feat: make VCsWatcher, a watch channel of VCs

src/command/join.rs

@@ -67,10 +67,10 @@ fn get_guild_and_voice_channel_id(
         .and_then(|member| member.user.as_ref().map(|user| user.id))
         .context(NoUserSnafu)?;
 
-    let voice_channel_id = vcs
-        .with_guild(guild_id, |guild_vcs| {
-            guild_vcs.get_left_for(&user_id).copied()
-        })
+    let &voice_channel_id = vcs
+        .get(&guild_id)
+        .context(UserNotInVCSnafu)?
+        .get_left_for(&user_id)
         .context(UserNotInVCSnafu)?;
 
     Ok((guild_id, voice_channel_id))
@@ -236,33 +236,33 @@ impl EventHandler for Handler {
 
 #[tracing::instrument(skip(state))]
 pub async fn handle(state: State, interaction: Interaction) {
-    let vcs = state.vcs;
+    let guild_and_voice_channel_id_res = get_guild_and_voice_channel_id(&interaction, &state.vcs_watcher.borrow());
+    let (guild_id, voice_channel_id) =
+        match guild_and_voice_channel_id_res {
+            Ok((guild_id, voice_channel_id)) => (guild_id, voice_channel_id),
+            Err(error) => {
+                state
+                    .discord_client
+                    .interaction(state.discord_application_id)
+                    .create_response(
+                        interaction.id,
+                        &interaction.token,
+                        &InteractionResponse {
+                            kind: InteractionResponseType::ChannelMessageWithSource,
+                            data: Some(
+                                InteractionResponseDataBuilder::new()
+                                    .embeds([get_guild_and_vc_error_to_embed(error)])
+                                    .flags(MessageFlags::EPHEMERAL)
+                                    .build(),
+                            ),
+                        },
+                    )
+                    .await
+                    .expect("TODO");
 
-    let (guild_id, voice_channel_id) = match get_guild_and_voice_channel_id(&interaction, &vcs) {
-        Ok((guild_id, voice_channel_id)) => (guild_id, voice_channel_id),
-        Err(error) => {
-            state
-                .discord_client
-                .interaction(state.discord_application_id)
-                .create_response(
-                    interaction.id,
-                    &interaction.token,
-                    &InteractionResponse {
-                        kind: InteractionResponseType::ChannelMessageWithSource,
-                        data: Some(
-                            InteractionResponseDataBuilder::new()
-                                .embeds([get_guild_and_vc_error_to_embed(error)])
-                                .flags(MessageFlags::EPHEMERAL)
-                                .build(),
-                        ),
-                    },
-                )
-                .await
-                .expect("TODO");
-
-            return;
-        }
-    };
+                return;
+            }
+        };
 
     state
         .discord_client