Dependency Inversion: Point the Arrow at an Abstraction

You sit down to unit-test orderShipped() and hit a wall: the first line of the service is new EmailSender(), so the test either sends a real email or fights a framework to mock a constructor. The high-level policy — when an order ships, tell the customer — is welded to a low-level detail: SMTP. Change transports, and the policy changes. Test the policy, and you test SMTP.

The members-only walkthrough is where the arrow actually flips: an interactive graph you toggle between the welded and inverted wiring, the before/after refactor in code, the inversion-vs-injection distinction, the real-world frameworks that are inversion engines, the tradeoffs, and an interview corner with a coding challenge and quiz.