For anyone who's interested, I've got the bundle working in Symfony 4 in a very 'hacky' way.
Good for a short-term patch/solution.

See the basic installation instructions on how to get bundle overrides for templates working in Symfony 4 with this, that was the only major hurdle I had after changing the installation requirements and renaming template locations to match Symfony 4.

https://github.com/hex333ham/FOSMessageBundle/tree/symfony4

I need it too :) Hacky way from @hairybobbins don't work for me, composer can't resolve paths.

@kubaceg Can you post the full error message from composer please?

Hi, from user entity

Fatal error: Interface 'FOS\MessageBundle\Model\ParticipantInterface' not found in /var/www/symfony/src/Entity/User/User.php on line 10

I'm thinking now that probably after my changes in composer.json (repository + new bundle) and composer install command nothing happens. I must check it once again

@kubaceg It looks to me like your application is failing before it reaches composer, it might be worth commenting out that reference then seeing if it works.

@kubaceg hey, just realised that target-dir was accidentally removed in the composer.json file due to the deprecation of psr-0, this means that if you move the hacky versions code under the correct directory structure within the bundle (FOS\MessageBundle) it will work.

If anyone knows of a way to fix this please let me know, I'll be researching the problem but I am not the best when it comes to composer.

if you're experiencing issues, run composer require friendsofsymfony/message-bundle:dev-symfony4, this will reload the package and has fixed all issues on my end

Thanks! I'll try it :)

It works fine :) only what I have to do was add own entities mapping in yaml + define aliases:

    fos_user.user_to_username_transformer:
        alias: App\Service\UserToUsernameTransformer

    FOS\MessageBundle\Composer\Composer: '@fos_message.composer'

    FOS\MessageBundle\Sender\Sender: '@fos_message.sender'

@kubaceg good spot, I'll add these notes to the install instructions on my fork later

cheers!

@hairybobbins I'm using your fork but it's giving the service error

The "fos_message.provider" service or alias has been removed or inlined when the container was compiled. You should either make it public, or stop using the container directly and use dependency injection instead.

It ran when I put the services as public in the class FOS\MessageBundle\DependencyInjection\FOSMessageExtension.php

Ex.:

class FOSMessageExtension extends Extension
{
//...
    $container->setAlias('fos_message.message_manager', new Alias($config['message_manager'], true));
//...
}

https://pastebin.com/1LWseFuu

@hairybobbins I just submitted a pull request to your fork implementing the fix by @afranioce for all the Aliases.

Thanks for all the suggestions. I used these to get the bundle to work, but now I can't get it to use my custom twig templates. In Symfony 4 they should be placed in /templates/bundles/... Unfortunately, this does not work at all. Any suggestions?

@finkbg This works for me, edit "templates/bundles/FOSMessageBundle/layout.html.twig" to extend your base template. Then change the extends path in the other files (templates/bundles/FOSMessageBundle/Message/.) to
{% extends 'bundles/FOSMessageBundle/layout.html.twig' %}
What errors are you getting?