PassthroughSubject vs CurrentValueSubject?

Middle PRO

The essence

A Subject is a Publisher you feed imperatively via send(_:); that is how delegates, callbacks and user actions get wrapped. Two built-in kinds:

PassthroughSubject — stores no value. A subscriber receives only what is sent after subscribing; anything sent earlier is lost. "Event" semantics: a tap, a notification, a command.
CurrentValueSubject — holds the current value and requires an initial one in init. A new subscriber immediately receives the current value, then the updates. Accessible via .value (read and write — writing also emits). "State" semantics: current user, loading flag.

The rest of this answer, with code and interview follow-ups, is in the DevPrep app.