Fetching the Catalog

  override suspend fun loadBooks(): List<Book> {
    when (val result = apiCall { api.books() }) {
      is ApiResult.Success -> {
        val now = System.currentTimeMillis()
        dao.upsertAll(result.data.map { it.toEntity(fetchedAt = now) })
        sync.updateData { SyncState(source = "network", books = result.data.size, at = now) }
        online.value = true
        Log.d("BookNestNet", "refreshed ${result.data.size} books from the network")
      }
      ...
    }
    val cached = dao.getAll()                              // the cache answers either way
    if (cached.isEmpty()) throw IOException("No connection and nothing saved yet")
    return cached.map { it.toBook() }
  }