A wizard input: the user picks a Home Assistant entity, and the client sends its entity id to the device.

• id — a string you choose, unique in the document. Its key on the wire is Espex.Wizard.fnv1_hash(id), which Espex.Wizard.Document.new/1 fills in as key.
• description — optional text shown with it, up to 255 bytes
• multiple — the user may pick several entities (default false)
• max_length — only with multiple: true: the longest list, in bytes joined by ,, the device accepts. 255-4095; 255 when nil.
• entity_filters — Espex.Wizard.EntityFilter structs limiting what the user may pick; any entity when empty
• links — Home Assistant state subscriptions that follow the picked entity (see "Linked inputs" below); none by default

espex hands the chosen ids to Espex.Wizard.input_set/2 by key.

Linked inputs

An input with links also subscribes to the picked entity's state, like ESPHome's homeassistant sensors linked to a wizard input. Each link is an Espex.HomeAssistantStates.Subscription without an entity_id (or a keyword list of its fields); the picked entity id fills it in:

%Input{
  id: "outdoor_temperature",
  entity_filters: [%EntityFilter{domain: ["sensor"]}],
  links: [[], [attribute: "unit_of_measurement"]]
}

When the client sets the input, the filled-in entries join the subscription table, replacing the ones from the input's previous value. Clients that subscribed to Home Assistant states, the one that set the input included, are sent the new entries, and the states arrive at Espex.HomeAssistantStates.handle_state/3.

• A multiple: true input cannot have links.
• Setting the value the input already has sends nothing. ESPHome firmware sends the entries again, which makes Home Assistant track the entity twice.
• Changing the value from A to B and back to A sends A's entries again, so the device gets A's current state. Home Assistant then tracks A twice on that connection, as with firmware. Every new value adds a tracked entity on each subscribed connection until it reconnects, as the protocol has no unsubscribe.
• Links live in memory and belong to the device, not to the client that set them: they stay after it disconnects, until a client sets the input again or Espex.update_adapters/2 sets a new :wizard module. Clients send every input again when they connect, before subscribing to states, so nothing is stored.