3.1.7. Instance Factories

Occasionally, a class will need to receive not just an instance, but a factory that is capable of creating a new instance over and over. For example, say we have a class like the following:

class ExampleNeedsFactory
{
    protected $struct_factory;

    public function __construct($struct_factory)
    {
        $this->struct_factory = $struct_factory;
    }

    public function getStruct(array $data)
    {
        $struct = $this->struct_factory->__invoke($data);
        return $struct;
    }
}

class ExampleStruct
{
    public function __construct(array $data)
    {
        foreach ($data as $key => $val) {
            $this->$key = $val;
        }
    }
}

We can inject a Factory that creates only ExampleStruct objects using $di->newFactory().

$di->params['ExampleNeedsFactory']['struct_factory'] = $di->newFactory('ExampleStruct');

Note that the arguments passed to the factory __invoke() method will be passed to the underlying instance constructor sequentially, not by name. This means the __invoke() method works more like the native new keyword, and not like $di->lazyNew(). These arguments override any $di->params values that have been set for the class being factoried; without the overrides, all existing $di->params values for that class will be honored. (Values from $di->setter for the class will also be honored, but cannot be overridden.)

Do not feel limited by the Factory implementation. We can create and inject factory objects of our own if we like. The Factory returned by the $di->newFactory() method is an occasional convenience, nothing more.

3.1.7.1. Broader Instantiation

Sometimes you may want to more broadly leverage a configured containers ability to instantiate objects in your classes, for example in some kind of service locator. The ResolutionHelper provides a callable object which takes a "specification" as a parameter and will retrieve a service if the container has one appropriate to the specification or instantiate a new instance of the class if there is no matching service name. If an array is passed as the specification, the array is returned with the first value "resolved". This array functionality allows for a callable to be returned referencing an method on an instance.

$helper = $di->newResolutionHelper();

$foo = $helper(\My\Foo::class);
// will return service named "My\Foo" if on is named as such
// or else a new instance of "My\Foo"

$fooBar = $helper([\My\Foo::class, 'bar']);
// will return the equivalent of [$foo, 'bar']

Using the ResolutionHelper, you can write consuming code that type hints a callable, rather than the container itself.