understanding dependency injection in PHP

Question

class Author {
    private $firstName;
    private $lastName;

    public function __construct($firstName, $lastName) {
        $this->firstName = $firstName;
        $this->lastName = $lastName;
    }

    public function getFirstName() {
        return $this->firstName;
    }

    public function getLastName() {
        return $this->lastName;
    }
}

class Question {
    private $author;
    private $question;

    public function __construct($question, Author $author) {
        $this->author = $author;
        $this->question = $question;
    }

    public function getAuthor() {
        return $this->author;
    }

    public function getQuestion() {
        return $this->question;
    }
}

Class author is injected into the constructor of Question class am I correct? but how to call the Question class to get the author's name?

$question = new Question('What is PHP', 'Adam');
$question->getFirstname;

like this? I assume Question class inherited Author class so Question's instance can use the function of Author Class?

Phil · Accepted Answer · 2014-12-11 04:15:59Z

2

Simple

echo $question->getAuthor()->getFirstName();

Think of it this way if it helps

$author = $question->getAuthor();
echo $author->getFirstName();

Also note that you can't construct a Question with the string "Adam", you need to pass an instance of Author

$question = new Question('What is PHP', new Author('Adam', 'Lastname'));

answered Dec 11, 2014 at 4:15

Phil

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2 Comments

james lebron Over a year ago

so the final would be like this? $question = new Question('What is PHP', new Author('Adam', 'Lastname')); echo $question->getAuthor()->getFirstName();

Phil Over a year ago

@jameslebron yes. You could also construct author beforehand, eg $author = new Author('Foo', 'Bar'); $q = new Question('Eh?', $author);

Ja͢ck · Accepted Answer · 2014-12-11 04:27:58Z

2

You could create a new method:

class Question 
{
    // ...
    function getAuthorFirstname()
    {
        return $this->author->getFirstname();
    }
}

$question = new Question(.., new Author(..., ...));
echo $question->getAuthorFirstname();

Or, if you don't really care about Law of Demeter or feel that it doesn't apply:

$question = new Question(.., new Author(..., ...));
echo $question->getAuthor()->getFirstname();

In the end it all comes down to striking a balance between information hiding and pragmatism.

edited Dec 11, 2014 at 4:27

answered Dec 11, 2014 at 4:12

Ja͢ck

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13 Comments

james lebron Over a year ago

ah this is method chaining right? so my code of calling the getFirstname is wrong? what if I wish not to to change anything in my Question class?

Phil Over a year ago

Is that really violating LoD? getAuthor() returns the Author instance so at that point, we're out of Question

Phil Over a year ago

In fact, using your getAuthorFirstName example implies that Question has knowledge of Author

deceze Over a year ago

getAuthor() simply returns a value of type Author. I don't see the LoD specifying that only primitive types may be returned. It doesn't make a lot of sense to "unwrap" all complex types and provide direct access methods to their primitive values, that just increases complexity and dependencies.

deceze Over a year ago

Qualifier: it does make sense to implement something like getAuthorFirstname() if you want to hide the internal implementation completely and not propagate knowledge of an Author object beyond this class. However, if the consumer of a Question object is expected to have knowledge of the Author type, it doesn't make sense to hide it at the cost of increased complexity.

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