Exposing custom data to Views
To get a better understanding of how Views works, we are going to look at an example of totally custom data and how we can expose it to Views. Based on that, we will begin to understand the role of various plugins and can begin to create our own. Additionally, we'll be able to expand on our product entity type data to enrich its Views interaction.
To exemplify all of this, we are going to revisit our sports module where we declared the players and teams tables of data and that we will now be exposing to Views. The goal is to allow site builders to create dynamic listings of this data as they see fit. The lessons learned from this example can be applied to other data sources as well, even things such as remote APIs (with some extra work).
Views data
Whenever we want to expose data to Views, we need to define this data in a way Views can understand it. That is actually what EntityViewsData::getViewsData() does for content entities. However...