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apps/desktop/src/lib/usePeerPresence.ts

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+import type { PresenceState } from '@chat-app/shared/supabase';
+import { useEffect, useState } from 'react';
+
+import { supabase } from './supabase';
+
+// Subscribe to a single peer's presence_state via Supabase realtime.
+// Returns null until the first row arrives, or when userId is undefined.
+export function usePeerPresence(userId: string | undefined): PresenceState | null {
+  const [presence, setPresence] = useState<PresenceState | null>(null);
+
+  useEffect(() => {
+    if (!userId) {
+      setPresence(null);
+      return;
+    }
+    let cancelled = false;
+
+    void supabase
+      .from('profiles')
+      .select('presence_state')
+      .eq('user_id', userId)
+      .maybeSingle()
+      .then(({ data }) => {
+        if (!cancelled) setPresence(data?.presence_state ?? null);
+      });
+
+    const channel = supabase
+      .channel('peer-presence:' + userId)
+      .on(
+        'postgres_changes',
+        {
+          event: 'UPDATE',
+          schema: 'public',
+          table: 'profiles',
+          filter: 'user_id=eq.' + userId,
+        },
+        (payload: { new: Record<string, unknown> }) => {
+          const next = payload.new['presence_state'];
+          if (typeof next === 'string') setPresence(next as PresenceState);
+        },
+      )
+      .subscribe();
+
+    return () => {
+      cancelled = true;
+      void supabase.removeChannel(channel);
+    };
+  }, [userId]);
+
+  return presence;
+}