Optional layout surface. Callers may select a view or Hiccup template. Custom views accept a spec with :props and content forms. :container/content places content inside a template; without a slot it is appended at the root.

Optional source sentinel; shared visibility owns acquisition and cleanup.

Runtime constructor; static definitions should use defc for the inline path.

Start a component’s declared dependencies without constructing or mounting it.

Resolve exported Vars at render time; Reagent 2 defc values are descriptors, not callable functions. Keep module Vars for development hot reloading.