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An Aside: Dependency Injection Parameters

An Aside: Dependency Injection Parameters

Cleanse the palette of all the forms stuff and open config.yml. Under the doctrine key, we see a bunch of percent sign values:

# app/config/config.yml
# ...

        driver:   "%database_driver%"
        host:     "%database_host%"
        port:     "%database_port%"
        dbname:   "%database_name%"
        user:     "%database_user%"
        password: "%database_password%"
        # ...

Whenever you see something surrounded by two percent signs in a config file, it’s a parameter. Parameters are like variables: you set them somewhere and then use them with this syntax. So where are these being set?

Open up parameters.yml to find the answer:

# app/config/parameters.yml
# ...

# This file is auto-generated during the composer install
    database_driver: pdo_mysql
    database_port: null
    database_name: knp_events
    database_user: root
    database_password: null
    # ...

In episode 1, we talked about how this file is special because it holds any server-specific configuration. This works because it’s in our .gitignore file so that every developer and server can have their own. So we set parameters here and use them anywhere else.

Adding More Parameters

But technically, we can add parameters to any configuration file. Go back to config.yml and add a new parameters key anywhere in the file. Below it, create a new parameter called our_assets_version, and set it to the assets_version value we’re using below:

# app/config/config.yml
    - { resource: parameters.yml }
    - { resource: security.yml }
    - { resource: "@EventBundle/Resources/config/services.yml" }
    - { resource: "@UserBundle/Resources/config/services.yml" }

    our_assets_version: 5-return-of-the-jedi

    # ...

Now, just use it under the framework key:

# app/config/config.yml
# ...

    # ...
        engines: ['twig']
        assets_version: %our_assets_version%
        assets_version_format: "%%s?v=%%s"
    # ...

See, they work just like variables. Refresh to make sure we didn’t break anything.

So now you know what these percent signs are all about. Spoiler alert! You can also access parameters from a controller using $this->container->getParameter, which might come in handy.

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