A Dependency Injection Primer
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A Dependency Injection Primer

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A gentle introduction to dependency injection as a general software engineering best practice.

A gentle introduction to dependency injection as a general software engineering best practice.

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  • 1. A Dependency Injection Primer In which @rowan_m discovers a  gateway drug to quality.
  • 2. Chapter 1  The Green Field In which our heroine begins a new  endeavour for a local publican
  • 3. Developers shouldnt couple How one might start to write their application code:$bar = new DrinkMapper();$drink = $bar->getCocktail(bloody mary);
  • 4. Naïve use of patternsSupporting code as written by our eager, young protagonist: // our data mapperclass DrinkMapper {    public function getCocktail($name) {        $bar = new PitcherAndPiano();        $row = $bar->getCocktail($name);        return new Cocktail($row);    }} // our domain objectclass Cocktail extends ArrayObject {}
  • 5. Naïve use of patterns (continued)Supporting code as written by our eager, young protagonist: // our storageclass PitcherAndPiano {    public function getCocktail($name) {        $db = new PDO(mysql:host=localhost;.            dbname=pnp, puser, ppass);        $row = $db            ->query("SELECT * FROM cocktails.                WHERE name = $name")            ->fetch();        return $row;    }}
  • 6. Chapter 2  Foreboding Clouds In which the gruff senior developer  mutters darkly about unit tests 
  • 7. Constructors for a house of cards Our heroine attempts to test from the deepest class upwards.     Emboldened by her success with PHPUnit_Extensions_ Database_TestCase on the PitcherAndPiano class, she  moves on...
  • 8. Foundations begin to crumble This method  cannot  be unit tested due to these concrete dependencies within the code. // our data mapperclass DrinkMapper {    public function getCocktail($name) {        $bar = new PitcherAndPiano();        $row = $bar->getCocktail($name);        return new Cocktail($row);    }}
  • 9. Filling in the cracks After some research, our intrepid heroine tries Setter Injection:class DrinkMapper {    private $_bar;    public function setBar(            PitcherAndPiano $bar) {        $this->_bar = $bar;    }    public function getCocktail($name) {        $row = $this->_bar->getCocktail($name);        return new Cocktail($row);    }}
  • 10. Setter Injection Abuse Still the saturnine senior shows scorn for the implied improvements.      Setters should be used with caution as they introduce  mutability. Immutable classes are simpler and more efficient.      This setter requires that it is called before the finder method. The object cannot be constructed in a valid state.
  • 11. Constructing correctly Enlightened, our heroine progresses to Constructor Injection:class DrinkMapper {    private $_bar;    public function setBar__construct(            PitcherAndPiano $bar) {        $this->_bar = $bar;    }    public function getCocktail($name) {        $row = $this->_bar->getCocktail($name);        return new Cocktail($row);    }}
  • 12. Understand rules to break them Of course, our senior now provides contrary observations.     A long list of constructor parameters may indicate the class has  too many responsibilities. However, it may be valid. Setter  injection is the right solution in places.       Constructor parameters are not self­documenting. One must  refer to the code of the called class.
  • 13. Chapter 3   Weather the Storm In which our heroine puts her new skills  into practice via a training montage.
  • 14. The other classOur heroine ponders why her new skills do not fit the other class.class DrinkMapper {    private $_bar;    public function __construct(            PitcherAndPiano $bar) {        $this->_bar = $bar;    }    public function getCocktail(             $name, Cocktail $cocktail) {        $row = $this->_bar->getCocktail($name);        return $cocktail->exchangeArray($row);    }}
  • 15. An awkward feeling Without disturbing the seniors slumber, our heroine ponders...  That is now dependent on another mutable class. It does not  make sense for a Cocktail to exist sans content.    Callers must now construct the return type too, leading to a  great deal of repeated code.
  • 16. The industrial revolution Inspiration strikes and our heroine recalls the Factory Pattern:class DrinkFactory {    public static $ctClass = Cocktail;       public static function getCocktail(array $data) {        return new self::$ctClass($data);    }} class DrinkMapper { [...]    public function getCocktail($name) {        $row = $this->_bar->getCocktail($name);        return DrinkFactory::getCocktail($row);    }}
  • 17. Mass production With a few tweaks, our heroine obtains her holy grail of 100% coverage! Factories can replace constructors within libraries. Controllers may arguably use constructors directly due to their specialised  nature.   Factories may also detect the presence of an object over a class  name to aid in injecting mock objects.
  • 18. Chapter 4   Pastures New In which our heroine travels on her  new wheel, only to discover... she did not invent it first!
  • 19. Exploring the frontier Our heroine begins to push the boundaries.  Define interfaces for injected classes for even looser coupling.  Swap implementations in real­life, not just mocks.
  • 20. Founding fathers Martin Fowler first coined the term as a specialisation of Inversion of Control: http://martinfowler.com/articles/injection.html    He additionally covers Interface Injection and Service Locators  Goodness, hes a sharp fellow: http://twitter.com/martinfowler
  • 21. They are among us! Richard Miller has written rather extensively on the subject with respect to Symfony 2: http://miller.limethinking.co.uk/tag/dependency­injection/  Take advantage of his delicious brain.  He may also be found on Twitter: http://twitter.com/mr_r_miller
  • 22. Questions & Discussion   fin