Zend Framework 2 Cookbook

We can clone the skeleton—and almost everything else on Github for that matter — through use of the following command:

$ git clone git://github.com/zendframework/
  ZendSkeletonApplication.git

Once finished we can go into the newly created folder called ZendSkeletonApplication, and copy and paste everything in there over to our web server document root. On a Linux system this is usually /var/www (this is also the case when we use Zend Server, as described in the Appendix, Setting up the Essentials). We can do this, for example, by typing the following commands:

$ cd ZendSkeletonApplication
$ mv ./* /var/www –f
$ cd /var/www

When everything is copied over, we are going to initialize the project by typing the following command:

$ php composer.phar install

Now the Command Line Interface (CLI) of PHP executes composer.phar, which will in this instance, download and install the Zend Framework 2 library and set up a simple project for us to be able to work in.

This command can take a long time before it is successfully executed, as Composer needs to do a lot of things before it tells you that Zend Framework 2 is ready for use, we won't go into the details of the workings of Composer here, as it is already discussed in the Appendix, Setting up the Essentials.

Once this command has been completed we need to make sure our web server document root is changed to match the layout of the skeleton. It is common practice that Zend Framework 2 uses the public folder as a main landing point for the application. The structure of the Zend Framework 2 skeleton allows us to bind the user to the public folder, while all our logic is safely outside the public area.

In essence this means we need to root or jail the web server in using the public folder first before we can actually see anything that we just installed. We want to root or jail the web server because we don't want the outside world to be able to abuse our web server more than necessary, and rooting or jailing makes sure that the web server itself has no access to any other folders than what it is jailed to, thus making our server a bit more secure.

In my personal case this means changing the Apache 2 configuration. In most Linux-based systems it will be the Apache web server that is serving our web requests.

The easiest thing that you can do is find your web server configuration (usually located in /etc/apache2 and append the DocumentRoot with /public. For me this would change the document root from /var/www' to '/var/www/public.

AllowOverride FileInfo

Finally we need to restart the Apache web server, which can be done by the following command if you are logged in as a root user or invoke it by prepending the command with sudo, which tells the server that we want to execute it as a super user.

$ apache2ctl restart

Now we are able to check our browser and see what we have actually done. We now simply go with a web browser to the project created by typing in the URL, in my case this would be the following:

http://localhost/

This will result in the following screen:

Congratulations, you have now set up a basic Zend Framework 2 application.

Let's look at our simple (Segment) configuration as follows (file: /module/Application/config/module.config.php):

return array(
  // Here we define our route configuration
  'routes' => array( 

    // We give this route the name 'website'
    'website' => array( 

      // The route type is of the class:
      // Zend\Mvc\Router\Http\Segment
      'type' => 'segment', 

        // Lets set the options for this route
        'options' => array( 

          /*
            The route that we want to match is /website
            where we can optionally add a controller name
            and an action name. For example:
              /website/index/index
          */ 
          'route' => '/website[/:controller[/:action]]',

          /*
            We don't want to accept everything, but this
            regex makes sure we only accept alpha-
            numeric characters and a dash and underscore.

            In our instance we want to check this for the
            action and the controller.
          */
          'constraints' => array( 
            'controller' => '[a-zA-Z][a-zA-Z0-9_-]*',
            'action' => '[a-zA-Z][a-zA-Z0-9_-]*'
          ),

          /*
           We want to make sure that if the user only 
           types /website in the URL bar it will actually
           go somewhere. We defined that here.
          */
          'defaults' => array( 
            'controller' => 'Website\Controller\Index', 
            'action' => 'index'
          ),
        ),
      ),
    ),
  ),
);

With this basic configuration we can easily define routes in our application, and in this instance we have configured a route that responds to the /website URL. When we would go to the /website URL, we would be routed to the Website\Controller\Index::indexAction by default. If we however use the route /website/another/route, we would be routed to the Website\Controller\Another::routeAction, as we have defined that the controller and action can be parsed behind that. If we omit the route path and put in /website/another, we would be redirected to the Website\Controller\Another::indexAction, as that is used by default by the framework.

The preceding example has only one really major drawback, which is, when we decide to use anonymous function in the configuration to create more dynamic routes, we would not be able to cache the route as closures are not serializeable by the cache.

However, there is another method of declaring the route, and that is in the code. The need to create the route functionality in the code could (obviously everyone has their own reasons and requirements) arise because we want to cache the configuration in a later stage (as we cannot cache anonymous function, for example) or when we want to load up a route dynamically from a database.

Let's take a look at the /module/Application/Module.php example:

<?php

// We are working in the Application module
namespace Application;

// Our main imports that we want to use
use Zend\Mvc\ModuleRouteListener;
use Zend\Mvc\MvcEvent;

// Define our module class, this is always 'Module', but 
// needs to be specifically created by the developer.
class Module
{
  public function onBootstrap(MvcEvent $e)
  {
    // First we want to get the ServiceManager
    $sm = $e->getApplication()->getServiceManager();
    
    /*
      Say our logged in user is 'gdog' and we want
      him to be able to go to /gdog to see his profile.
    */
    $user = 'gdog';

    // Now get the router
    $router = $sm->get('router');

    // Lets add a route called 'member' to our router
    $router->addRoute('member', array(

      /*
        We want to make /$user the main end point, with 
        an optional controller and action.
      */
     'route' => '/'. $user. '[/:controller[/:action]]',

      /*
        We want a default end point (if no controller
        and action is given) to go to the index action
        of the index controller.
      */
      'defaults' => array( 
        'controller' => 'Member\Controller\Index', 
        'action' => 'index' 
      ), 

      /*
        We only want to allow alphanumeric characters
        with an exception to the dash and underscore.
      */
      'constraints' => array( 
          'controller' => '[a-zA-Z][a-zA-Z0-9_-]*', 
          'action' => '[a-zA-Z][a-zA-Z0-9_-]*' 
      ), 
    ));
  }
}

Naturally there are more ways of adding a route, but the method mentioned in the preceding code for adding a route displays a canny way of dynamically adding a route. What we created there is that whenever Gdog goes to his profile, he can simply type in http://example.ext/gdog and end up on his profile.

Even more wonderful is that if our friend Gdog wants to see his friends, he is able to do that by just typing in for example, http://example.ext/gdog/my/friends, which will resolve to the Member module and then go to the My controller, lastly executing the Friends action.

This route stack is—as the name implies—the simplest router around and is basically a list with routes that is being parsed to see which route matches, by default this type of router is not used in Zend Framework 2. The general rule of thumb is that if we want to add a route with a high priority, we give it a high index number for example, 100, or 200. If we want to give the route a very low priority, we would give it an index number of, for example, 5 or 10.

Giving priorities to routes comes in handy when we have very specific routes (which usually have a high priority) and less specific routes (low priority). If we, for example, want to make /website/url redirect to a completely different module, controller, and action, but not affect the other website routes, we need to give the /website/url route a higher priority so that when it is found, it will not search for the lower priority routes.

If we, by accident, turn the priorities around, we would find our /website/url always redirect to the route that contains all the /website routes.

SimpleRouteStack uses a Zend\Mvc\Router\PriorityList class to manage its routes priorities.

We need to consider routing before we want to start creating our application, as when the application grows we might get into trouble with our routing if we haven't considered 'how to route' beforehand. It would therefore be wise for us to 'sitemap' the application before coding the routes to make sure we have a correct route list and are not creating any conflicting routes.

The SimpleRouteStack class has a number of methods defined that are very useful for us:

addRoute($name, $route, $priority) / addRoutes($routes): We can use this to add a new route or an array of routes to our route type by simply adding it through this method. A route requires a name, route (which can be a string or an instance of RouteInterface) and if we fancy a priority, we can give that as the third parameter.

hasRoute($name): If we would want to check whether a specific route already exists, we can search using its name and find out if it does or doesn't.

Routers are not restricted to using the URI path to find out how to route a request. They can also use other information such as the query parameters, headers, methods, or hostnames to find a match.

A wonderful set of HTTP routers can be found in the Zend\Mvc\Router\Http namespace and we will take a quick look at the different classes that reside within this namespace.

The Zend\Mvc\Router\Http\Method class is used when we want to match against an HTTP method instead of a segment or path. This could be, for example, a POST, DELETE and so on. The method is also called verb by Zend Framework 2, which means that instead of a route parameter, it requests a verb parameter when adding the route, which is an excellent way to create RESTful APIs.

The Zend\Mvc\Router\Http\Part class is used to describe child_routes in our routing configuration. This means that—although never used directly—we can define that /user/profile is being redirected to use the UserController, with the profile action.

Let's consider the following configuration:

return array(
  // We begin our router configuration
  'router' => array(

    // Define our routes 
    'routes' => array(

      // We are defining a route named 'Example'
      'Example' => array(
        'type' => 'Literal',
        'options' => array(
  
           /*
            This route will resolve to /recipe 
            which will resolve to the Example 
            module's IndexController and execute 
            the IndexAction.
           */
           'route' => 'recipe',
           'defaults' => array(
             '__NAMESPACE__' => 'Example\Controller',
             'controller' => 'Index',
           ),
         ),
                
         'may_terminate' => true,

          /*
            Here we begin to define our Part route, 
            which always begins with the 
            'child_routes' configuration.
          */
          'child_routes' => array(
            'client' => array(
              'type' => 'Literal',
              'options' => array(
           
              /*
                This child route (or Part) 
                will resolve to /recipe/foo       
                and will call the fooAction in  
                the IndexController.
              */
              'route' => '/foo',
              'defaults' => array(
               'action' => 'fooAction'
              ),
            ),
          ),
        ),
      ),
    ),
  ),
);

The Zend\Mvc\Router\Http\Regex class would be used when we have a complex routing structure that requires us to dynamically create the route. This would, for example, come in handy when we look at News sites, where posts are built up like /archive/some-subject-2013.html. This fairly complex route (as some-subject-2013.html is dynamic in our case) would require a Regex router that can resolve the Controller, Action, and in our case also the output format.

Let's consider the following example:

// We begin our router configuration
'router' => array(

  // Define our routes 
  'routes' => array(

    // We are defining a route named 'Archive'
    'Archive' => array(
      'type' => 'Literal',
      'options' => array(

        /*
          This route will resolve to /archive 
          which will resolve to the Archive 
          module's IndexController and execute 
          the IndexAction.
        */
        'regex' => '/archive/(?<id>[a-zA-Z0-9_-
  ]+)(\.(?<format>(html|xml)))?',
        'defaults' => array(
            '__NAMESPACE__' => 'Archive\Controller',
            'controller' => 'Index',
            'action' => 'indexAction',
            'format' => 'html',
        ), 
        'spec' => '/archive/%id%.%format%',
      ),
    ),
  ),
),

In the preceding example, it is important to note that /archive/%id%.%format% tells us that we will receive two parameters in our method called indexAction that is, id and format.

The Zend\Mvc\Router\Http\Scheme class is always using the defaults parameter and will accept only one other parameter, which is called scheme and can only contain one of the following options, that is, http, https, and mailto.

The Zend\Mvc\Router\Http\Segment class is probably one of the most-used routers that we would use, as you can dynamically define the route and controller for any module by using, for example, /:controller/:action, which is easily recognizable by the colon separation. We can define any constraints to the segment by configuring only the use of alphanumeric characters or another definition that we would like to use.

An example of Segment is given in the first example in the How to do it... section.

Let's take a look at this DI example, considering that we have the same classes as the preceding example:

namespace OneNamespace
{
  class FirstClass 
  {
    [..] 
  }

  class SecondClass 
  {
    [..]  
  }
}

namespace AnotherNamespace 
{
  class ThirdClass 
  {
    [..]
  }
}

// Instead of configuring all the classes, we will now 
// simply configure the Di, and only instantiate the 
// class that we want to use.
$di = new \Zend\Di\Di();
$lister = $di->get(
    'OneNamespace\FirstClass',
    array(
        'first_name' => 'Jane',
        'last_name' => 'Doe',
        'vehicle' => 'Car',
    )
);

In the preceding example, we simply say to the DI that AnotherNamespace\ThirdClass has two parameters in its __construct method. The DI will then utilize Reflection to find out what parameters are present there, and will then give any class that has a first_name, vehicle, or last_name parameter in its constructor that parameter.

Of course we will see a potential flaw here, as you might need to utilize multiple instantiations, one can presume that at some point the same parameter name will be used. In our example, it would cause a problem if another class also has a $first_name parameter but requires a different input, as the DI will simply give the one that is in its list.

One good thing about this is that this can flaw only happens when we use the DI at a call-time level, and not in a global configuration level as we will see now. That is why it isn't recommended to use the DI at call-time level at all.

What we also can do to create a more specific (or accurate) initialization of our object – and to make sure classes with the same property names don't conflict – is initializing the DI with a configuration object.

The idea behind this is that we first create a configuration object (or array) that defines which classes need which properties set, and then use that to initialize the DI, which in its turn finds out when it needs to initiate what.

Take a look at the following example, which shows you the exact thing we just explained:

<?php
// We are assuming that we are using the same classes as 
// in the previously shown examples.
namespace OneNamespace 
{
  class FirstClass 
  {
    [..] 
  }

  class SecondClass 
  {
    [..]  
  }
}

namespace AnotherNamespace 
{
  class ThirdClass 
  {
    [..]
  }
}

// After defining our classes we now begin to create our 
// configuration array which we will use to initialize 
// the DI.
$configuration = array(

  // We want to use this specific configuration at 
  // initialization of our class.
  'instance' => array(
    
    // We specify the class name to use here
    'SecondClass' => array(
       
      // We want to use this as a parameter
      'parameters' => array(

        // The property name to fill is vehicle.
        'vehicle' => 'Airplane'
      ),
    ),

    'FirstClass' => array(
      // Again we want to use this as a parameter
      'parameters' => array(

        // The property name to fill is first name and 
        //last name.
        'first_name' => 'Neil',
        'last_name' => 'deGrasse Tyson',
      ),
    ),
  ),
);

// We want to instantiate the Di\Configuration now.
use \Zend\Di\Configuration; 

$diConfiguration = new Configuration($configuration);

// Now instantiate the Di itself, with the configuration 
// attached.
$di = new \Zend\Di\Di($configuration);

// And to get the object we want to use, we just do the 
//same as before.
$firstClass = $di->get('OneNamespace\FirstClass');

To make everything even nicer, we would just put the Zend\Di\Configuration of the DI in the bootstrap of our module, so that we can use it easily throughout the namespace. This way we can simply put the configuration of the DI in our module.config.php and let the framework take care of it.

This means that every get() call will result in the same instantiation over and over again. If we would like a new instance, we would need to call newInstance() as the DI implements the singleton pattern, which means that all the data persists every time we call the get() method unless we force a new instance of the DI.

When our class has more properties than we define, we will find out that the DI will use the last value for every other property in the class. Of course this is unwanted, and if we wrote the class ourselves we should consider refactoring the configuration and/or class.

However, when there is no other way we can define the right properties only by using a Fully Qualified (FQ) setter parameter.

In our configuration we would then define a very specific property name, for example, class::method:paramPos. If we take our ThirdClass example from earlier on, this would then be ThirdClass::setFirstName:0 and ThirdClass::setLastName:0 respectively.

When beginning to code in Zend Framework 2 there is some misunderstanding as to what the different configuration files do. By default we have multiple configuration files, and it might not always be simple to understand where things need to go. That is why we like to apply a simple rule:

The configuration that we usually place in the global.php file can be, for example, the caching method and configuration, the database configuration. Normally we would like to place items in there that are environment related, but nothing that is security sensitive.

Let's take a look at a bad example of global.php:

<?php
return array(

  // We want to create a new database connection
  'db' => array(

    // The driver we want to use is the Pdo, our  
    // favorite
    'driver' => 'Pdo',

    // This is our connection url, defining a MySQL 
    // connection, with database 'somename' which is 
    // available on the localhost server.
    'dsn' => 'mysql:dbname=somename;host=localhost',

    // This is exactly what we should NOT do in this  
    // file, shame on you developer!
    'username' => 'terribleuser',
    'password' => 'evenworsepassword',
  ),

  // We need a database adapter defined as well, 
  // otherwise we can't use it at all.
  'service_manager' => array(
    'factories' => array(
      'Zend\Db\Adapter\Adapter' => 
  'Zend\Db\Adapter\AdapterServiceFactory',
    ),
  ),
);

It is terrible practice to put the username and password in the global.php file. The global.php file is to be put in our version control, and therefore should contain only configuration items that are required to globally run the application, not specific information that is relevant per environment, such as database usernames and passwords.

One of the benefits of the ultra-many configuration files in Zend Framework 2, is that you are able to override your global configuration with your local configuration. This certainly comes in handy when developing and you find yourself in a position where your details are slightly different in configuration than your production environment.

Let's assume that we have the following /config/autoload/global.php configuration file:

<?php
return array(

  // We want to create a new database connection
  'db' => array(

    // The driver we want to use is the Pdo, our  
    // favorite
    'driver' => 'Pdo',

    // This is our connection url, defining a MySQL 
    // connection, with database 'somename' which is 
    // available on the localhost server.
    'dsn' => 'mysql:dbname=somename;host=localhost',
  ),

  // We need a database adapter defined as well, 
  // otherwise we can't use it at all.
  'service_manager' => array(
    'factories' => array(
      'Zend\Db\Adapter\Adapter' => 
  'Zend\Db\Adapter\AdapterServiceFactory',
    ),
  ),
);

As we can see in the preceding example, we create a nice and simple MySQL database connection to our somename database which resides on the localhost. But as good developers we have not defined our username and password in here. That is where the /config/autoload/local.php file comes in.

Let's take a look at how our local.php might look like:

<?php
return array(
  'db' => array(
    'username' => 'awesomeuser',
    'password' => 'terriblepassword',
  ),
);

If we are using a version control system (please say yes), we should not commit this file, not only for security reasons but also because this is a local configuration file and wouldn't be necessary on a live system, as we would create a new one with the right details for that environment.

If we look at our default config/application.config.php file we have only a few properties set, but loads of inline comments, which really come in handy when we can't remember the exact name or description of a property any more.

The main configuration that we will be changing the most in our application as we develop is the modules property. This specific property is a simple array with the different module namespaces that we have (and want to use) in our application. At default this looks somewhat like this:

<?php 

return array(
  // This should be an array of module namespaces used 
  // in the application.
  'modules' => array(
    'Application',
  ),
[..]

When we add or remove a module, this line needs to be modified as well and one can even suggest modifying this before starting a new module or removing one. The reason for this is simple, when we forget to modify this file when removing a module it will generate a 500 – Application Error when visiting the application in our browser. And because this configuration file is read quite early in the instantiation, it can sometimes be hard for the developer to pinpoint why the application fails to load all of a sudden.

The bootstrap is in our case the start of a module, whenever a module is requested it will use the onBootstrap() method located in the Module.php file. Although the method is not required, we usually want this method in our module as it is an easy method of making sure that some instances already exist or are configured before venturing further in our client request.

Sessions are a wonderful way of saving information about a user on a temporary basis. Think about saving the information of a logged-in user, or history on the pages they have been. Once we begin creating an application we find ourselves saving a lot of things in the session.

The first thing we need to do is modify the /module/Application/config/module.config.php file, and add another section called session to it. Let's assume that we have a completely empty module configuration:

<?php
return array(
  'service_manager' => array(
     // These are the factories needed by the Service 
     // Locator to load in the session manager
    'factories' => array(
      'Zend\Session\Config\ConfigInterface' => 
  'Zend\Session\Service\SessionConfigFactory',
      'Zend\Session\Storage\StorageInterface' => 
  'Zend\Session\Service\SessionStorageFactory',
      'Zend\Session\ManagerInterface' => 
  'Zend\Session\Service\SessionManagerFactory',
    ),
    'abstract_factories' => array(
  'Zend\Session\Service\ContainerAbstractFactory',
    ),
  ),
  'session_config' => array(
    // How long can the session be idle for in seconds 
    // before it is being invalidated
    'remember_me_seconds' => 3600,

    // What is the name of the session (can be anything)
    'name' => 'some_name',
  ),
  // What kind of session storage do we want to use, 
  // only SessionArrayStorage is available at the minute
  'session_storage' => array(
    'type' => 'SessionArrayStorage',
    'options' => array(), 
  ),
  // These are session containers we can use to store 
  // our information in
  'session_containers' => array(
    'ContainerOne',
    'ContainerTwo',
  ),
);

And that is it. Sessions are now useable in our controllers and models. We have now created two session containers that we can use to store our information in. We can access these containers in any Controller or Model that has a service locator available by doing the following (file: /module/Application/src/Application/Controller/IndexController.php):

<?php

namespace Application;

use Zend\Mvc\Controller\AbstractActionController;

class IndexController extends AbstractController
{
  public function indexAction()
  {
    // Every session container we define receives a 
    // SessionContainer\ prefix before the name
    $containerOne = $this->getServiceLocator()
  ->get('SessionContainer\ContainerOne');
  }
}

<?php
// Don't forget to add the namespace
namespace Application\Model;

// We shouldn't forget to add these!
use Zend\EventManager\EventManager;
 
class SwagMachine
{
  // This will hold our EventManager
  private $em;

  public function getEventManager() 
  {
    // If there is no EventManager, make one!
    if (!$this->em) {
      $this->em = new EventManager(__CLASS__);
    }
 
    // Return the EventManager.
    return $this->em;
  }
 
  public function findSwag($id)
  {
    // Trigger our findSwag.begin event 
    // and push our $id variable with it.
    $response = $this->getEventManager()
                     ->trigger(
           'findSwag.begin', 
           $this,
           array(
             'id' => $id
           )
    );
  
    // Make our last response, the final 
    // ID if there is a response.
    if ($response->count() > 0)
      $id = $response->last();
 
    // ********************************
    // In the meantime important code 
    // is happening...
    // ********************************
  
    // ...And that ends up with the 
    // folowing return value:
    $returnValue = 'Original Value ('. $id. ')';
    
    // Now let's trigger our last 
    // event called findSwag.end and 
    // give the returnValue as a 
    // parameter.
    $this->getEventManager()
         ->trigger(
           'findSwag.end', 
           $this, 
           array(
             'returnValue' => $returnValue
           )
    );
 
    // Now return our value.
    return $returnValue;
  }
}

<?php

namespace Application\Controller;

use Zend\Mvc\Controller\AbstractActionController;

class IndexController extends AbstractActionController
{
  public function indexAction() 
  {
    // Get our SwagMachine
    $machine = new SwagMachine();
    
    // Let's attach our first callback, 
    // which potentially will increase 
    // the $id with 10, which would 
    // make it result in 30!
    $machine->getEventManager()
            ->attach(
        'findSwag.begin',
        function(Event $e) 
        {
          // Get the ID from our findSwag() 
          // method, and add it up with 12.
          return $e->getParam('id') + 10;
        },
        200
    );
    
    // Now attach our second callback, 
    // which potentially will increase 
    // the value of $id to 60! We give 
    // this a *higher* priority then 
    // the previous attached event 
    // trigger.
    $machine->getEventManager()
            ->attach(
        'findSwag.begin',
        function(Event $e) 
        {
        // Get the ID from our findSwag() 
        // method, and add it up with 15.
        return $e->getParam('id') + 40;
      },
      100
    );
  
    // Now create a trigger callback 
    // for the end event called findSwag.end, 
    // which has no specific priority, 
    // and will just output to the screen.
    $machine->getEventManager()
            ->attach(
        'findSwag.end',
        function(Event $e) 
        {
          echo 'We are returning: '
             . $e->getParam('returnValue');
        }
    );

    // Now after defining the triggers, 
    // simply try and find our 'Swag'.      
    echo $machine->findSwag(20);
  }
}

We are returning: Original Value (60)
Original Value (60)

Module.php:
namespace Application;

// We are going to use events, and because we use a MVC, 
// we need to use the MvcEvent.
use Zend\Mvc\MvcEvent;

class Module
{
  public function onBootstrap(MvcEvent $e)
  {
    // Get our SharedEventManager from the MvcEvent $e 
    // that we got from the method
    $sharedEvents = $e->getApplication()
                      ->getEventManager()
                      ->getSharedManager();

    // Also retrieve the ServiceManager of the 
    // application.
    $sm = $e->getApplication()->getServiceManager();

    // Let's propose a new ViewStrategy to our 
    // EventManager.
    $sharedEvents->attach(

        // We are attaching the event to this namespace 
        // only.
        __NAMESPACE__, 

        // We want to attach to this very specific 
        // event, the Dispatch event of our controller.
        MvcEvent::EVENT_DISPATCH, 

        // The callback function of the event, used when 
        // the event we attached to happens. In our 
        // callback we also want our local variable $sm 
        // to be available for use.
        function($e) use ($sm) 
        {
          // Get our alternate view strategy from the 
          // ServiceManager and attach the EventManager     
          // to the strategy.
          $strategy = $sm->get('ViewJsonStrategy');
          $view = $sm->get('ViewManager')->getView();
          $strategy->attach($view->getEventManager());
        }, 

        // We want to give this a priority, so this will 
        // get more priority.
        100
    );
}

Simply said the Observer pattern means that there are several interested parties, called listeners that want to know when the application triggers a certain event. When a specific event is triggered, the listeners will be notified so that they can take their necessary actions.

If we want to explain what AOP is, we could say that in short it stands for writing clean code that have only function and are as isolated from the rest of the code as possible.