Mapping dependencies at construction time

This registers collaborators (dependencies) in the constructors in order to decouple the object creation in methods.

Keywords

inversion control, dependency injection, builder

Goal

Testable, reusable and maintainable code by avoiding hard-coded references in methods.

Benefits

• Creating collaborators (a.k.a. dependencies) only in constructors.
• Avoiding a global dependency injection container.
• Switching default implementations with alternatives and test's objects.

Advantages of the language

• Referencing of classes and interfaces with its namespace.

Alternatives

• Usage of mapping.
• Anonymous classes.
• Passing empty objects that create those which are fully identifiable.
• Defining and implementing an interface for each collaboration.

Here is an example that reconstitutes an entity from a data store.

You just need to pass the interface and its implementation.

    // It reconstitutes a person from a data store: PersonID is a kind 
    // of Identity as much as an AboutMe is a kind of About and both
    // of them are collaborators. Take note that the creation is
    // delayed until need it, but it is not a Singleton.

    // Testing purposes.
    $person = new PersonFromStore(
        new FetchedPerson($id), TestPersonID::class, AboutMe::class
    );   

    // Application.
    $person = new PersonFromStore(
        new FetchedPerson($id), PersonID::class, AboutMe::class
    );  

    // Description
    final class PersonFromStore implements Party
    {
        private $record;
        private $identity;
        private $about;

        public function __construct(IdentityRecord $record, string $identity, string $about)
        {
            $this->record = $record;        

            $this->identity = new Assignment(Identity::class, $identity);

            $this->about = new Assignment(About::class, $about);
        }

        // Please, your ID?
        public function identity() : Identity
        {
            // It calls the main constructor or the operator "new" in the ID class.
            // Same as: new PersonID(...)
            return $this->identity->new($this->record->key());
        }

        // Tell me something about you.
        public function about() : About
        {
            // It calls a convenience constructor in the AboutMe class.
            // Same as: AboutMe::withID(...)
            return $this->about->withID($this->identity());
        }
    }  

A proposal for the php community.

    // Definition
    final class PersonFromStore implements Party 
    {
        // It can use another word, such as join, to avoid conflicts with traits.
        use (Identity, About);

        public function identity() : Identity
        {
            return new Identity($record->key());
        }

        ...               
    }

    // Client
    $person = new PersonFromStore($fetchedPerson) use (PersonID, AboutMe);
    // Test
    $person = new PersonFromStore($fetchedPerson) use (TestPersonID, AboutMe);

Now, we can create objects from their interfaces and thus, we have no more hard-coded dependencies in the methods.