# ChatReaction

One person's reaction to one message, with one emoji. A
[`ChatMessage`](ChatMessage.md) holds its reactions, in the order they were
added, as `reactions`.

```ts
// Shown for illustration only.
interface ChatReaction {
  /** The profile the reactor reacted under. */
  reactorProfile: Cell<ChatProfile>;

  /** A single emoji. */
  emoji: string;

  /** The room's handler clock when it recorded the reaction. */
  sentAt: FabricEpochNsec;
}
```

A reaction is stored as
`AuthoredByCurrentUser<TrustedActionWrite<ChatReaction, …>>`: an object pushed
onto its message's reaction list, a document of its own, admitted only through
the room's `sendReaction` stream, as a trusted gesture on `ChatReactSurface`,
and removed only through its `deleteReaction` stream, on the same surface (see
[`ChatRoomOutput`](ChatRoomOutput.md#streams)). Its identity is its cell.

## Fields

- **`reactorProfile`** links the reactor's profile
  ([`ChatProfile.md`](ChatProfile.md)), which a client reads when it draws, as
  it does for a message's author.
- **`emoji`** is any single emoji: exactly one emoji sequence that [Unicode
  Technical Standard #51](https://www.unicode.org/reports/tr51/) recommends for
  general interchange (`RGI_Emoji`), with its modifiers and joiners included.
  The room refuses anything else, including text, and two emoji written as one.
  A room has no fixed list. Which emoji a client puts within easy reach is the
  client's choice.
- **`sentAt`** is the handler clock when the room recorded the reaction, at
  whatever resolution the system provides (see
  [times](ChatMessage.md#times)). A reaction removed and added again is a new
  reaction, with a new `sentAt`.

## Uniqueness

A message shows at most one reaction per reactor and emoji. The room adds none
where the sender already has one with that emoji, so adding a reaction that's
already there, or removing one that isn't, changes nothing, and either can be
sent again safely. Adding or removing one never changes anyone else's, and two
people reacting at once both land. Two sessions of one person reacting with one
emoji at the same moment can each add one; a client tallies by reactor and
emoji, so the pair shows once, and removing takes the one the room finds. A
reaction is part of its message, but a separately authorized part: only its
reactor writes it, and it goes only by its reactor, or out of reach with its
message when the message is deleted or obliterated (see
[`ChatMessage`](ChatMessage.md#who-wrote-what)).