decision/frontend/app/composables/useWebSocket.ts

/**
 * Composable for WebSocket connectivity to receive live vote updates.
 *
 * Connects to the backend WS endpoint and allows subscribing to
 * individual vote session channels for real-time tally updates.
 */
export function useWebSocket() {
  const config = useRuntimeConfig()
  let ws: WebSocket | null = null
  let reconnectTimer: ReturnType<typeof setTimeout> | null = null
  const connected = ref(false)
  const lastMessage = ref<any>(null)

  /**
   * Open a WebSocket connection to the backend live endpoint.
   */
  function connect() {
    if (ws && (ws.readyState === WebSocket.OPEN || ws.readyState === WebSocket.CONNECTING)) {
      return
    }

    const wsUrl = config.public.apiBase
      .replace(/^http/, 'ws')
      .replace(/\/api\/v1$/, '/api/v1/ws/live')

    ws = new WebSocket(wsUrl)

    ws.onopen = () => {
      connected.value = true
      if (reconnectTimer) {
        clearTimeout(reconnectTimer)
        reconnectTimer = null
      }
    }

    ws.onclose = () => {
      connected.value = false
      reconnect()
    }

    ws.onerror = () => {
      connected.value = false
    }

    ws.onmessage = (event: MessageEvent) => {
      try {
        lastMessage.value = JSON.parse(event.data)
      } catch {
        lastMessage.value = event.data
      }
    }
  }

  /**
   * Subscribe to real-time updates for a vote session.
   */
  function subscribe(sessionId: string) {
    if (ws?.readyState === WebSocket.OPEN) {
      ws.send(JSON.stringify({ action: 'subscribe', session_id: sessionId }))
    }
  }

  /**
   * Unsubscribe from a vote session's updates.
   */
  function unsubscribe(sessionId: string) {
    if (ws?.readyState === WebSocket.OPEN) {
      ws.send(JSON.stringify({ action: 'unsubscribe', session_id: sessionId }))
    }
  }

  /**
   * Gracefully close the WebSocket connection.
   */
  function disconnect() {
    if (reconnectTimer) {
      clearTimeout(reconnectTimer)
      reconnectTimer = null
    }
    if (ws) {
      ws.onclose = null
      ws.close()
      ws = null
    }
    connected.value = false
  }

  /**
   * Schedule a reconnection attempt after a delay.
   */
  function reconnect() {
    if (reconnectTimer) return
    reconnectTimer = setTimeout(() => {
      reconnectTimer = null
      connect()
    }, 3000)
  }

  return { connected, lastMessage, connect, subscribe, unsubscribe, disconnect }
}