Register extension in config.neon:

extensions:
    autoRegister: Ublaboo\DirectoryRegister\DI\AutoRegisterExtension

With this extension, all classes within specified directories will be automatically registered in DIC. E.g.:

autoRegister:
    dirs: [
        App\Forms: %appDir%/Forms,
        App\ORM\Repository: %appDir%/ORM/Repository
    ]
    interfaces: [
        IFormFactory: %appDir%/Forms,
    ]
    skip: [App\Forms\ContactFormFactory]

Always you have to specify namespace for classes in partucular directory. Let's take following directorystructure as an example:

app/
    Forms/
        ContactFormFactory.php
        ProductFormFactory.php
        SignInFormFactory.php
        FooFormFactory.php
        BarFormFactory.php
        BazFormFactory.php

See, now above configuration will be the same as writing all these lines in config.neon:

services:
    - App\Forms\ProductFormFactory
    - App\Forms\SignInFormFactory
    - App\Forms\FooFormFactory
    - App\Forms\BarFormFactory
    - App\Forms\BazFormFactory

This extension is simple and fast, but only works with projects that stick to PSR-0 or PSR-4. So you have to have all automatically registered classes named same as the file is and neither of your files can contain other classes.