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Tutorial on Using Drupal 8 Plugin Derivatives Effectively

In an earlier tutorial, we looked at the Drupal 8 plugin system and how to create our very own custom plugin type. We’ve seen that much of the functionality declared via _info hooks in Drupal 7 has been replaced by these plugins. Our use case was very basic and it allowed each instance of such functionality to be declared manually via a new plugin class and associated form.

Drupal 8 logo

But what if we needed such instances declared dynamically depending on some factors external to our little subsystem? For example, when declaring _info hooks in Drupal 7, we can get a list of something, loop over it and declare a new item in the returned array for each individual something. The menu system does this in order to provide a new block for each menu that either comes with Drupal core or is later created through the UI.

So what about Drupal 8? We’ve seen that for each plugin of a certain type we need to declare a different PHP class. To create a new block, we need a new class. To create another block, we need another class. So where would that looping we see in Drupal 7 take place? The short answer to this is: within a plugin derivative.

In this article we will explore the long answer to that and learn what derivates are and how we can use them. For the latter, we will build an example inside the demo module that can be found in this git repository and which should hopefully help us better understand what’s going on. For a slightly more complex example, the Menu system is great as it provides an individual block for each of its menus (similar to Drupal 7 but using plugins).

What we are going to do is actually very simple. We are going to implement basic Node Block functionality by which for all the article nodes on our site we will have a block. Ridiculous? Sure. Should we be doing this for all the nodes on our site? Definitely not! But it’s a very basic implementation meant to keep things short and demonstrate the use of the plugin derivatives.

Plugin Derivatives

Plugin derivatives are the way through which a plugin of a certain type can be represented in the system as multiple instances of itself. In other words, a plugin can reference a deriver class which is responsible for providing a list of plugin definitions that are based on the initial plugin (start from the same base definition) but have slightly different configuration or definition data. The SystemMenuBlock we referred to above is a great example. It’s a single plugin which has as many derivatives as there are menus on the site.

To go a bit deeper, when a list of all the plugins of a certain type is requested, the plugin manager uses its discovery mechanism to load all the plugins of this type. If that mechanism is decorated with the DerivativeDiscoveryDecorator, the manager will be able to also retrieve derivatives. In order to do this, the derivative discovery looks for a deriver class on each plugin and, if it finds one, asks it for this list.

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