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Custom validation of multiple, dependent entity fields in Drupal 8

Drupal 8 custom validation of multiple entity fields

In a recent Drupal 8.6 project, I was using the media entity type to add custom fields to pdf documents. These pdf files contain two fields: a select list and a text field. When a specific select option is chosen, the text field becomes required. I could have used a form alter function to add my custom validation for these fields, but this would limit my validation to just one form. This wouldn't work for entities that were created with JSON API or other methods.