Method fetchOptimistic

packages/query-core/src/queryObserver.ts:395–446

* Fetches a query defined by the given options without affecting this * observer's own tracked query or result, and returns a promise that * resolves with the `QueryObserverResult` for that fetch. This is useful * for prefetching data that another observer (e.g. a query about to be * nav

(
    options: QueryObserverOptions<
      TQueryFnData,
      TError,
      TData,
      TQueryData,
      TQueryKey
    >,
  )

   * ```
   */
  fetchOptimistic(
    options: QueryObserverOptions<
      TQueryFnData,
      TError,
      TData,
      TQueryData,
      TQueryKey
    >,
  ): Promise<QueryObserverResult<TData, TError>> {
    const defaultedOptions = this.#client.defaultQueryOptions(options)

    const query = this.#client
      .getQueryCache()
      .build(this.#client, defaultedOptions)

    let unsubscribe = () => {}
    let resolveEarly:
      | ((result: QueryObserverResult<TData, TError>) => void)
      | undefined

    const cachePromise = new Promise<QueryObserverResult<TData, TError>>(
      (resolve) => {
        resolveEarly = resolve
        unsubscribe = this.#client.getQueryCache().subscribe((event) => {
          if (
            event.type === 'updated' &&
            event.query.queryHash === query.queryHash &&
            query.state.data !== undefined
          ) {
            unsubscribe()
            resolve(this.createResult(query, defaultedOptions))
          }
        })
      },
    )

    return Promise.race([
      query
        .fetch()
        .then(() => {
          const result = this.createResult(query, defaultedOptions)
          // Settle the subscriber promise so both branches always settle.
          // This value is ignored by Promise.race since the fetch branch already won.
          resolveEarly?.(result)
          return result
        })
        .finally(() => {
          unsubscribe()
        }),
      cachePromise,
    ])
  }

  protected fetch(
    fetchOptions: ObserverFetchOptions,