Merge branch 'hotfix/45' into develop

Forward port #45

doc/book/zend.stdlib.hydrator.aggregate.md

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+# AggregateHydrator
+
+`Zend\Stdlib\Hydrator\Aggregate\AggregateHydrator` is an implementation of
+`Zend\Stdlib\Hydrator\HydratorInterface` that composes multiple hydrators via event listeners.
+
+You typically want to use an aggregate hydrator when you want to hydrate or extract data from
+complex objects that implement multiple interfaces, and therefore need multiple hydrators to handle
+that in subsequent steps.
+
+## Installation requirements
+
+The `AggregateHydrator` is based on the `Zend\EventManager` component, so be sure to have it
+installed before getting started:
+
+```php
+php composer.phar require zendframework/zend-eventmanager:2.*
+```
+
+## Basic usage
+
+A simple use case may be hydrating a `BlogPost` object, which contains data for the user that
+created it, the time it was created, the current publishing status, etc:
+
+```php
+use Zend\Stdlib\Hydrator\Aggregate\AggregateHydrator;
+
+$hydrator = new AggregateHydrator();
+
+// attach the various hydrators capable of handling simpler interfaces
+$hydrator->add(new My\BlogPostHydrator());
+$hydrator->add(new My\UserAwareObjectHydrator());
+$hydrator->add(new My\TimestampedObjectHydrator());
+$hydrator->add(new My\PublishableObjectHydrator());
+// ...
+
+// Now retrieve the BlogPost object
+// ...
+
+// you can now extract complex data from a blogpost
+$data = $hydrator->extract($blogPost);
+
+// or you can fill the object with complex data
+$blogPost = $hydrator->hydrate($data, $blogPost);
+```
+
+> ## Note
+#### Hydrator priorities
+`AggregateHydrator::add` has a second optional argument `$priority`. If you have two or more
+hydrators that conflict with each other for same data keys, you may decide which one has to be
+executed first or last by passing a higher or lower integer priority to the second argument of
+`AggregateHydrator::add`
+
+In order to work with this logic, each of the hydrators that are attached should just ignore any
+unknown object type passed in, such as in following example:
+
+```php
+namespace My;
+
+use Zend\Stdlib\Hydrator\HydratorInterface
+
+class BlogPostHydrator implements HydratorInterface
+{
+    public function hydrate($data, $object)
+    {
+        if (!$object instanceof BlogPost) {
+            return $object;
+        }
+
+        // ... continue hydration ...
+    }
+
+    public function extract($object)
+    {
+        if (!$object instanceof BlogPost) {
+            return array();
+        }
+
+        // ... continue extraction ...
+    }
+}
+```
+
+## Advanced use cases
+
+Since the `AggregateHydrator` is event-driven, you can use the `EventManager` API to tweak its
+behaviour.
+
+Common use cases are:
+
+> -   Removal of hydrated data keys (passwords/confidential information) depending on business rules
+- Caching of the hydration/extraction process
+- Transformations on extracted data, for compatibility with third-party APIs
+
+In the following example, a cache listener will be introduced to speed up hydration, which can be
+very useful when the same data is requested multiple times:
+
+```php
+use Zend\Stdlib\Hydrator\Aggregate\AggregateHydrator;
+use Zend\Stdlib\Hydrator\Aggregate\ExtractEvent;
+use Zend\Cache\Storage\Adapter\Memory;
+
+$hydrator = new AggregateHydrator();
+
+// attach the various hydrators
+$hydrator->add(new My\BlogPostHydrator());
+$hydrator->add(new My\UserAwareObjectHydrator());
+$hydrator->add(new My\TimestampedObjectHydrator());
+$hydrator->add(new My\PublishableObjectHydrator());
+// ...
+
+$cache             = new Memory();
+$cacheReadListener = function (ExtractEvent $event) use ($cache) {
+    $object = $event->getExtractionObject();
+
+    if (!$object instanceof BlogPost) {
+        return;
+    }
+
+    if ($cache->hasItem($object->getId())) {
+        $event->setExtractedData($cache->getItem($object->getId()));
+        $event->stopPropagation();
+    }
+};
+$cacheWriteListener = function (ExtractEvent $event) use ($cache) {
+    $object = $event->getExtractionObject();
+
+    if (!$object instanceof BlogPost) {
+        return;
+    }
+
+    $cache->setItem($object->getId(), $event->getExtractedData());
+};
+
+// attaching a high priority listener executed before extraction logic
+$hydrator->getEventManager()->attach(ExtractEvent::EVENT_EXTRACT, $cacheReadListener, 1000);
+// attaching a low priority listener executed after extraction logic
+$hydrator->getEventManager()->attach(ExtractEvent::EVENT_EXTRACT, $cacheWriteListener, -1000);
+```
+
+With an aggregate hydrator configured in this way, any `$hydrator->extract($blogPost)` operation
+will be cached