command bus

[gnugat]

command bus

[gnugat]

Hi @FlorianCcj, I'm glad I can help!

If you consider the following interfaces:

<?php

interface CommandHandler
{
    public function supports($command): bool
    public function handle($command);
}

interface CommandBus
{
    public function register(CommandHandler $commandHandler);
    public function handle($command);
}

You can have a "base" CommandBus implementation that registers CommandHandlers, and then when given a Command will try to find a Handler that supports it:

<?php

class RegisteringCommandBus implements CommandBus
{
    private $commandHandlers = [];

    public function add(CommandHandler $commandHandler)
    {
        $this->commandHandlers[] = $commandHandler;
    }

    public function handle($command)
    {
        foreach ($this->commandHandlers as $commandHandler) {
            if ($commandHandler->supports($command)) {
                return $commandHandler->handle($command);
            }
        }
        throw new NotSupportedException('The given Command is not supported by any registered CommandHandlers.');
    }
}

For most use cases, this is enough, but sometimes you want to always execute some code before or after all or some CommandHandlers, which is where the "Middleware" pattern is used: you create an implementation that does that extra task, and you inject in it the "base" implementation so you can call it before or after that task.

For example here's a CommandBus implementation that always log a message after any command handler have been executed:

<?php

class LogCommandBus implements CommandBus
{
    private $commandBus;
    private $logger;

    public function __construct(
        CommandBus $commandBus,
        Logger $logger
    ) {
        $this->commandBus = $commandBus;
        $this->logger = $logger;
    }

    public function handle($command)
    {
        $value = $this->commandBus->handle($command);
        $this->logger->log("C'est pas faux");
 
        return $value;
    }
}

In most of my use cases, I have no use for Middleware, so I usually skip the CommandBus entitrely but still stick to the Command / CommandHandler. You can read more about this here: http://gnugat.github.io/2017/09/20/pragmaticlean-command-bus.html

I hope this is useful, let me know if you have more questions.