Porting Rules code from Drupal 7

Porting code to Drupal 8 or Drupal 9 is a big deal - there are so many changes between D7 and D8/9 it's tempting to re-write your code from scratch rather than try to port it. This is hindered by lack of documentation for a lot of Drupal 8/9 - this lack of documentation is much much worse for contributed modules like Rules. Many times you have to resort to reading code to figure out how things are supposed to work, because there is no documentation and no examples!

By providing some examples here, I hope I can ease this painful process. These examples complement the more detailed description found later in this documentation - I'm not going to try to explain too much here, just present both 'before' and 'after' code that covers many typical use cases, so you can shortcut much of the process.