Doing Work in the Handler

Video not working?

It looks like your browser may not support the H264 codec. If you're using Linux, try a different browser or try installing the gstreamer0.10-ffmpeg gstreamer0.10-plugins-good packages.

Thanks! This saves us from needing to use Flash or encode videos in multiple formats. And that let's us get back to making more videos :). But as always, please feel free to message us.

Inside our controller, after we save the new file to the filesystem, we're creating a new AddPonkaToImage object and dispatching it to the message bus... or technically the "command" bus... because we're currently using it as a command bus. The end result is that the bus calls the __invoke() method on our handler and passes it that object. Messenger understands the connection between the message object and handler thanks to the argument type-hint and this interface.

Command Bus: Beautifully Disappointing

By the way, you might be thinking:

Wait... the whole point of a "command" bus is to... just "call" this __invoke() method for me? Couldn't I just... ya know... call it myself and skip a layer?

And... yes! It's that simple! It should feel completely underwhelming at first!

But having that "layer", the "bus", in the middle gives us two nice things. First, out code is more decoupled: the code that creates the "command" - our controller in this case - doesn't know or care about our handler. It dispatches the message and moves on. And second, this simple change is going to allow us to execute handlers asynchronously. More on that soon.

Moving code into the Handler

Back to work: all the code to add Ponka to the image is still done inside our controller: this gets an updated version of the image with Ponka inside, another service actually saves the new image onto the filesystem, and this last bit - $imagePost->markAsPonkaAdded() - updates a date field on the entity. It's only a few lines of code... but that's a lot of work!

Copy all of this, remove it, and I'll take my comments out too. Paste all of that into the handler. Ok, no surprise, we have some undefined variables. $ponkaficator, $photoManager and $entityManager are all services.

... lines 1 - 7
class AddPonkaToImageHandler implements MessageHandlerInterface
{
public function __invoke(AddPonkaToImage $addPonkaToImage)
{
$updatedContents = $ponkaficator->ponkafy(
$photoManager->read($imagePost->getFilename())
);
$photoManager->update($imagePost->getFilename(), $updatedContents);
$imagePost->markAsPonkaAdded();
$entityManager->flush();
}
}

In the controller... on top, we were autowiring those services into the controller method. We don't need $ponkaficator anymore.

... lines 1 - 20
class ImagePostController extends AbstractController
{
... lines 23 - 37
public function create(Request $request, ValidatorInterface $validator, PhotoFileManager $photoManager, EntityManagerInterface $entityManager, MessageBusInterface $messageBus)
{
... lines 40 - 63
}
... lines 65 - 95
}

Anyways, how can we get those services in our handler? Here's the really cool thing: the "message" class - AddPonkaToImage is a simple, "model" class. It's kind of like an entity: it doesn't live in the container and we don't autowire it into our classes. If we need an AddPonkaToImage object, we say: new AddPonkaToImage(). If we decide to give that class any constructor arguments - more on that soon - we pass them right here.

But the handler classes are services. And that means we can use, good, old-fashioned dependency injection to get any services we need.

Add public function __construct() with, let's see here, PhotoPonkaficator $ponkaficator, PhotoFileManager $photoManager and... we need the entity manager: EntityManagerInterface $entityManager.

... lines 1 - 5
use App\Photo\PhotoFileManager;
use App\Photo\PhotoPonkaficator;
use Doctrine\ORM\EntityManagerInterface;
... lines 9 - 10
class AddPonkaToImageHandler implements MessageHandlerInterface
{
... lines 13 - 16
public function __construct(PhotoPonkaficator $ponkaficator, PhotoFileManager $photoManager, EntityManagerInterface $entityManager)
{
... lines 19 - 21
}
... lines 23 - 32
}

I'll hit Alt + Enter and select Initialize Fields to create those properties and set them.

... lines 1 - 10
class AddPonkaToImageHandler implements MessageHandlerInterface
{
private $ponkaficator;
private $photoManager;
private $entityManager;
public function __construct(PhotoPonkaficator $ponkaficator, PhotoFileManager $photoManager, EntityManagerInterface $entityManager)
{
$this->ponkaficator = $ponkaficator;
$this->photoManager = $photoManager;
$this->entityManager = $entityManager;
}
... lines 23 - 32
}

Now... let's use them: $this->ponkaficator, $this->photoManager, $this->photoManager again... and $this->entityManager.

... lines 1 - 10
class AddPonkaToImageHandler implements MessageHandlerInterface
{
... lines 13 - 23
public function __invoke(AddPonkaToImage $addPonkaToImage)
{
$updatedContents = $this->ponkaficator->ponkafy(
$this->photoManager->read($imagePost->getFilename())
);
$this->photoManager->update($imagePost->getFilename(), $updatedContents);
... line 30
$this->entityManager->flush();
}
}

Message Class Data

Nice! This leaves us with just one undefined variable: the actual $imagePost that we need to add Ponka to. Let's see... in the controller, we create this ImagePost entity object... which is pretty simple: it holds the filename on the filesystem... and a few other minor pieces of info. This is what we store in the database.

Back in AddPonkaToImageHandler, at a high level, this class needs to know which ImagePost it's supposed to be working on. How can we pass that information from the controller to the handler? By putting it on the message class! Remember, this is our class, and it can hold whatever data we want.

So now that we've discovered that our handler needs the ImagePost object, add a public function __construct() with one argument: ImagePost $imagePost. I'll do my usual Alt+Enter and select "Initialize fields" to create and set that property.

... lines 1 - 4
use App\Entity\ImagePost;
class AddPonkaToImage
{
private $imagePost;
public function __construct(ImagePost $imagePost)
{
$this->imagePost = $imagePost;
}
... lines 15 - 19
}

Down below, we'll need a way to read that property. Add a getter: public function getImagePost() with an ImagePost return type. Inside, return $this->imagePost.

... lines 1 - 4
use App\Entity\ImagePost;
class AddPonkaToImage
{
private $imagePost;
public function __construct(ImagePost $imagePost)
{
$this->imagePost = $imagePost;
}
... lines 15 - 19
}

And really... you can make this class look however you want: we could have made this a public property with no need for a constructor or getter. Or you could replace the constructor with a setImagePost(). This is the way I like to do it... but it doesn't matter: as long as it holds the data you want to pass to the handler... you're good!

Anyways, now we're dangerous! Back in ImagePostController, down here, AddPonkaToImage now needs an argument. Pass it $imagePost.

... lines 1 - 20
class ImagePostController extends AbstractController
{
... lines 23 - 37
public function create(Request $request, ValidatorInterface $validator, PhotoFileManager $photoManager, EntityManagerInterface $entityManager, MessageBusInterface $messageBus)
{
... lines 40 - 59
$message = new AddPonkaToImage($imagePost);
... lines 61 - 63
}
... lines 65 - 95
}

Then, over in the handler, finish this with $imagePost = $addPonkaToImage->getImagePost().

... lines 1 - 10
class AddPonkaToImageHandler implements MessageHandlerInterface
{
... lines 13 - 23
public function __invoke(AddPonkaToImage $addPonkaToImage)
{
$imagePost = $addPonkaToImage->getImagePost();
... lines 27 - 32
}
}

I love it! So that's the power of the message class: it really is like you're writing a message to someone that says:

I want you to do a task and here's all the information that you need to know to do that task.

Then, you hand that off to the message bus, it calls the handler, and the handler has all the info it needs to do that work. It's a simple... but really neat idea.

Let's make sure it all works: move over and refresh just to be safe. Upload a new image and... it still works!

Next: there's already one other job we can move to a command-handler system: deleting an image.

Leave a comment!

This tutorial is built with Symfony 4.3, but will work well on Symfony 4.4 or 5.

What PHP libraries does this tutorial use?

// composer.json
{
    "require": {
        "php": "^7.1.3",
        "ext-ctype": "*",
        "ext-iconv": "*",
        "intervention/image": "^2.4", // 2.4.2
        "league/flysystem-bundle": "^1.0", // 1.1.0
        "sensio/framework-extra-bundle": "^5.3", // v5.3.1
        "symfony/console": "4.3.*", // v4.3.2
        "symfony/dotenv": "4.3.*", // v4.3.2
        "symfony/flex": "^1.1", // v1.4.4
        "symfony/framework-bundle": "4.3.*", // v4.3.2
        "symfony/messenger": "4.3.*", // v4.3.4
        "symfony/orm-pack": "^1.0", // v1.0.6
        "symfony/serializer-pack": "^1.0", // v1.0.2
        "symfony/validator": "4.3.*", // v4.3.2
        "symfony/webpack-encore-bundle": "^1.5", // v1.6.2
        "symfony/yaml": "4.3.*" // v4.3.2
    },
    "require-dev": {
        "symfony/debug-pack": "^1.0", // v1.0.7
        "symfony/maker-bundle": "^1.0", // v1.12.0
        "symfony/test-pack": "^1.0" // v1.0.6
    }
}