Doctrine 2 features a lightweight event system that is part of the Common package.

++ The Event System

The event system is controlled by the `EventManager`. It is the central point 
of Doctrine's event listener system. Listeners are registered on the manager 
and events are dispatched through the manager.

    [php]
    $evm = new EventManager();

Now we can add some event listeners to the `$evm`. Let's create a `EventTest` class
to play around with.

    [php]
    class EventTest
    {
        const preFoo = 'preFoo';
        const postFoo = 'postFoo';

        private $_evm;

        public $preFooInvoked = false;
        public $postFooInvoked = false;

        public function __construct($evm)
        {
            $evm->addEventListener(array(self::preFoo, self::postFoo), $this);
        }

        public function preFoo(EventArgs $e)
        {
            $this->preFooInvoked = true;
        }

        public function postFoo(EventArgs $e)
        {
            $this->postFooInvoked = true;
        }
    }

    // Create a new instance
    $test = new EventTest($evm);

Events can be dispatched by using the `dispatchEvent()` method.

    [php]
    $evm->dispatchEvent(EventTest::preFoo);
    $evm->dispatchEvent(EventTest::postFoo);

You can easily remove a listener with the `removeEventListener()` method.

    [php]
    $evm->removeEventListener(array(self::preFoo, self::postFoo), $this);

The Doctrine 2 event system also has a simple concept of event subscribers. We
can define a simple `TestEventSubscriber` class which implements the
`\Doctrine\Common\EventSubscriber` interface and implements a `getSubscribedEvents()`
method which returns an array of events it should be subscribed to.

    [php]
    class TestEventSubscriber implements \Doctrine\Common\EventSubscriber
    {
        const preFoo = 'preFoo';

        public $preFooInvoked = false;

        public function preFoo()
        {
            $this->preFooInvoked = true;
        }

        public function getSubscribedEvents()
        {
            return array(self::preFoo);
        }
    }

    $eventSubscriber = new TestEventSubscriber();
    $evm->addEventSubscriber($eventSubscriber);

Now when you dispatch an event any event subscribers will be notified for that event.

    [php]
    $evm->dispatchEvent(TestEventSubscriber::preFoo);

Now you can test the `$eventSubscriber` instance to see if the `preFoo()` method was invoked.

    [php]
    if ($eventSubscriber->preFooInvoked) {
        echo 'pre foo invoked!';
    }

++ Lifecycle Events

The EntityManager and UnitOfWork trigger a bunch of events during the life-time of their registered entities.

* preRemove - The preRemove event occurs for a given entity before the respective EntityManager remove operation for that entity is executed.
* postRemove - The postRemove event occurs for an entity after the entity has been deleted. It will be invoked after the database delete operations.
* prePersist - The prePersist event occurs for a given entity before the respective EntityManager persist operation for that entity is executed.
* postPersist - The postPersist event occurs for an entity after the entity has been made persistent. It will be invoked after the database insert operations. Generated primary key values are available in the postPersist event.
* preUpdate - The preUpdate event occurs before the database update operations to entity data.
* postUpdate - The postUpdate event occurs after the database update operations to entity data.
* postLoad - The postLoad event occurs for an entity after the entity has been loaded into the current EntityManager from the database or after the refresh operation has been applied to it.
* loadClassMetadata - The loadClassMetadata event occurs after the mapping metadata for a class has been loaded from a mapping source (annotations/xml/yaml).
* onFlush - The onFlush event occours after the change-sets of all managed entities are computed. This event is not a lifecycle callback.

> **CAUTION**
> Note that the postLoad event occurs for an entity before any associations have been
> initialized. Therefore it is not safe to access associations in a postLoad callback
> or event handler.

You can access the Event constants from the `Events` class in the ORM package.

    [php]
    use Doctrine\ORM\Events;
    echo Events::preUpdate;

These can be hooked into by two different types of event listeners:

* Lifecycle Callbacks are methods on the entity classes that are called when the event is triggered. They recieve absolutely no arguments and are specifically designed to allow changes inside the entity classes state.
* Lifecycle Event Listeners are classes with specific callback methods that recieves some kind of `EventArgs` instance which give access to the entity, EntityManager or other relevant data.

> **NOTE**
> All Lifecycle events that happen during the `flush()` of an EntityManager have very specific constraints on the allowed
> operations that can be executed. Please read the *Implementing Event Listeners* section very carefully to understand
> which operations are allowed in which lifecycle event.

++ Lifecycle Callbacks

A lifecycle event is a regular event with the additional feature of providing 
a mechanism to register direct callbacks inside the corresponding entity classes
that are executed when the lifecycle event occurs.

    [php]
    
    /** @Entity @HasLifecycleCallbacks */
    class User
    {
        // ...

        /**
         * @Column(type="string", length=255)
         */
        public $value;

        /** @Column(name="created_at", type="string", length=255) */
        private $createdAt;

        /** @PrePersist */
        public function doStuffOnPrePersist()
        {
            $this->createdAt = date('Y-m-d H:m:s');
        }

        /** @PrePersist */
        public function doOtherStuffOnPrePersist()
        {
            $this->value = 'changed from prePersist callback!';
        }

        /** @PostPersist */
        public function doStuffOnPostPersist()
        {
            $this->value = 'changed from postPersist callback!';
        }

        /** @PostLoad */
        public function doStuffOnPostLoad()
        {
            $this->value = 'changed from postLoad callback!';
        }

        /** @PreUpdate */
        public function doStuffOnPreUpdate()
        {
            $this->value = 'changed from preUpdate callback!';
        }
    }

Note that when using annotations you have to apply the @HasLifecycleCallbacks marker annotation on the entity class.

If you want to register lifecycle callbacks from YAML or XML you can do it with
the following.

    [yml]
    User:
      type: entity
      fields:
    # ...
        name:
          type: string(50)
      lifecycleCallbacks:
        doStuffOnPrePersist: prePersist
        doStuffOnPostPersist: postPersist

XML would look something like this:

    [xml]
    <?xml version="1.0" encoding="UTF-8"?>

    <doctrine-mapping xmlns="http://doctrine-project.org/schemas/orm/doctrine-mapping"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="http://doctrine-project.org/schemas/orm/doctrine-mapping
                              /Users/robo/dev/php/Doctrine/doctrine-mapping.xsd">

        <entity name="User">

            <lifecycle-callbacks>
                <lifecycle-callback type="prePersist" method="doStuffOnPrePersist"/>
                <lifecycle-callback type="postPersist" method="doStuffOnPostPersist"/>
            </lifecycle-callbacks>

        </entity>

    </doctrine-mapping>

You just need to make sure a public `doStuffOnPrePersist()` and `doStuffOnPostPersist()` method is defined on your `User` model.

    [php]
    // ...

    class User
    {
        // ...

        public function doStuffOnPrePersist()
        {
            // ...
        }

        public function doStuffOnPostPersist()
        {
            // ...
        }
    }

The `key` of the lifecycleCallbacks is the name of the method and the value is
the event type. The allowed event types are the ones listed in the previous Lifecycle Events section.

++ Listening to Lifecycle Events

Lifecycle event listeners are much more powerful than the simple lifecycle callbacks that are defined on the entity
classes. They allow to implement re-usable behaviours between different entity classes, yet require much more detailed
knowledge about the inner workings of the EntityManager and UnitOfWork. Please read the *Implementing Event Listeners*
section carefully if you are trying to write your own listener.

To register an event listener you have to hook it into the EventManager that is passed to the EntityManager factory:

    [php]
    $eventManager = new EventManager();
    $eventManager->addEventListener(array(Events::preUpdate), MyEventListener());
    $eventManager->addEventSubscriber(new MyEventSubscriber());

    $entityManager = EntityManager::create($dbOpts, $config, $eventManager);

You can also retrieve the event manager instance after the EntityManager was created:

    [php]
    $entityManager->getEventManager()->addEventListener(array(Events::preUpdate), MyEventListener());
    $entityManager->getEventManager()->addEventSubscriber(new MyEventSubscriber());

++ Implementing Event Listeners

This section explains what is and what is not allowed during specific lifecycle events of the UnitOfWork.
Although you get passed the EntityManager in all of these events, you have to follow this restrictions very
carefully since operations in the wrong event may produce lots of different errors, such as inconsistent data and
lost updates/persists/removes.

For the described events that are also lifecycle callback events the restrictions
apply as well, with the additional restriction that you do not have access to the EntityManager
or UnitOfWork APIs inside these events.

+++ prePersist

There are two ways for the `prePersist` event to be triggered. One is obviously
when you call `EntityManager#persist()`. The event is also called for all
cascaded associations.

There is another way for `prePersist` to be called, inside the `flush()`
method when changes to associations are computed and this association
is marked as cascade persist. Any new entity found during this operation
is also persisted and `prePersist` called on it. This is called "persistence by reachability".

In both cases you get passed a `LifecycleEventArgs`
instance which has access to the entity and the entity manager.

The following restrictions apply to `prePersist`:

* If you are using a PrePersist Identity Generator such as sequences the ID value
  will *NOT* be available within any PrePersist events.
* Doctrine will not recognize changes made to relations in a pre persist event
  called by "reachability" through a cascade persist unless you use the internal
  `UnitOfWork` API. We do not recommend such operations in the persistence by
  reachability context, so do this at your own risk and possibly supported by unit-tests.

+++ preRemove

The `preRemove` event is called on every entity when its passed to
the `EntityManager#remove()` method. It is cascaded for all
associations that are marked as cascade delete.

There are no restrictions to what methods can be called inside
the `preRemove` event, except when the remove method itself was
called during a flush operation.

+++ onFlush

OnFlush is a very powerful event. It is called inside `EntityManager#flush()`
after the changes to all the managed entities and their associations have
been computed. This means, the `onFlush` event has access to the sets of:

* Entities scheduled for insert
* Entities scheduled for update
* Entities scheduled for removal
* Collections scheduled for update
* Collections scheduled for removal

To make use of the onFlush event you have to be familiar with the internal UnitOfWork API,
which grants you access to the previously mentioned sets. See this example:

    [php]
    class FlushExampleListener
    {
        public function onFlush(OnFlushEventArgs $eventArgs)
        {
            $em = $eventArgs->getEntityManager();
            $uow = $em->getUnitOfWork();

            foreach ($uow->getScheduledEntityInsertions() AS $entity) {

            }

            foreach ($uow->getScheduledEntityUpdates() AS $entity) {

            }

            foreach ($uow->getScheduledEntityDeletions() AS $entity) {

            }

            foreach ($uow->getScheduledCollectionDeletions() AS $col) {

            }

            foreach ($uow->getScheduledCollectionUpdates() AS $col) {

            }
        }
    }

The following restrictions apply to the onFlush event:

* Calling `EntityManager#persist()` does not suffice to trigger a persist on an entity.
  You have to execute an additional call to `$unitOfWork->computeChangeSet($classMetadata, $entity)`.
* Changing primitive fields or associations requires you to explicitly trigger
  a re-computation of the changeset of the affected entity. This can be done
  by either calling `$unitOfWork->computeChangeSet($classMetadata, $entity)`
  or `$unitOfWork->recomputeSingleEntityChangeSet($classMetadata, $entity)`. The second
  method has lower overhead, but only re-computes primitive fields, never associations.

+++ preUpdate

PreUpdate is the most restrictive to use event, since it is called right before
an update statement is called for an entity inside the `EntityManager#flush()`
method.

Changes to associations of the updated entity are never allowed in this event, since Doctrine cannot guarantee to
correctly handle referential integrity at this point of the flush operation. This
event has a powerful feature however, it is executed with a `PreUpdateEventArgs`
instance, which contains a reference to the computed change-set of this entity.

This means you have access to all the fields that have changed for this entity
with their old and new value. The following methods are available on the `PreUpdateEventArgs`:

* `getEntity()` to get access to the actual entity.
* `getEntityChangeSet()` to get a copy of the changeset array. Changes to this returned array do not affect updating.
* `hasChangedField($fieldName)` to check if the given field name of the current entity changed.
* `getOldValue($fieldName)` and `getNewValue($fieldName)` to access the values of a field.
* `setNewValue($fieldName, $value)` to change the value of a field to be updated.

A simple example for this event looks like:

    [php]
    class NeverAliceOnlyBobListener
    {
        public function preUpdate(PreUpdateEventArgs $eventArgs)
        {
            if ($eventArgs->getEntity() instanceof User) {
                if ($eventArgs->hasChangedField('name') && $eventArgs->getNewValue('name') == 'Alice') {
                    $eventArgs->setNewValue('name', 'Bob');
                }
            }
        }
    }

You could also use this listener to implement validation of all the fields that have changed.
This is more efficient than using a lifecycle callback when there are expensive validations
to call:

    [php]
    class ValidCreditCardListener
    {
        public function preUpdate(PreUpdateEventArgs $eventArgs)
        {
            if ($eventArgs->getEntity() instanceof Account) {
                if ($eventArgs->hasChangedField('creditCard')) {
                    $this->validateCreditCard($eventArgs->getNewValue('creditCard'));
                }
            }
        }

        private function validateCreditCard($no)
        {
            // throw an exception to interupt flush event. Transaction will be rolled back.
        }
    }

Restrictions for this event:

* Changes to associations of the passed entities are not recognized by the flush operation anymore.
* Changes to fields of the passed entities are not recognized by the flush operation anymore, use the computed change-set passed to the event to modify primitive field values.
* Any calls to `EntityManager#persist()` or `EntityManager#remove()`, even in combination with the UnitOfWork API are strongly discouraged and don't work as expected outside the flush operation.

+++ postUpdate, postRemove, postPersist

The three post events are called inside `EntityManager#flush()`. Changes in here
are not relevant to the persistence in the database, but you can use this events
to

++ Load ClassMetadata Event

When the mapping information for an entity is read, it is populated in to a 
`ClassMetadataInfo` instance. You can hook in to this process and manipulate
the instance.

    [php]
    $test = new EventTest();
    $metadataFactory = $em->getMetadataFactory();
    $evm = $em->getEventManager();
    $evm->addEventListener(Events::loadClassMetadata, $test);

    class EventTest
    {
        public function loadClassMetadata(\Doctrine\ORM\Event\LoadClassMetadataEventArgs $eventArgs)
        {
            $classMetadata = $eventArgs->getClassMetadata();
            $fieldMapping = array(
                'fieldName' => 'about',
                'type' => 'string',
                'length' => 255
            );
            $classMetadata->mapField($fieldMapping);
        }
    }