Dependency Injection (2010)

The fourth in a short series of presentations given at a PHP development shop.

Transcript

1. Dependency Injection
   2010/05/25
   

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2. Dependency Injection
   
   For classes with dependencies on other
   classes.
   
   When an object is used, we pass their
   dependencies to it.
   

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3. Dependency Injection
   
   When we refer to a class from inside another
   one, we don't rely care what that class is
   exactly, only what its interface is. For example:
   class Car
   {
   protected $engine;
   public function __construct() {
   $this->engine = new Engine();
   }
   public function go() {
   $this->engine->on();
   $this->engine->applyTorque();
   }
   }
   

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4. Dependency Injection
   
   Engine could be V8Engine, V6Engine,
   whatever.
   
   So long as we can do the same sort of actions
   with each, we're set.
   

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5. Dependency Injection
   
   Say we want to test cars with different engines.
   class RaceTest() {
   public function testEngines() {
   $car1 = new Car();
   $car2 = new Car();
   // Uh ...
   }
   }
   class Car {
   protected $engine;
   public function __construct() {
   $this->engine = new Engine();
   }
   public function go() {
   $this->engine->on();
   $this->engine->applyTorque();
   }
   }
   

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6. Dependency Injection
   
   Let's change Car a bit.
   class Car {
   protected $engine;
   public function __construct(IEngine $engine) {
   $this->engine = $engine;
   }
   public function go() {
   $this->engine->on();
   $this->engine->applyTorque();
   }
   }
   

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7. Dependency Injection
   
   Back to the race.
   class RaceTest() {
   public function testEngines() {
   $car1 = new Car(new V8Engine);
   $car2 = new Car(new LawnmowerEngine);
   // calls go() on each in a thread, performs timing tests,
   // yadda yadda don't care. :-|
   $test = new SpeedComparison($car1, $car2);
   return $test->run()
   ->getResult();
   }
   }
   class Car { … }
   

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8. So What Do This Give Us?
   
   We're giving Car its dependencies.
   
   Benefit: Flexibility. Dependencies are now
   based on interfaces, not specific classes.
   
   Benefit: Hey, now it's easier to unit
   test. ;-) ;-) :-|
   

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9. The Not-So-Good
   
   Drawback: Now you have to give classes what
   they need.
   
   I'm a programmer, Jim, not a babysitter!
   
   Tentatively suggest using defaults, eg.
   class Car {
   protected $engine;
   public function __construct(IEngine $engine=null) {
   if (is_null($engine)) {
   $this->engine = new Engine;
   } else {
   $this->engine = $engine;
   }
   }
   }
   

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10. Discussion: Go!
    

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