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Getting Results from your Handler

5.0 version
Maintained Unmaintained
4.2

Getting Results from your Handler

When a message is handled, the HandleMessageMiddleware adds a HandledStamp for each object that handled the message. You can use this to get the value returned by the handler(s):

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use Symfony\Component\Messenger\MessageBusInterface;
use Symfony\Component\Messenger\Stamp\HandledStamp;

$envelope = $messageBus->dispatch(SomeMessage());

// get the value that was returned by the last message handler
$handledStamp = $envelope->last(HandledStamp::class);
$handledStamp->getResult();

// or get info about all of handlers
$handledStamps = $envelope->all(HandledStamp::class);

A HandleTrait also exists in order to ease leveraging a Messenger bus for synchronous needs. The handle() method ensures there is exactly one handler registered and returns its result.

Working with Command & Query Buses

The Messenger component can be used in CQRS architectures where command & query buses are central pieces of the application. Read Martin Fowler's article about CQRS to learn more and how to configure multiple buses.

As queries are usually synchronous and expected to be handled once, getting the result from the handler is a common need.

To avoid boilerplate code, you can leverage the HandleTrait in any class that has a $messageBus property:

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// src/Action/ListItems.php
namespace App\Action;

use App\Message\ListItemsQuery;
use App\MessageHandler\ListItemsQueryResult;
use Symfony\Component\Messenger\HandleTrait;
use Symfony\Component\Messenger\MessageBusInterface;

class ListItems
{
    use HandleTrait;

    public function __construct(MessageBusInterface $messageBus)
    {
        $this->messageBus = $messageBus;
    }

    public function __invoke()
    {
        $result = $this->query(new ListItemsQuery(/* ... */));

        // Do something with the result
        // ...
    }

    // Creating such a method is optional, but allows type-hinting the result
    private function query(ListItemsQuery $query): ListItemsResult
    {
        return $this->handle($query);
    }
}

Hence, you can use the trait to create command & query bus classes. For example, you could create a special QueryBus class and inject it wherever you need a query bus behavior instead of the MessageBusInterface:

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// src/MessageBus/QueryBus.php
namespace App\MessageBus;

use Symfony\Component\Messenger\Envelope;
use Symfony\Component\Messenger\HandleTrait;
use Symfony\Component\Messenger\MessageBusInterface;

class QueryBus
{
    use HandleTrait;

    public function __construct(MessageBusInterface $messageBus)
    {
        $this->messageBus = $messageBus;
    }

    /**
     * @param object|Envelope $query
     *
     * @return mixed The handler returned value
     */
    public function query($query)
    {
        return $this->handle($query);
    }
}

This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.