studio/src/hooks/useSession.js

import useAppStore from '../store/appStore'
import { getProcessStatus, getTree, readFile } from '../api/projects'

// When opening a session we default to this file if it exists in the tree.
// Just a convention — the React template's entry is here; other templates
// will eventually carry the path in their manifest (Phase 5a).
const DEFAULT_ENTRY = 'frontend/src/App.jsx'

export function useSession() {
  const {
    setSessions, setActiveSessionId, setMessages,
    setProject, resetProject,
    resetSessionTokens, resetInferenceStats,
    setRuntime, resetRuntime, stopStream, clearPacing,
  } = useAppStore()

  async function loadSessions() {
    const res = await fetch('/api/sessions')
    const data = await res.json()
    const sessions = Array.isArray(data) ? data : []
    setSessions(sessions)
    return sessions
  }

  async function createSession(name = 'New Session', template) {
    const body = { name }
    if (template) body.template = template
    const res = await fetch('/api/sessions', {
      method: 'POST',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify(body),
    })
    const session = await res.json()
    await loadSessions()
    return session
  }

  async function openSession(id) {
    // Mid-stream session switch: abort the in-flight chat so it doesn't
    // append to a session the user is no longer looking at.  Same for the
    // pacing pill, which would otherwise stay visible across the switch.
    stopStream()
    clearPacing()

    const res = await fetch(`/api/sessions/${id}`)
    const data = await res.json()
    setActiveSessionId(data.id)
    // Rehydrate historical assistant turns with their tool log so old
    // conversations render the same way they originally streamed.
    const messages = (data.messages || []).map((m) => {
      if (m.role !== 'assistant' || !m.tool_calls_json) return m
      try {
        return { ...m, toolCalls: JSON.parse(m.tool_calls_json) }
      } catch {
        return m
      }
    })
    setMessages(messages)
    resetSessionTokens()
    resetInferenceStats()

    // Reset runtime so the Run button shows the correct state for the new project.
    resetRuntime()

    const projectId = data.project_id
    if (!projectId) {
      resetProject()
      return
    }

    const tree = await getTree(projectId)

    // Prefetch every file.  Scaffold templates are small (<20 files); Phase 5
    // will switch to lazy load when project sizes grow.
    const fileEntries = tree.filter((e) => !e.is_dir)
    const contents = await Promise.all(
      fileEntries.map((e) =>
        readFile(projectId, e.path).then((c) => [e.path, c]).catch(() => null),
      ),
    )
    const files = Object.fromEntries(contents.filter(Boolean))

    // Open the entry file by default if present, otherwise the first file in tree.
    const activeFile = files[DEFAULT_ENTRY] != null
      ? DEFAULT_ENTRY
      : (fileEntries[0]?.path ?? null)

    setProject({ id: projectId, tree, files, activeFile, dirty: {} })

    // Restore process status if dev servers are already running from a previous visit.
    try {
      const processStatus = await getProcessStatus(projectId)
      const { overall, frontend, backend } = processStatus
      if (overall !== 'idle') {
        setRuntime({
          status:         overall,
          frontendPort:   frontend.port,
          backendPort:    backend.port,
          frontendStatus: frontend.status,
          backendStatus:  backend.status,
        })
      }
    } catch { /* not fatal — user can click Run again */ }
  }

  async function deleteSession(id) {
    await fetch(`/api/sessions/${id}`, { method: 'DELETE' })
    const sessions = await loadSessions()
    if (sessions.length > 0) {
      await openSession(sessions[0].id)
    } else {
      setActiveSessionId(null)
      setMessages([])
      resetProject()
    }
  }

  async function renameSession(id, name) {
    await fetch(`/api/sessions/${id}`, {
      method: 'PATCH',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify({ name }),
    })
    await loadSessions()
  }

  return { loadSessions, createSession, openSession, deleteSession, renameSession }
}