Ecosyste.ms: Awesome

An open API service indexing awesome lists of open source software.

Awesome Lists | Featured Topics | Projects

https://github.com/amphp/injector

A recursive dependency injector used to bootstrap and wire together S.O.L.I.D., object-oriented PHP applications.
https://github.com/amphp/injector

auryn injector

Last synced: about 1 month ago
JSON representation

A recursive dependency injector used to bootstrap and wire together S.O.L.I.D., object-oriented PHP applications.

Awesome Lists containing this project

README 

        
# injector



A recursive dependency injector to bootstrap and wire together S.O.L.I.D., object-oriented PHP applications.



##### How It Works



Among other things, the injector recursively instantiates class dependencies based on the parameter

type-hints specified in class constructor signatures. This requires the use of Reflection. You may

have heard that "reflection is slow". Let's clear something up: *anything* can be "slow" if you're

doing it wrong. Reflection is an order of magnitude faster than disk access and several orders of

magnitude faster than retrieving information (for example) from a remote database. Additionally,

each reflection offers the opportunity to cache the results if you're worried about speed. The injector

caches any reflections it generates to minimize the potential performance impact.



> The injector **is NOT** a Service Locator. DO NOT turn it into one by passing the injector into your

> application classes. Service Locator is an anti-pattern; it hides class dependencies, makes code

> more difficult to maintain and makes a liar of your API! You should *only* use an injector for

> wiring together the disparate parts of your application during your bootstrap phase.



## The Guide



**Basic Usage**



* [Basic Instantiation](#basic-instantiation)

* [Injection Definitions](#injection-definitions)

* [Type-Hint Aliasing](#type-hint-aliasing)

* [Non-Class Parameters](#non-class-parameters)

* [Global Parameter Definitions](#global-parameter-definitions)



**Advanced Usage**



* [Instance Sharing](#instance-sharing)

* [Instantiation Delegates](#instantiation-delegates)

* [Prepares and Setter Injection](#prepares-and-setter-injection)

* [Injecting for Execution](#injecting-for-execution)

* [Dependency Resolution](#dependency-resolution)



**Example Use Cases**



* [Avoiding Evil Singletons](#avoiding-evil-singletons)

* [Application Bootstrapping](#app-bootstrapping)



## Requirements and Installation



- Requires PHP 8.0 or higher.



#### Installation



###### Composer



You may also use composer to include the project as a dependency in your projects `composer.json`. The relevant package is `amphp/injector`.



Alternatively require the package using composer cli:



```bash

composer require amphp/injector

```



## Basic Usage



To start using the injector, simply create a new instance of the `Amp\Injector\Injector` ("the Injector") class:



```php

make(SomeNamespace\MyClass::class);



var_dump($obj2 instanceof SomeNamespace\MyClass); // true

```



###### Concrete Type-hinted Dependencies



If a class only asks for concrete dependencies you can use the Injector to inject them without

specifying any injection definitions. For example, in the following scenario you can use the

Injector to automatically provision `MyClass` with the required `SomeDependency` and `AnotherDependency`

class instances:



```php

dep1 = $dep1;

        $this->dep2 = $dep2;

    }

}



$injector = new Amp\Injector\Injector;

$myObj = $injector->make(MyClass::class);



var_dump($myObj->dep1 instanceof SomeDependency); // true

var_dump($myObj->dep2 instanceof AnotherDependency); // true

```



###### Recursive Dependency Instantiation



One of the Injector's key attributes is that it recursively traverses class dependency trees to

instantiate objects. This is just a fancy way of saying, "if you instantiate object A which asks for

object B, the Injector will instantiate any of object B's dependencies so that B can be instantiated

and provided to A". This is perhaps best understood with a simple example. Consider the following

classes in which a `Car` asks for `Engine` and the `Engine` class has concrete dependencies of its

own:



```php

engine = $engine;

    }

}



class Engine {

    private $sparkPlug;

    private $piston;

    public function __construct(SparkPlug $sparkPlug, Piston $piston) {

        $this->sparkPlug = $sparkPlug;

        $this->piston = $piston;

    }

}



$injector = new Amp\Injector\Injector;

$car = $injector->make(Car::class);

var_dump($car instanceof Car); // true

```



### Injection Definitions



You may have noticed that the previous examples all demonstrated instantiation of classes with

explicit, type-hinted, concrete constructor parameters. Obviously, many of your classes won't fit

this mold. Some classes will type-hint interfaces and abstract classes. Some will specify scalar

parameters which offer no possibility of type-hinting in PHP. Still other parameters will be arrays,

etc. In such cases we need to assist the Injector by telling it exactly what we want to inject.



###### Defining Class Names for Constructor Parameters



Let's look at how to provision a class with non-concrete type-hints in its constructor signature.

Consider the following code in which a `Car` needs an `Engine` and `Engine` is an interface:



```php

engine = $engine;

    }

}

```



To instantiate a `Car` in this case, we simply need to define an injection definition for the class

ahead of time:



```php

define(Car::class, ['engine' => 'V8']);

$car = $injector->make(Car::class);



var_dump($car instanceof Car); // true

```



The most important points to notice here are:



1. A custom definition is an `array` whose keys match constructor parameter names

2. The values in the definition array represent the class names to inject for the specified

   parameter key



Because the `Car` constructor parameter we needed to define was named `$engine`, our definition

specified an `engine` key whose value was the name of the class (`V8`) that we want to inject.



Custom injection definitions are only necessary on a per-parameter basis. For example, in the

following class we only need to define the injectable class for `$arg2` because `$arg1` specifies a

concrete class type-hint:



```php

arg1 = $arg1;

        $this->arg2 = $arg2;

    }

}



$injector = new Amp\Injector\Injector;

$injector->define(MyClass::class, ['arg2' => 'SomeImplementationClass']);



$myObj = $injector->make(MyClass::class);

```



> **NOTE:** Injecting instances where an abstract class is type-hinted works in exactly the same way

as the above examples for interface type-hints.



###### Using Existing Instances in Injection Definitions



Injection definitions may also specify a pre-existing instance of the requisite class instead of the

string class name:



```php

dependency = $dependency;

    }

}



$injector = new Amp\Injector\Injector;

$dependencyInstance = new SomeImplementation;

$injector->define(MyClass::class, [':dependency' => $dependencyInstance]);



$myObj = $injector->make(MyClass::class);



var_dump($myObj instanceof MyClass); // true

```



> **NOTE:** Since this `define()` call is passing raw values (as evidenced by the colon `:` usage),

you can achieve the same result by omitting the array key(s) and relying on parameter order rather

than name. Like so: `$injector->define('MyClass', [$dependencyInstance]);`



###### Specifying Injection Definitions On the Fly



You may also specify injection definitions at call-time with `Amp\Injector\Injector::make`. Consider:



```php

dependency = $dependency;

    }

}



$injector = new Amp\Injector\Injector;

$myObj = $injector->make('MyClass', ['dependency' => 'SomeImplementationClass']);



var_dump($myObj instanceof MyClass); // true

```



The above code shows how even though we haven't called  the Injector's `define` method, the

call-time specification allows us to instantiate `MyClass`.



> **NOTE:** on-the-fly instantiation definitions will override a pre-defined definition for the

specified class, but only in the context of that particular call to `Amp\Injector\Injector::make`.



### Type-Hint Aliasing



Programming to interfaces is one of the most useful concepts in object-oriented design (OOD), and

well-designed code should type-hint interfaces whenever possible. But does this mean we have to

assign injection definitions for every class in our application to reap the benefits of abstracted

dependencies? Thankfully the answer to this question is, "NO."  The Injector accommodates this goal

by accepting "aliases". Consider:



```php

engine = $engine;

    }

}



$injector = new Amp\Injector\Injector;



// Tell the Injector class to inject an instance of V8 any time

// it encounters an Engine type-hint

$injector->alias('Engine', 'V8');



$car = $injector->make('Car');

var_dump($car instanceof Car); // bool(true)

```



In this example we've demonstrated how to specify an alias class for any occurrence of a particular

interface or abstract class type-hint. Once an implementation is assigned, the Injector will use it

to provision any parameter with a matching type-hint.



> **IMPORTANT:** If an injection definition is defined for a parameter covered by an implementation

assignment, the definition takes precedence over the implementation.



### Non-Class Parameters



All of the previous examples have demonstrated how the Injector class instantiates parameters based

on type-hints, class name definitions and existing instances. But what happens if we want to inject

a scalar or other non-object variable into a class? First, let's establish the following behavioral

rule:



> **IMPORTANT:** The Injector assumes all named-parameter definitions are class names by default.



If you want the Injector to treat a named-parameter definition as a "raw" value and not a class name,

you must prefix the parameter name in your definition with a colon character `:`. For example,

consider the following code in which we tell the Injector to share a `PDO` database connection

instance and define its scalar constructor parameters:



```php

share('PDO');

$injector->define('PDO', [

    ':dsn' => 'mysql:dbname=testdb;host=127.0.0.1',

    ':username' => 'dbuser',

    ':passwd' => 'dbpass'

]);



$db = $injector->make('PDO');

```



The colon character preceding the parameter names tells the Injector that the associated values ARE

NOT class names. If the colons had been omitted above, the injector would attempt to instantiate classes of

the names specified in the string and an exception would result. Also, note that we could just as

easily specified arrays or integers or any other data type in the above definitions. As long as the

parameter name is prefixed with a `:`, the injector will inject the value directly without attempting to

instantiate it.



> **NOTE:** As mentioned previously, since this `define()` call is passing raw values, you may opt to

assign the values by parameter order rather than name. Since PDO's first three parameters are `$dsn`,

`$username`, and `$password`, in that order, you could accomplish the same result by leaving out the

array keys, like so:

`$injector->define('PDO', ['mysql:dbname=testdb;host=127.0.0.1', 'dbuser', 'dbpass']);`



### Global Parameter Definitions



Sometimes applications may reuse the same value everywhere. However, it can be a hassle to manually

specify definitions for this sort of thing everywhere it might be used in the app. The injector mitigates

this problem by exposing the `Injector::defineParam()` method. Consider the following example ...



```php

myValue = $myValue;

    }

}



$injector = new Amp\Injector\Injector;

$injector->defineParam('myValue', $myUniversalValue);

$obj = $injector->make('MyClass');

var_dump($obj->myValue === 42); // bool(true)

```



Because we specified a global definition for `myValue`, all parameters that are not in some other

way defined (as below) that match the specified parameter name are auto-filled with the global value.

If a parameter matches any of the following criteria the global value is not used:



- A typehint

- A predefined injection definition

- A custom call time definition



## Advanced Usage



### Instance Sharing



One of the more ubiquitous plagues in modern OOP is the Singleton anti-pattern. Coders looking to

limit classes to a single instance often fall into the trap of using `static` Singleton

implementations for things like configuration classes and database connections. While it's often

necessary to prevent multiple instances of a class, the Singleton method spells death to testability

and should generally be avoided. `Amp\Injector\Injector` makes sharing class instances across contexts a

triviality while allowing maximum testability and API transparency.



Let's consider how a typical problem facing object-oriented web applications is easily solved by

wiring together your application using the injector. Here, we want to inject a single database connection

instance across multiple layers of an application. We have a controller class that asks for a

DataMapper that requires a `PDO` database connection instance:



```php

pdo = $pdo;

    }

}



class MyController {

    private $mapper;

    public function __construct(DataMapper $mapper) {

        $this->mapper = $mapper;

    }

}



$db = new PDO('mysql:host=localhost;dbname=mydb', 'user', 'pass');



$injector = new Amp\Injector\Injector;

$injector->share($db);



$myController = $injector->make('MyController');

```



In the above code, the `DataMapper` instance will be provisioned with the same `PDO` database

connection instance we originally shared. This example is contrived and overly simple, but the

implication should be clear:



> By sharing an instance of a class, `Amp\Injector\Injector` will always use that instance when

> provisioning classes that type-hint the shared class.



###### A Simpler Example



Let's look at a simple proof of concept:



```php

share('Person');



$person = $injector->make('Person');

var_dump($person->name); // John Snow



$person->name = 'Arya Stark';



$anotherPerson = $injector->make('Person');

var_dump($anotherPerson->name); // Arya Stark

var_dump($person === $anotherPerson); // bool(true) because it's the same instance!

```



Defining an object as shared will store the provisioned instance in the Injector's shared cache and

all future requests to the provider for an injected instance of that class will return the

originally created object. Note that in the above code, we shared the class name (`Person`)

instead of an actual instance. Sharing works with either a class name or an instance of a class.

The difference is that when you specify a class name, the Injector

will cache the shared instance the first time it is asked to create it.



> **NOTE:** Once the Injector caches a shared instance, call-time definitions passed to

`Amp\Injector\Injector::make` will have no effect. Once shared, an instance will always be returned for

instantiations of its type until the object is un-shared or refreshed:



### Instantiation Delegates



Often factory classes/methods are used to prepare an object for use after instantiation. The injector

allows you to integrate factories and builders directly into the injection process by specifying

callable instantiation delegates on a per-class basis. Let's look at a very basic example to

demonstrate the concept of injection delegates:



```php

verification = true;

    }

}



$complexClassFactory = function() {

    $obj = new MyComplexClass;

    $obj->doSomethingAfterInstantiation();



    return $obj;

};



$injector = new Amp\Injector\Injector;

$injector->delegate('MyComplexClass', $complexClassFactory);



$obj = $injector->make('MyComplexClass');

var_dump($obj->verification); // bool(true)

```



In the above code we delegate instantiation of the `MyComplexClass` class to a closure,

`$complexClassFactory`. Once this delegation is made, the Injector will return the results of the

specified closure when asked to instantiate `MyComplexClass`.



###### Available Delegate Types



Any valid PHP callable may be registered as a class instantiation delegate using

`Amp\Injector\Injector::delegate`. Additionally you may specify the name of a delegate class that

specifies an `__invoke` method and it will be automatically provisioned and have its `__invoke`

method called at delegation time. Instance methods from uninstantiated classes may also be specified

using the `['NonStaticClassName', 'factoryMethod']` construction. For example:



```php

dependency = $dep;

    }

    function __invoke() {

        $obj = new SomeClassWithDelegatedInstantiation;

        $obj->value = 1;

        return $obj;

    }

    function factoryMethod() {

        $obj = new SomeClassWithDelegatedInstantiation;

        $obj->value = 2;

        return $obj;

    }

}



// Works because MyFactory specifies a magic __invoke method

$injector->delegate('SomeClassWithDelegatedInstantiation', 'MyFactory');

$obj = $injector->make('SomeClassWithDelegatedInstantiation');

var_dump($obj->value); // int(1)



// This also works

$injector->delegate('SomeClassWithDelegatedInstantiation', 'MyFactory::factoryMethod');

$obj = $injector->make('SomeClassWithDelegatedInstantiation');

$obj = $injector->make('SomeClassWithDelegatedInstantiation');

var_dump($obj->value); // int(2)

```



### Prepares and Setter Injection



Constructor injection is almost always preferable to setter injection. However, some APIs require

additional post-instantiation mutations. The injector accommodates these use cases with its

`Injector::prepare()` method. Users may register any class or interface name for post-instantiation

modification. Consider:



```php

prepare('MyClass', function($myObj, $injector) {

    $myObj->myProperty = 42;

});



$myObj = $injector->make('MyClass');

var_dump($myObj->myProperty); // int(42)

```



While the above example is contrived, the usefulness should be clear.



### Injecting for Execution



In addition to provisioning class instances using constructors, the injector can also recursively instantiate

the parameters of any [valid PHP callable](http://php.net/manual/en/language.types.callable.php).

The following examples all work:



```php

execute(function(){});

$injector->execute([$objectInstance, 'methodName']);

$injector->execute('globalFunctionName');

$injector->execute('MyStaticClass::myStaticMethod');

$injector->execute(['MyStaticClass', 'myStaticMethod']);

$injector->execute(['MyChildStaticClass', 'parent::myStaticMethod']);

$injector->execute('ClassThatHasMagicInvoke');

$injector->execute($instanceOfClassThatHasMagicInvoke);

$injector->execute('MyClass::myInstanceMethod');

```



Additionally, you can pass in the name of a class for a non-static method and the injector will

automatically provision an instance of the class (subject to any definitions or shared instances

already stored by the injector) before provisioning and invoking the specified method:



```php

execute('Example::myMethod', $args = [':arg2' => 42]));

```



### Dependency Resolution



`Amp\Injector\Injector` resolves dependencies in the following order:



1. If a shared instance exists for the class in question, the shared instance will always be returned

2. If a delegate callable is assigned for a class, its return result will always be used

3. If a call-time definition is passed to `Amp\Injector\Injector::make`, that definition will be used

4. If a pre-defined definition exists, it will be used

5. If a dependency is type-hinted, the Injector will recursively instantiate it subject to any implementations or definitions

6. If no type-hint exists and the parameter has a default value, the default value is injected

7. If a global parameter value is defined that value is used

8. Throw an exception because you did something stupid



## Example Use Cases



Dependency Injection Containers (DIC) are generally misunderstood in the PHP community. One of the

primary culprits is the misuse of such containers in the mainstream application frameworks. Often,

these frameworks warp their DICs into Service Locator anti-patterns. This is a shame because a

good DIC should be the exact opposite of a Service Locator.



###### The injector is NOT a service locator!



There's a galaxy of differences between using a DIC to wire together your application versus

passing the DIC as a dependency to your objects (Service Locator). Service Locator (SL) is an

anti-pattern -- it hides class dependencies, makes code difficult to maintain and makes a liar of

your API.



When you pass a SL into your constructors it makes it difficult to determine what the class dependencies

really are. A `House` object depends on `Door` and `Window` objects. A `House` object DOES NOT depend

on an instance of `ServiceLocator` regardless of whether the `ServiceLocator` can provide `Door` and

`Window` objects.



In real life you wouldn't build a house by transporting the entire hardware store (hopefully) to

the construction site so you can access any parts you need. Instead, the foreman (`__construct()`)

asks for the specific parts that will be needed (`Door` and `Window`) and goes about procuring them.

Your objects should function in the same way; they should ask only for the specific dependencies

required to do their jobs. Giving the `House` access to the entire hardware store is at best poor

OOP style and at worst a maintainability nightmare. The takeaway here is this:



> **IMPORTANT:** do not use the injector like a Service Locator!



### Avoiding Evil Singletons



A common difficulty in web applications is limiting the number of database connection instances.

It's wasteful and slow to open up new connections each time we need to talk to a database.

Unfortunately, using singletons to limit these instances makes code brittle and hard to test. Let's

see how we can use the injector to inject the same `PDO` instance across the entire scope of our application.



Say we have a service class that requires two separate data mappers to persist information to a database:



```php

pdo = $pdo;

    }

    public function find($houseId) {

        $query = 'SELECT * FROM houses WHERE houseId = :houseId';



        $stmt = $this->pdo->prepare($query);

        $stmt->bindValue(':houseId', $houseId);



        $stmt->setFetchMode(PDO::FETCH_CLASS, 'Model\\Entities\\House');

        $stmt->execute();

        $house = $stmt->fetch(PDO::FETCH_CLASS);



        if (false === $house) {

            throw new RecordNotFoundException(

                'No houses exist for the specified ID'

            );

        }



        return $house;

    }



    // more data mapper methods here ...

}



class PersonMapper {

    private $pdo;

    public function __construct(PDO $pdo) {

        $this->pdo = $pdo;

    }

    // data mapper methods here

}



class SomeService {

    private $houseMapper;

    private $personMapper;

    public function __construct(HouseMapper $hm, PersonMapper $pm) {

        $this->houseMapper = $hm;

        $this->personMapper = $pm;

    }

    public function doSomething() {

        // do something with the mappers

    }

}

```



In our wiring/bootstrap code, we simply instantiate the `PDO` instance once and share it in the

context of the `Injector`:



```php

setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);



$injector = new Amp\Injector\Injector;



$injector->share($pdo);

$mapper = $injector->make('SomeService');

```



In the above code, the DIC instantiates our service class. More importantly, the data mapper classes

it generates to do so are injected *with the same database connection instance we originally shared*.



Of course, we don't have to manually instantiate our `PDO` instance. We could just as easily seed

the container with a definition for how to create the `PDO` object and let it handle things for us:



```php

define('PDO', [

    ':dsn' => 'sqlite:some_sqlite_file.db'

]);

$injector->share('PDO');

$service = $injector->make('SomeService');

```



In the above code, the injector will pass the string definition as the `$dsn` argument in the

`PDO::__construct` method and generate the shared PDO instance automatically only if one of the

classes it instantiates requires a `PDO` instance!



### App-Bootstrapping



DICs should be used to wire together the disparate objects of your application into a cohesive

functional unit (generally at the bootstrap or front-controller stage of the application). One such

usage provides an elegant solution for one of the thorny problems in object-oriented (OO) web

applications: how to instantiate classes in a routed environment where the dependencies are not

known ahead of time.



Consider the following front controller code whose job is to:



1. Load a list of application routes and pass them to the router

2. Generate a model of the client's HTTP request

3. Route the request instance given the application's route list

4. Instantiate the routed controller and invoke a method appropriate to the HTTP request



```php

loadFromXml(CONTROLLER_ROUTES);

$router = new Router($routes);



$requestDetector = new RequestDetector();

$request = $requestDetector->detectFromSuperglobal($_SERVER);



$requestUri = $request->getUri();

$requestMethod = strtolower($request->getMethod());



$injector = new Amp\Injector\Injector;

$injector->share($request);



try {

    if (!$controllerClass = $router->route($requestUri, $requestMethod)) {

        throw new NoRouteMatchException();

    }



    $controller = $injector->make($controllerClass);

    $callableController = array($controller, $requestMethod);



    if (!is_callable($callableController)) {

        throw new MethodNotAllowedException();

    } else {

        $callableController();

    }



} catch (NoRouteMatchException $e) {

    // send 404 response

} catch (MethodNotAllowedException $e) {

    // send 405 response

} catch (Exception $e) {

    // send 500 response

}

```



And elsewhere we have various controller classes, each of which ask for their own individual

dependencies:



```php

request = $request;

        $this->mapper = $mapper;

    }

    public function get() {

        // do something for HTTP GET requests

    }

    public function post() {

        // do something for HTTP POST requests

    }

}

```



In the above example the injector DIC allows us to write fully testable, fully OO controllers that ask

for their dependencies. Because the DIC recursively instantiates the dependencies of objects it

creates we have no need to pass around a Service Locator. Additionally, this example shows how we can

eliminate evil Singletons using the sharing capabilities of the injector DIC. In the front controller

code, we share the request object so that any classes instantiated by the `Amp\Injector\Injector` that ask

for a `Request` will receive the same instance. This feature not only helps eliminate Singletons,

but also the need for hard-to-test `static` properties.