README

A purely functional Dependency Injection Container for PHP

Install

Add fundic as a dependency to your project using Composer running

composer require marcosh/fundic

Tests

Run the tests using

php vendor\bin\phpunit

Theory

In its essence a dependency injection container is just a component which is able, from a key, to retrieve a corresponding working object.

A common approach to do this is to configure how the corresponding object should be contructed or even relay on autowiring based on class naming.

Another approach si to see a dependency injection container as a map that associates to a key a factory which builds the object identified by the key, possibly using recursively the container itself.

fundic takes this idea to its core and, in fact, if you look at the essence, it is just a map that associates keys to factories of the form

interface ValueFactory
{
    /**
     * @return mixed
     */
    public function __invoke(ContainerInterface $container, string $name);
}

Basic usage

Instantiate

Actually fundic gives you two containers:

Psr11Container which implements the specifications of PSR-11;
TypedContainer, a more type safe version

You can create a new instance of them just by calling their static method create:

$container = TypedContainer::create();

This will create an empty instance of the container that you can fill as you like.

Configure

An empty container is not really useful. You could add new entries to the container using

$container = $container->add($key, $factory);

where $key is a string and $factory is an instance of ValueFactory.

Be aware that $container is immutable and that add returns a new instance. Therefore it is really important that you remember to assign its result to a variable.

Retrieve

Both Psr11Container and TypedContainer implement Psr\Container\ContainerInterface (even if TypedContainer is just respecting the signature of the methods and not conforming to the annotations), therefore you can query them using the set and has methods as follows

// create a new empty container
$container = Psr11Container::create();

// instructs the container on how to build the object
// associated with the provided key
$container = $container->add('foo', $factory);

$container->has('foo'); // returns true

$object = $container->get('foo'); // retrieves the object associated to the key

Container return values and exceptions

Psr11Container works as a standard container and conforms completely to the specifications of PSR-11. Hence, the result of any call to the get method is the expected object.

Psr11Container::get also throws exceptions if a key is not found or if there is an error while building the return value.

On the other hand, TypedContainer alone does not totally conform to the specifications of PSR-11, specifically in the return values of get and the handling of the exceptions.

In order to make the code purely functional and to avoid unwanted side effects, the result of get is not directly the desired object, but a Result data structure which could have the following values:

Just, which is a wrapper around the desired value that can be retrieved using Just::get;
NotFound, which represents the fact that such an entry is not present in the container;
Exception, which represents the fact that something wrong happened while invoking the factory;

These above are just values and you could do whatever you want with them (immediately react to them, pass them around, etc ...)

Factories

Some factories are provided to ease the creation of Fundic\Factory\ValueFactory instances.

It goes without saying that you could provide your own ad-hoc implementations of Fundic\Factory\ValueFactory.

ConstantFactory

If you need to store in the container a constant value, may it be a native data type, an array or an object, you could use the Fundic\Factory\ConstantFactory as follows:

$value = // your constant that needs to be stored in the container

$container->add('foo', new ConstantFactory($value));

$container->get('foo');

The ConstanctFactory class wraps the value in a Fundic\Factory\ValueFactory which always returns the provided value.

ClassNameFactory

If you need to retrieve from the container an object with no (or only optional) dependencies, you could use a Fundic\Factory\ClassNameFactory, passing to it just the class name, as follows:

class Foo { ... } // no non optional dependencies in the constructor

$container->add(Foo::class, new ClassNameFactory(Foo::class));

$container->get(Foo::class);

The ClassNameFactory just calls new on the provided class name and returns a new instance of the class.

Callable factory

The most generic Fundic\Factory\ValueFactory implementation that we provide is Fundic\Factory\CallableFactory, which just wraps any callable with the same signature of ValueFactory (i.e. it needs to have as input parameters a Psr\Container\ContainerInterface and a string which is the class name). For example:

$callable = function (ContainerInterface $container, string $name) { ... };

$container->add(Foo::class, new CallableFactory($callable));

$container->get(Foo::class);

Factory decorators

Sometimes you want to modify how a specific key is built and retrieved from the container without touching the provided factory.

An easy mechanism to allow this possibility is to use the decorator pattern. This means that we wrap our factory with another factory which receives the first factory as a constructor argument. In functional terms, suppose we have a factory f for a specific foo key (i.e. f : (ContainerInterface, string) -> foo); what we do is passing the whole f to g where g(f) : (ContainerInterface, string) -> foo.

This allows us to modify the result of the inner factory before returning it, or even avoiding to call the inner factory and return a newly built value.

You could provide your own factory decorators to create complex workflows for object creations. Decorators are highly composable, so you could use several of them to build a single object.

Some decorators of common use are provided by the library.

Memoize

If you want to retrieve the same instance of an object every time you ask a particular key to the container, you need to store the result obtained the first time somewhere and the return that instead of creating a new instance every time.

This is exactly what the Memoize decorator does. The first time it calls the inner factory to build the object, and then always returns that particular instance.

class Foo { ... }

$container->add(Foo::class, new Memoize(new ClassNameFactory(Foo::class)));

$container->get(Foo::class); // a new instance of Foo is built and returned
$container->get(Foo::class); // the same instance of Foo is returned

Proxy

If the building process of a object is particularly heavy, you could desire to postpone it until the very last moment when you are sure you need an instance of that particular object.

To do this you could proxy your object and initially return a wrapper that will build the actual object only once a method is called on it.

You could do this using the Proxy decorator, as follows:

class Foo { ... } // class which is heavy to build

$container->add(Foo::class, new Proxy(new ClassNameFactory(Foo::class)));

$foo = $container->get(Foo::class); // returns a proxy

$foo->bar(); // here we really instantiates Foo and call the bar method on it

marcosh / fundic

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