Dependency Injection and Service Locator

Dependency Injection as known as DI, is an implementation for the Inversion of Control (aka. IoC), a design principle for object-oriented programming. By using DI, classes can be highly decoupled, making the debug and testing life easier.

Dependency Injection

Blink provides DI container feature through the the class blink\di\Container, it supports the following kinds of dependency injection:

  • Constructor injection
  • Setter and property injection
  • PHP callable injection
  • Class method injection

Constructor Injection

The DI container supports constructor injection with the help of type hits for constructor parameters, the type hits tell the container which classes or interfaces are dependent when it is used to created a new object. The container will try to get the instance of the dependent classes or interfaces and then inject them into the new object through constructor. For example:

class Foo
    public function __construct(Bar $bar)

$foo = $container->get('Foo');
// which is equivalent to the following:
$bar = new Bar;
$foo = new Foo($bar);

Setter and Property injection

Setter and property injection is supported through object configurations. When registering a dependency or when creating a new object, you can provide a configuration which will be used by the container to inject the dependencies through the corresponding setters or properties. For example:

use blink\core\Object;

class Foo extends Object
    public $bar;

    private $_qux;

    public function getQux()
        return $this->_qux;

    public function setQux(Qux $qux)
        $this->_qux = $qux;

$container->get('Foo', [], [
    'bar' => $container->get('Bar'),
    'qux' => $container->get('Qux'),

PHP Callable Injection

In this case, the container will use a registered PHP callable to build new instances of a class. Each time when blink\di\Container::get() is called, the corresponding callable will be invoked. The callable is responsible to resolve the dependencies and inject them appropriately to the newly created objects. For example:

$container->set('Foo', function () {
    return new Foo(new Bar);

$foo = $container->get('Foo');

Class method injection

Class method injection is a special type of DI where dependencies are declared using the type hits of method signature and resolved in the runtime when the method is actually called. The main use case in Blink is in controller actions. In this case, it is useful to keeping the MVC controller slim and light-weighted since it doesn't requires you to configure all the possible dependencies of the controller beforehand.

use blink\http\Request;

public function actionLogin($email, Request $request)

Service Locator

Service locator is an object that knows how to provide all sorts of services that an application might need. Within the service locator, each service exists as only a single instance and uniquely identified by an ID. You can use the ID to retrieve a service instance from the service locator.

In Blink, a service locator is simply an instance of blink\core\ServiceLocator or a child class.

The most commonly used service locator is the Blink application object, which can be accessed through app() helper function. The services it provides are called application services, such as the errorHandler, logging, and auth.

To use a service locator, the first step is to register services with it, a service can be registered via blink\core\ServiceLocator::bind(), the following code shows different ways of registering services:

use blink\core\ServiceLocator;

$locator = new ServiceLocator;

// 1. register "log" service using a class name
$locator->bind('log', 'blink\log\Logger');

// 2. register "log" service using a configuration array
$locator->bind('log', [
    'class' => 'blink\log\Logger',
    'targets' => [],

// 3. register "log" using an anonymous function
$locator->bind('log', function () {
    return new blink\log\Logger([]);

// 4. resgister "log" using an object directly
$locator->bind('log', new blink\log\Logger());

Once a service has been registered, you can access ti using its ID, in one of the two following ways:

$log = $locator->get('log');

// or

$log = $locator->log;

Helper Functions

Creating Instances

Blink provides a helper function make($type, $params = []) which build upon DI to create class instances and implement dependency injection very conveniently. Here is the example:

$object = make([
    'class' => 'blink\log\Logger',
    'prop1' => 'val1',

// and

$object = make('blink\log\Logger');

Retrieving Service Instances

Besides accessing Blink application object, we can also using app() function to access service instances by ID, for example:

$log = app('log');

// which is enquivalent to

$log = app()->get('log');

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