Injecting dependencies

A complex collaborator is a required parameter. Services, engines, controllers, coordinators, repositories, clients — whoever configures the object supplies them, not the object itself.

• Defaults are for simple leaves: a scheduler, a clock, a timer source, a formatter — no dependencies of their own, and the default is the answer rather than a guess.

The sentinel default is the anti-pattern

init(engine: Engine? = nil) {
    self.engine = engine ?? Engine(...)     // ← this
}

nil here means "construct it yourself" — the one thing the parameter existed to prevent.

• The dependency is invisible. Nothing at the call site says this type needs an Engine.
• Two construction paths. Production takes one, tests the other, so the shipped path is the least exercised.
• The default outlives its reason, and nothing fails when it stops being right.
• The constraint is the signal. A default that would need another parameter's value to build is the language saying the object cannot construct this itself. Pass it in.

Move construction, do not add an escape hatch

• When a test needs to see something a type builds internally, move the construction to the call site. Do not add a parameter only tests use — the test then verifies the path nobody ships.
• Move the new/init to whoever already decides how the object is configured; test and production become one path.

Two parameters that must be one instance

When two collaborators must share the identical object — a cache for a reader and a writer, a session for two clients — bind it to a local at the call site and pass it to both:

let session = Session(...)
let reader = Reader(session: session)
let writer = Writer(session: session)

The sharing is visible where it is established. Constructing twice, or documenting the requirement in a comment, leaves it for a refactor to break silently.

What this buys

• Every dependency in the signature is one a reader sees, a test replaces and a change traces.
• Wiring moves out to a composition point — where that knowledge belongs.
• A parameter list that feels too long is information: the type has too many responsibilities. Defaults hide the evidence, not the responsibilities.