In this post we’ll see how to save temporary values from a form and how to retrieve or process them later in a controller. To do that, we’ll use the Form API and the private tempstore (the temporary store storage system of Drupal 8).
The use case is the following: we need to build a simple RSS reader (a form) where the user could introduce the URL of an RSS file and the number of items to retrieve from that file. Next, in a new page (a controller), the application should display the list of items with a link to each syndicated page .
The easiest way to achieve it would be to retrieve the values in our buildForm() method, process them and display the result thought a specific field of the form. But that’s not our use case.