nip17: "seen" event.

17.md

@@ -8,7 +8,11 @@ Private Direct Messages
 
 This NIP defines an encrypted direct messaging scheme using [NIP-44](44.md) encryption and [NIP-59](59.md) seals and gift wraps.
 
-## Direct Message Kind
+## Message kinds
+
+Each of these message kinds can be gift-wrapped as described afterwards. Clients have to first unwrap them to then see their kinds.
+
+### Direct Message Kind
 
 Kind `14` is a chat message. `p` tags identify one or more receivers of the message.
 
@@ -31,7 +35,7 @@ Kind `14` is a chat message. `p` tags identify one or more receivers of the mess
 
 `.content` MUST be plain text. Fields `id` and `created_at` are required.
 
-An `e` tag denotes the direct parent message this post is replying to. 
+An `e` tag denotes the direct parent message this post is replying to.
 
 `q` tags MAY be used when citing events in the `.content` with [NIP-21](21.md).
 
@@ -41,7 +45,7 @@ An `e` tag denotes the direct parent message this post is replying to.
 
 Kind `14`s MUST never be signed. If it is signed, the message might leak to relays and become **fully public**.
 
-## File Message Kind
+### File Message Kind
 
 ```jsonc
 {
@@ -59,7 +63,6 @@ Kind `14`s MUST never be signed. If it is signed, the message might leak to rela
     ["decryption-key", "<decryption-key>"],
     ["decryption-nonce", "<decryption-nonce>"],
     ["x", "<the SHA-256 hexencoded string of the file>"],
-    // rest of tags...
   ],
   "content": "<file-url>"
 }
@@ -82,6 +85,46 @@ Kind `15` is used for sending encrypted file event messages:
 
 Just like kind `14`, kind `15`s MUST never be signed.
 
+### "Seen" event
+
+An event of kind `30016` that CAN be emitted automatically by a client whenever a chat window is displayed to the user, even if the user hasn't acted on it.
+
+```jsonc
+{
+  "id": "<usual hash>",
+  "pubkey": "<sender-pubkey>",
+  "created_at": "<current-time>",
+  "kind": 30016,
+  "tags": [
+    ["d", "<subject>"],
+    ["seen", "<latest_message_id>", "<previous_id>", "<...>"]
+  ],
+  "content": ""
+}
+```
+
+- `d` must be set to the conversation subject, or it can be empty or omitted in the most common case of a `<subject>` not existing.
+- `seen` must be set to the id of the last messages seen. It can contain any number of ids, ordered from the newest to the oldest.
+
+This event SHOULD be discarded whenever a new event is received for the same conversation.
+
+Any messages with timestamp before the last item in the `seen` array are assumed to have been seen.
+
+If there is a gap in the `seen` array that indicates a message may have been missed for whatever reason.
+
+For example:
+
+- `alice` sends message `aaaa`
+- `alice` sends message `bbbb`
+- `alice` sends message `cccc`
+- `bob` sends event with `["seen", "bbbb", "aaaa"]`
+- at this point `alice`'s client should display `aaaa` and `bbbb` as seen, `cccc` as unseen
+- `alice` sends message `dddd`
+- `alice` sends message `eeee`, which is lost due to relay malfunction
+- `alice` sends message `ffff`
+- `bob` sends event with `["seen", "ffff", "dddd", "cccc"]`
+- at this point `alice`'s client should display all messages as seen, except for `eeee` which should be displayed with an error indicator
+
 ## Chat Rooms
 
 The set of `pubkey` + `p` tags defines a chat room. If a new `p` tag is added or a current one is removed, a new room is created with a clean message history.