I'm building a reusable library in PHP which will be used by multiple in-house applications (some web some not). I use constructor dependency injection mostly for my classes, for example injecting my Repository classes into business logic classes:
namespace \Company\Project\BusinessLogic;
class MyComplexBusinessWorkflow
{
private $_repository;
public function __construct(IMyEntityRepository $repository)
{
$this->_repository = $repository
}
...
}
So with Dice I'd like to have something like this which inject default for the library IMyEntityRepository implementation:
$my_complex_workflow_instance = $dice->create('MyComplexBusinessWorkflow');
instead of:
$my_complex_workflow_instance = new MyComplexBusinessWorkflow(new MyEntityMySQLRepository(new MysqlConnectionWrapper()));
I'm ok with embedding particular DI container library into my library code, but not sure how to manage container instance ($dice in the code above) itself.
Didn't get an answer, so provide my "solution". I've implemented a separate Builder class for instantiating DI container.
Code that will use a library will have a dependency on the particular container, this can be absracted by an interface. Unfortunatelly different DI containers work in different way, so that's not a complete abstraction.
In comments I was pointed to this interesting initiative to unify DI containers in PHP: https://github.com/container-interop/container-interop
Related
I'm building a MVC PHP framework and I wonder which are the best practices to load what I need in my classes, be it other classes or plain configuration.
Till today I've used singletons, registry and lately a dependency injection container.
While many people claim that DI is the way to go, it seems to me like it justs moves the problem of coupling between components into another place.
Singletons introduce global state, registry introduces tight coupling and DI introduces... well, lots of complexity. I am still confused and can't find a proper way to wire my classes each other.
In the meanwhile, I came up with a custom solution. Actually it's not a solution, it just abstracts the implementation of service loading from my code.
I built an abstract class with _load_service and _load_config methods which all the components of my framework extend in order to load other services or configuration.
abstract class Base_Component {
function _load_service($service) {
// could be either
return DI_container::getInstance()->$service;
// or
$class = '\services\\'.$service;
return new $class;
// or other implementation
}
}
The implementation of loading them is now implemented in only one place, the base class, so at least I got rid of code lines like the following into my components:
$database = new Database(Registry::getInstance()->load('db_config'));
or
$database = DI_container::getInstance()->database;
Now if want a database instance I do this
$database = $this->_load_service('database');
and the implementation of service loader, container, registry or whatever can be easily changed in a single class method without having to search through all my code to change calls to whatever container implementation I was using before.
But as I said I'm not even close to sure about what method I will use for loading classes and configuration.
What are your opinions?
Why reinvent the wheel? Use Pimple as your DI container, and learn how to use it from its documentation.
Or, use Silex microframework as a base to create your own framework. It extends Pimple functionality, so you can use dependency injection.
To answer your question, this is how you use a DI without coupling your classes to it:
interface ContainerInterface {
public function getService($service_name);
public function registerService($service_name,Closure $service_definition);
}
class Application {
public function __construct(ContainerInterface $container) {
$this->container= $container;
}
public function run() {
// very simple to use!
$this->container->getService('db')->someDatabaseQuery();
}
}
$c = new My_DI_Container;
// Service definitions could be in a separate file
$c->registerService('db',function() { return new Database('some config'); });
// Then you inject your DI container into the objects that need it
$app = new Application($c);
$app->run(); // or whatever
This way, the DI container is decoupled and in the future you could use a different implementation. The only requirement is that it implements the ContainerInterface.
Note that the container object is being pushed, and not pulled. Avoid using singleton. To get/set single-instance objects, use the container (that's its responsibility). And to get the container instance, just push it through constructors.
Answer to your question; Look at PHP autoloading. Registering classes via autoloading makes it so you don't have to put require/includes everywhere, which really has a positive impact on RAD (rapid application development).
My thoughts:
Kudos for attempting such a daunting task, your approach appears to based on good practices such as singletons and factories.
I don't care for dependency injection. OOP is based on encapsulation, injecting one object into another, imo, breaks that encapsulation. When you inject an object into another object, the target object has to 'trust' that nothing regarding the injected object has changed, otherwise you may get unusual behavior.
Consider name-spacing your classes (not PHP name-spacing, but prefix your framework like Zend does, Zend_), this will help so you can register a namespace, then when a class is called the autoloader will ensure that the proper class is loaded. This is how Zend_Framework works. For specifics check out Zend_Loader_Autoloader. The Symfony framework actually takes this one step further; during the first request it will go through all known locations looking for class files, it will then build out an array of the classes and paths to the files then save the array to a file (file caching), so subsequent requests won't have the same overhead. Something to consider for your framework.
As far as config files go, Symfony uses YAML files, which I have found to be extremely flexible. You can even include PHP code for increased flexibility. Symfony has provided a stand-alone YAML parser that is easy to use. You can increase performance by adding a caching layer and caching parsed YAML files so you don't have to parse the files for every request.
I am assuming you are building your framework on top of an ORM. My recommendation would be not to any functionality specific to a version of the ORM, otherwise your framework becomes coupled with that version and you will have to upgrade both the ORM and the framework at the same time.
I would suggest looking under the hood at other frameworks and see if you can pick the best of each; resulting in a solid, easy to use framework.
I'm making my own primitive MVC framework with PHP, and I'm wondering where I should load/instantiate corresponding controller dependencies?
In the constructor of each controller (tightly coupled) or inject them (loosely coupled)?
The only part of the latter that I'm not too sure of is for the dependencies to be instantiated on bootstrap level, outside of the MVC paradigm, before being injected. Not every controller uses the exact same dependencies besides the default parent ones. I would have to instantiate them all, which would also create a lot of overhead.
I've seen some existing frameworks do it like $this->load->model('model'); // CodeIgniter in the constructor, but I have no clue on why they're doing it like that.
I would suggest you inject the dependencies, so your controllers are less coupled to your framework. This will make a switch to another framework easier.
About instantiating dependencies: I suggest you use (or implement) a dependency injection container. This container should contain factories that can instantiate services.
In an ideal situation your controllers are services too (meaning they too have factories in the dependency injection container).
This way only the controller you need for a particular request will be instantiated, and therefor only its dependencies are instantiated.
When building you own framework, this means that after the routing phase (when the correct controller is known), the framework should grab that controller from the container. The container itself will make sure all dependencies that are needed will be provided.
Have a look at Pimple for an example of a simple dependency injection container.
PS: That line from CodeIgniter looks a lot like the service locator pattern. This pattern is similar to dependency injection, but does not provide full inversion of control.
Q: Where should i load/instantiate corresponding controller dependencies?
There are multiple ways.
The load and instantiation concepts are basically "before/outside" and "after/inside".
Before and outside means, that you load the file containing a class (which you want to instantiate and pass to the controller), before you load the controller.
But how do you know, what the controller needs, before loading the controller? Uh..
Dependency Description Files
A description file comes into play, describing the wiring between your controller and it's dependencies. In other words, you can see the dependencies of your controller by looking at it's dependency description file. This concept is often used by Dependency Injection tools, which analyze the object and pull the dependencies names out automatically. It's also possible to maintain such a wiring configuration file manually. But it's tedious.
Service Locator
A Service Locator is a instantiation helper for dependencies.
Basically, it contains the same information like a dependency description file, but this time in form of a registry. The link between parts of your application becomes this registry.
Both strategies introduce overhead. It's a trade-off. When you change the perspective and look at things from an application with maybe 500+ classes, then you realize that a dependency injection tool is sometimes worth it.
Manual Injection
via Constructor Injection.
After and inside means, that you load the file containing your controller and then start to care about the dependencies.
At this point the class is not instantiated, yet, but the autoloader might do it's dirty deeds behind the scene. He evaluates the use statements at the top of your controller file. The use statements declare namespaced classes, which the autoloader resolves to actuall files and loads them. You might then start to use these classes as dependencies in your controller. This is probably the easiest way to solve your problem and i strongly suggest looking into the topics autoloading with namespaces and use-statements.
When the class is instantiated, you have the following possiblities:
use might use Setter Injection or Reference Injection to set the dependencies to the object. This requires that your Constructor Dependencies are already solved or your constructor is empty.
It's possible to combine these strategies.
Q: What does this do $this->load->model('model'); // CodeIgniter?
CodeIgniter is a legacy application framework. It was created in times, when namespaced autoloading wasn't available. $this->load is a basic class loading helper. This is the opposite of an "auto"loader, (which surprise, surprise) loads things automatically.
CodeIgniters loader class is used to load various other classes, like libraries or files from the view, helpers, models or user defined stuff. This is again the concept of a registry. Here the registry just knowns where things are in your application layout and resolves them. So $this->load->model('model'); means that the modelfunction must have some piecies of information, about the position of model files in your application.
You provide a model name and the path for the file is constructed by model.
And this is exaclty what it does (except a bit of overhead): https://github.com/EllisLab/CodeIgniter/blob/develop/system/core/Loader.php#L223.
Since I'm a Symfony developer, I can only give you a reference to Symfony.
I think you should do like they are doing in Symfony by thinking about what you need in each
Controller object.
At least, you need :
a Request object
and a Model loader object that gives you every Model you need.
Create a BaseController that implements these few functions and then extend it with custom Controllers.
You can also take a look on Silex : http://silex.sensiolabs.org/ a Micro Framework
Hope it helps.
When do you say "In the constructor" you mean to pass in the conatiner and pull the dependencies from them (in the constructor)?
<?php
class SomeController
{
public function __construct($container)
{
$this->service1 = $contanier->get('service1);
}
//...
}
I advice against that, though simpler and easier you will be coupling your controllers to the container thus using a ServiceLocator instead of truly inversion of control.
If you want your controllers to be easy unit-testable you should use inversion of control:
class SomeController
{
public function __construct($service1)
{
$this->service1 = $service1;
}
//...
}
And you can even create your controller as a service inside the container:
// this uses Pimple notation, I hope you get the point
$container['controller'] = function($c) {
return SomeController($c['service1']);
}
Use proxy services to lazy load them
Also if your controllers needs more than some services and you won't be using all of them you can:
1) Use proxy services in order to lazy load the service only when they are really needed
<?php
class ProxyService
{
/**
* #var Service1Type
*/
private $actualService;
public function __construct()
{
$this->actualService = null;
}
private function initialize()
{
$this->actualService = new Service1(); // This operation may take some time thus we deferred as long as possible
}
private function isInitialized()
{
return $this->actualService === null;
}
public function someActionOnThisService()
{
if (!$this->isInitalized()) {
$this->initalize();
}
$this->actualService->someActionOnThisService();
}
There you have a simple proxy object with lazy loading. You may want to check the fantastic Proxy Manager Library if you want to go that route
2) Split your controller
If your contoller has too many dependencies, you may want to split it.
In fact you may want to read the proposal by Paul M. Jones (lead developer of Aura Framework) about MVC-Refinement, IMHO is a good read even though you may not fully agree with it.
Even if you split your controller in order to reduce the dependencies, lazy loading your dependencies is a good idea (obviously you'll have to check weather if its doable in your context: more work in order to gain more speed).
Maybe you need to define __autoload() function before you try to load the Classes which is not loaded yet. Like:
function __autoload($className) {
require "/path/to/the/class/file/$className.php";
}
My example is very very simple to auto require the file which the class definition is in.
You can also use if-else statement or switch statement in that function to fit your own situations smartly.
The __autoload() function is called when PHP doesn't find the class definition, works for new, class_exists(), call_user_method(), etc, and absolutely for your dependences/parents classes. If there is still no class definition after __autoload() is called, PHP will generate an error.
Or you can use spl_autoload_register() function instead of __autoload() more gracefully.
For more information, you might want to see:
http://php.net/manual/en/function.autoload.php
http://php.net/manual/en/function.spl-autoload-register.php
I am using a modified copy of yadif as my dependency injection framework.
Currently, I have created an abstract class called AContainerAware which is similiar to symfony2's ContainerAware:
abstract class AContainerAware{
protected $_container;
public function setContainer(Container $container){
$this->_container = $container;
}
protected function get($component){
//return a component from $this->_container;
}
}
This works well in most cases. I just have class who require the container to extend AContainerAware, and the container is automatically injected by the DI framework during object creation using setContainer(). I can then easily get components from the container.
The issue is when working with third party vendor packages. In those cases, for example writing an extension for twig requires me to extend the Twig_Extension class, which means I can't extend AContainerAware to get access to the container.
I am considering whether to turn AContainerAware into an interface IContainerAware. Since classes can implement multiple interfaces, IContainerAware should be implementable in most cases. The only issue with this approach is that one cannot write any code for setContainer() and get() in an interface, so it is repetitive to have to implement the exact code for those functions in each class that requires the container. It also poses maintainance issues if setContainer() and get() were to change in the future.
Is there a better way to mark a class for container injection?
You can use PHP traits which were introduced in PHP 5.4.
Traits enable horizontal reuse of code, whereas inheritance is vertical reuse. A class can use multiple traits, whereas you can only inherit one parent class.
I am currently building an MVC application in PHP (not using any frameworks). I am using yadif (https://github.com/beberlei/yadif) for dependency injection.
I would like to build a login module. It should be able to use adapters, for example one might be able to set that logins are authenticated using the MySql database or some LDAP directory. This setting would be done in the admin area and is stored in a database.
I imagine that I would have an abstract adapter:
<?php
abstract AbstractLoginAdapter{
abstract function login($username, $pass){}
}
I would then just implement adapters like so:
<?php
MySQLLoginAdapter extends AbstractLoginAdapter{
public function login($username, $pass){
//do stuff
}
}
That's all nice and well, but how do I create an instance of the adapter? Usually, dependencies would be injected using yadif via the constructor:
<?php
class loginController{
private $_adapter;
public function __construct(AbstractLoginAdapter $adapter){
$this->_adapter = $adapter;
}
}
However, since I don't know which concrete adapter will be injected, I can't set that in a configuration before hand. Yadif allows me to create a configuration which I then need to pass to the container:
$builder = new Yadif_Builder();
$builder->bind("loginController")
->to("loginController")
->args($SelectedLoginAdapter);
Since the application uses a front controller, a DI container is created there. It then creates a routing object etc.
In light of this, should I pass a reference of that container down to the loginController object, and then use that container to instantiate my adapter?
Or should I instantiate a new container within my loginController object and then just load in an instance of the adapter?
I would do the first: pass a reference down to your controller. You'll want to use a single Dependency Injector Container (DIC) in your application. You don't want to create a new DIC whenever you need access to it. That would lead to duplication of objects stored in the DIC.
I know this is how Symfony 2 does it. All controllers (and many other classes) implement the ContainerAware interface. That interface has a single method setContainer() that is used to pass down a reference to the DIC.
I don't know about your specific DI tool but from a DI point of view you would be specifying which type to use. The container itself is responsible for instantiating a new instance (and possibly of all the dependencies of that type as well) of the configured type.
The benefit of DI in your example would be that you could deploy exactly the same code with a different configuration with 1 installation using LDAP and the other using MySQL authentication.
Refactor type hinting ("AbstractLoginAdapter") to ("MySQLLoginAdapter").
If you call abstract class method in the new __CLASS__ // Fatal Error.
I have a class which depends on 3 classes, all 3 of which have other classes they rely on. Currently, I'm using a container class to build up all the required classes, inject them into one another and return the application.
The simplified version of the container looks something like this:
class Builder
{
private $_options;
public function __construct($options)
{
$this->_options = $options;
}
public function build()
{
$cache = $this->getCache();
$response = $this->getResponse();
$engine = $this->getEngine();
return new Application($cache,$response,$engine);
}
public function getResponse()
{
$encoder = $this->getResponseEncoder();
$cache = $this->getResponseCache();
return new Response($encoder,$cache);
}
// Methods for building each object
}
I'm not sure if this would be classified as FactoryMethod or a DI Container. They both seem to solve the same problem in the same way - They build objects and inject dependencies. This container has some more complicated building methods, like loading observers and attaching them to observable objects.
Should factories be doing all the building (loading extensions etc) and the DI container should use these factories to inject dependencies? That way the sub-packages, like Cache, Response etc, can each have their own specialised factories.
A DI Container is definitely a Factory, but it's a general-purpose factory.
However, if you use it in a pull-based way by asking it to create dependencies for you every time you need them, you would be employing the Service Locator anti-pattern. That's just a general-purpose factory and actually has little to do with DI.
True Dependency Injection is, as the name implies, push based. You write all your code using simple patterns like Constructor Injection, and use the DI Container to resolve your entire dependency graph in one go in the application's Composition Root, injecting all dependencies into their respective consumers.