nullables-testing-style

To write state-based tests without mocks, use the Nullables style by instantiating classes with `new`, wiring dependencies via `.createNull()`, and verifying observable output through output tracking instead of spying on internal calls.

The Nullables testing pattern is an approach that uses infrastructure wrappers and configurable null dependencies to isolate I/O. Use it when refactoring code or creating sociable, example-driven tests to avoid mocks and simulate environment-bound behavior safely.

Configure null dependencies by implementing `.create()` and `.createNull()` factory methods on your infrastructure wrappers. This provides configurable responses for external events and isolates the class under test from real I/O environments during testing.

Verify external event behavior without spies by using behavior simulation within infrastructure wrappers and output tracking. This mechanism records observable changes and state transitions, allowing you to assert results directly rather than inspecting mock interactions.

The best way to remove mock dependencies is introducing infrastructure wrappers at the environment boundary. Replace mock setups with explicit instances and `.createNull()` methods, enabling state-based tests that are easier to read and reason about.

Avoid the Nullables testing style when you cannot modify the environment boundary to introduce infrastructure wrappers, or when your codebase strictly requires traditional mock frameworks to verify internal function calls rather than observable state.