The last thing we are going to talk about in this chapter is entity validation and how we can make sure that field and entity data as a whole contains valid data. When I say valid, I don't mean whether it complies with the strict TypedData definition but whether, within that, it complies with certain restrictions (constraints) we impose on it.
Drupal 8 uses the Symfony Validator component for applying constraints and then validating entities, fields and any other data against those constraints. I do recommend that you check out the Symfony documentation page on this component to better understand its principles. For now, let's quickly see how it is applied in Drupal 8.
There are three main parts to a validation--a ...
