Dependency Injection - PHPUG Cologne 2013/8

In this talk, I introduce the concept of Dependency Injection as well as Pimple as an example for a Dependency Injection Container. I also talk about one thing that you absolutely do *not* want to do when using Dependency Injection Containers.

Transcript

1. Dependency Injection
   

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2. inject dependencies!
   

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3. class Foo
   {
   private $db;
   private $memcache;
   public function __construct()
   {
   $this->db = new DatabaseConnection();
   $this->memcache = new MemcacheClient();
   }
   }
   

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4. class Foo
   {
   private $db;
   private $memcache;
   public function __construct(
   DatabaseConnection $db,
   MemcacheClient $memcache
   )
   {
   $this->db = $db;
   $this->memcache = $memcache;
   }
   }
   

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5. class Foo
   {
   private $db;
   private $memcache;
   public function __construct(
   DatabaseConnectionInterface $db,
   CacheClientInterface $memcache
   )
   {
   $this->db = $db;
   $this->memcache = $memcache;
   }
   }
   

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6. Optional dependencies
   

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7. use Psr\Log\LoggerInterface;
   use Psr\Log\NullLogger;
   class Bar
   {
   private $logger;
   public function __construct()
   {
   $this->logger = new NullLogger();
   }
   public function setLogger(LoggerInterface $logger)
   {
   $this->logger = $logger;
   }
   }
   

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8. Dependency Injection
   ●
   Use constructor injection for required
   dependencies – services your class needs to
   run
   ●
   Use setter injection for optional dependencies
   – services your class may use if available
   ●
   Program against interfaces, not concrete
   implementations!
   

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9. That's it!
   

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10. Advantages
    

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11. $foo = new Foo(); // fatal error
    $db = new DatabaseConnection();
    $memcache = new MemcacheClient();
    $foo = new Foo($db, $memcache); // works
    

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12. $db = new PostgreSqlConnection();
    $memcache = new MemcacheClient();
    $foo = new Foo($db, $memcache);
    

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13. $db = new DatabaseConnectionMock();
    $memcache = new CacheClientMock();
    $foo = new Foo($db, $memcache);
    

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14. Advantages
    ●
    Dependencies of classes are obvious, your IDE
    will point them out to you
    ●
    Dependencies can easily be replaced
    ●
    Easy replacement also means easier to test
    

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15. Problems?
    

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16. // Create a new instance of Foo
    $db = new DatabaseConnection();
    $memcache = new MemcacheClient();
    $foo = new Foo($db, $memcache);
    

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17. // Create a new instance of Foo
    $mysqli = new mysqli();
    $db = new DatabaseConnection($mysqli);
    $memcache = new MemcacheClient();
    $foo = new Foo($db, $memcache);
    

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18. // Create a new instance of Foo
    list($host, $user, $pw, $db) = getDbConfig();
    $mysqli = new mysqli($host, $user, $pw, $db);
    $db = new DatabaseConnection($mysqli);
    $memcache = new MemcacheClient();
    $foo = new Foo($db, $memcache);
    

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19. // Create a new instance of Foo
    list($host, $user, $pw, $db) = getDbConfig();
    $mysqli = new mysqli($host, $user, $pw, $db);
    $db = new DatabaseConnection($mysqli);
    list($mcHost, $mcPort) = getMemcacheConfig();
    $memcache = new MemcacheClient($mcHost, $mcPort);
    $foo = new Foo($db, $memcache);
    

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20. Problems?
    ●
    Construction can turn into a major PITA.
    

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21. DI Container
    To the rescue!
    

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22. DIC Example: Pimple
    ●
    http://pimple.sensiolabs.org
    ●
    MIT License
    ●
    Single-class solution
    ●
    About 50 SLOC
    

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23. $dic = new Pimple();
    $dic['mysqli.config'] = getDbConfig();
    

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24. $dic = new Pimple();
    $dic['mysql.config'] = getDbConfig();
    $dic['mysqli'] = new mysqli(/* ... */);
    $dic['db'] = new DatabaseConnection($dic['mysqli']);
    

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25. Lazy initialisation
    Watch closely, here's where the magic happens.
    

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27. $dic = new Pimple();
    $dic['mysql.config'] = getDbConfig();
    $dic['mysqli'] = function($dic) {
    return new mysqli(/* ... */);
    };
    $dic['db'] = function($dic) {
    return new DatabaseConnection($dic['mysqli']);
    };
    

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28. // DIC setup for db and cache here...
    $dic['foo'] = function($dic) {
    return new Foo($dic['db'], $dic['cache']);
    };
    // Later in your application:
    $foo = $dic['foo'];
    var_dump($foo instanceof Foo); // true
    

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29. Lazy initialisation in Pimple
    ●
    Pimple::offsetGet() calls closure we saved as
    $dic['foo']
    ●
    Closure needs $dic['db'] and $dic['cache'],
    means more calls to Pimple::offsetGet()
    ●
    Cascade closure / Pimple::offsetGet() calls
    until dependencies are resolved
    

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30. More Pimple features
    // To have one global instance of a class:
    $dic['example'] = $dic->share(function($dic) {
    return new Example();
    });
    // To return the closure instead of executing it:
    $dic['closure'] = $dic->protect(function() {
    // Do cool stuff here
    });
    

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31. DI Container
    ●
    Knows the services that are available in your
    application
    ●
    Knows how to create them using Builders
    ●
    Creates them as soon as you need them
    

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32. Cool!
    So now I can do this:
    

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33. class Controller
    {
    private $dic;
    public function __construct(Pimple $dic)
    {
    $this->dic = $dic;
    }
    public function doSomething()
    {
    $foo = $this->dic['foo'];
    // Do something here
    }
    }
    

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34. NOPE!
    

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35. $dic = new Pimple();
    $controller = new Controller($dic);
    $controller->doSomething();
    // InvalidArgumentException:
    // - Identifier "foo" is not defined.
    

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36. Stop hiding your
    dependencies!
    

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37. class Controller
    {
    private $foo;
    public function __construct(Foo $foo)
    {
    $this->foo = $foo;
    }
    public function doSomething()
    {
    // Do something here
    }
    }
    

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38. $dic = new Pimple();
    // DIC setup here...
    $name = determineControllerFromRequest();
    $controller = $dic[$name];
    echo $controller->doSomething();
    

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39. Don't abuse the DIC
    ●
    Do not hide your classes' dependencies by
    passing the DIC (or any other Service Locator)
    to them
    ●
    You get to pull exactly ONE object out of the
    DIC yourself: The controller that needs to be
    executed.
    

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40. Thank you!
    Any questions?
    

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