only knows about an interface, which has methods that are not implemented. The key is that it will receive a concrete implementation in the constructor,<\/strong> and we save it as an attribute of the class<\/strong> (lines 17, 18). We even specify that it must receive an implementation of StorageInterface<\/code>, even if in PHP is not necessary to declare types. But, in this cases, is a good practice to specify the interface we are expecting, to express that we will receive an unknown implementation of that interface.<\/p>\nSo, when we call those methods (lines 27, 36), we will be calling received instance’s implemented methods. What they do is not important, the only thing the class has to know is that it will have methods named as defined in the interface.<\/p>\n
We don’t have to even require those specific implementations classes, is enough to require only the interface (line 3).<\/p>\n
We could even add the possibility to change that implementation on runtime!<\/p>\n
StorageHandler.php<\/em><\/span><\/p>\npublic function setStorage(StorageInterface $storage) {\r\n\u00a0\u00a0\u00a0 $this->storage = $storage;\r\n}<\/pre>\nAdding a setter, we could change the implementation StorageHandler<\/code> class is using at any time.<\/p>\n4.4. Instantiating interface implementations<\/h3>\n
Last step is to control the StorageHandler<\/code>:<\/p>\nController.php<\/em><\/span><\/p>\n<?php\r\n\r\nrequire_once('StorageHandler.php');\r\nrequire_once('Textfile.php');\r\n\r\ndefine('ACTION_READ', 'read');\r\ndefine('ACTION_WRITE', 'write');\r\ndefine('WRITE_DATA', 'data');\r\n\r\n\/**\r\n\u00a0* Checks if the given parameters are set. If one of the specified parameters is not set,\r\n\u00a0* die() is called.\r\n\u00a0*\r\n\u00a0* @param $parameters The parameters to check.\r\n\u00a0*\/\r\nfunction checkGETParametersOrDie($parameters) {\r\n\u00a0\u00a0\u00a0 foreach ($parameters as $parameter) {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 isset($_GET[$parameter]) || die(\"Please, provide '$parameter' parameter.\");\r\n\u00a0\u00a0\u00a0 }\r\n}\r\n\r\n\/\/ Flow starts here.\r\ncheckGETParametersOrDie(['action']);\r\n\r\n$action = $_GET['action'];\r\n\r\n$storage = new Textfile();\r\n$handler = new StorageHandler($storage);\r\n\r\nswitch ($action) {\r\n\u00a0\u00a0\u00a0 case ACTION_READ:\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 $response = $handler->readData();\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 break;\r\n\r\n\u00a0\u00a0\u00a0 case ACTION_WRITE:\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 checkGETParametersOrDie([WRITE_DATA]);\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 $data = $_GET[WRITE_DATA];\r\n\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 $handler->writeData($data);\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 $response = 'Data written.';\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 break;\r\n\r\n\u00a0\u00a0\u00a0 default:\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 $response = 'Please, provide a valid \"action\" parameter.';\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 break;\r\n}\r\n\r\necho $response;<\/pre>\nAs in the first case in the previous approach without DI, we suppose that we are going to use the text file as storage. So, we require it (line 4), we instantiate it (line 27), and instantiate the StorageHandler<\/code> class passing the Textfile<\/code> implementation in the constructor (line 28). Or, in other words: we are injecting the Textfile<\/code> dependency to StorageHandler<\/code><\/strong>.<\/p>\nNow, let’s say that we want to use the database. The changes would be:<\/p>\n
Controller.php<\/em><\/span><\/p>\n\/\/...\r\n\r\nrequire_once('DB.php');\r\n\r\n\/\/ ...\r\n\r\n$storage = new DB();<\/pre>\nAnd that’s it. The $handler<\/code> object will be working without the need of suffering any changes. We just include the new class, and we change the instance of the variable passed to StorageHandler<\/code> instantiation; the injected dependency, now, is DB<\/code><\/strong>.<\/p>\nIsn’t it amazing?<\/p>\n
5. Second approach to DI: Injector class<\/h2>\n
In the first approach, we have seen how to perform a Dependency Injection, in such a disorganized way: we only have to change the instantiating class, yes, but we have to know exactly where is being instantiated. And this may not be easy in medium-large sized projects – in this case, we have only a dependency, but imagine finding each injection for tens of dependencies.<\/p>\n
So, let’s create a completely isolated wrapper for the injection to StorageHandler<\/code> – just as in the diagram we saw in section 1.1.<\/p>\n5.1. Creating the injector<\/h3>\n
As said, let’s define a class only for instantiating the dependency for StorageHandler<\/code>:<\/p>\nStorageInjector.php<\/em><\/span><\/p>\n<?php\r\n\r\nrequire_once('Textfile.php');\r\n\/**\r\n\u00a0* Storage dependency injector.\r\n\u00a0*\/\r\nclass StorageInjector {\r\n\r\n\u00a0\u00a0\u00a0 \/**\r\n\u00a0\u00a0 \u00a0 * Retrieves the specified instance of StorageInterface.\r\n\u00a0\u00a0 \u00a0 *\r\n\u00a0\u00a0 \u00a0 * @return StorageInterface implementation.\r\n\u00a0\u00a0 \u00a0 *\/\r\n\u00a0\u00a0 \u00a0public static function getStorageInstance() {\r\n\u00a0\u00a0 \u00a0\u00a0\u00a0 \u00a0return new Textfile();\r\n\u00a0\u00a0 \u00a0}\r\n}<\/pre>\nSo simple, just a function that retrieves the defined instance. We have defined as a static function, because there’s no real need to instantiate this class.<\/p>\n
5.2. Calling the injector<\/h3>\n
Now, the only thing we have to do is to pass the returned object by StorageInjector::getStorageInstance()<\/code> to StorageHandler<\/code> constructor:<\/p>\nController.php<\/em><\/span><\/p>\n$handler = new StorageHandler(StorageInjector::getStorageInstance());<\/pre>\nAnd the StorageHandler<\/code> will use the whatever dependency injected by the StorageInjector, which will have the responsibility of deciding the dependency to inject<\/strong>.<\/p>\nSo, now, if we have to inject the DB<\/code> dependency, we exactly know where to change that injection:<\/p>\nStorageInjector.php<\/em><\/span><\/p>\npublic static function getStorageInstance() {\r\n\u00a0\u00a0 \u00a0return new DB();\r\n}<\/pre>\nIncluding, of course, the file where DB is defined.<\/p>\n
We say that we will exactly know the location of the injection because we are supposing that we are looking for a Injector suffixed file, or any other agreed standard for injector files<\/strong>.<\/p>\n6. PHP-DI<\/h2>\n
The PHP community is so great. It is constantly developing useful components. And Dependency Injection is not an exception: PHP-DI. In this chapter we will see how we can use it.<\/p>\n
6.1. Installation<\/h3>\n
The installation is so simple, thanks to our beloved Composer. Just enter the following command in your working directory:<\/p>\n
composer require php-di\/php-di<\/pre>\nAnd it will download the required stuff. Note that a composer.json and composer.lock, and a vendor directory have been created. Don’t touch them. The first two define the component we are using, and the vendor folder contains the PHP-CI code itself.<\/p>\n
6.2. The Injection Container Container<\/h3>\n
PHP-CI uses a concept called container for the dependency injection, using a technique named autowiring<\/em>, inspired by Java’s Spring framework. Let’s see how it works.<\/p>\nFirst, in the Controller.php file, we need to require a new file in order to use PHP-CI’s classes:<\/p>\n
Controller.php<\/em><\/span><\/p>\nrequire_once('vendor\/autoload.php');\r\n\r\n\/\/ ...<\/pre>\nThen, we will replace the following lines:<\/p>\n
Controller.php<\/em><\/span><\/p>\n\/\/ ...\r\n\r\n$storage = new Textfile();\r\n$handler = new StorageHandler($storage);\r\n\r\n\/\/ ...<\/pre>\nWith the following:<\/p>\n
Controller.php<\/em><\/span><\/p>\n\/\/ ...\r\n$container = DI\\ContainerBuilder::buildDevContainer();\r\n$handler = $container->get('Textfile');\r\n\r\n\/\/ ...<\/pre>\nWhat