What is a Binding really, and how do derived, constant and transforming bindings behave?

A Binding stores no value: it is a getter and a setter (plus a transaction). $state hands you one that reads and writes the state’s storage; Binding(get:set:) builds one from any two closures, so the “source of truth” can be a model, a computed value or a database. The check shows the forms: writing through a binding derived from a struct binding goes through its setter (writes shows one set), .constant(7) accepts writes and silently ignores them (use it in previews and for read-only controls), a key path such as whole.tags[0] projects a binding to one element, and a transforming binding can store lowercase while presenting uppercase.

Because a binding is only closures, two things follow. Setters run on every write, so put validation and side effects there only if they are cheap, and remember a derived binding calls the parent’s setter with the whole modified value. And a binding does not keep the data alive or observe it: the view that owns the state still decides when to re-render. Converting an optional binding with Binding(optionalBinding) gives you a non-optional one for a sheet or a text field, but reading it after the underlying value became nil traps, so prefer an if let in the view or an item-based API (sheet(item:)) that removes the view when the value disappears. I did not run that crash here: outside a real view graph the conversion misbehaves, so the check does not cover it.