useTaskPrefetch

takprefetch.ts

export function reviveTaskInstance(instance: TaskInstance<any>) {
  if (instance.isError) {
    instance._deferredObject.promise = Promise.reject(instance.error);
  } else {
    // TODO: fix this
    instance._deferredObject.promise = Promise.resolve(instance.value) as unknown as Promise<void>; 
  }

  instance.cancel = () => {};
  instance.canceledOn = (signal) => instance;
  instance._run = () => {};
  instance.then = instance._deferredObject.promise.then;
  instance.catch = instance._deferredObject.promise.catch;
  instance.finally = instance._deferredObject.promise.finally;
}

export function useTaskPrefetch<T>(key : string, task: Task<T, any>) : TaskInstance<T> {
  let taskInstance;
  const { $ssrContext } = getCurrentInstance() as any;

  /* Server */

  if (process.server) {
    // perform, add to prefetch, add to ssrContext
    const taskInstance = task.perform();
    onServerPrefetch(async () => {
      try {
        await taskInstance;
        if ($ssrContext?.nuxt) {
          if (!$ssrContext.nuxt.vueConcurrency) {
            $ssrContext.nuxt.vueConcurrency = {};
          }
          
          $ssrContext.nuxt.vueConcurrency[key] = { instances: task._instances };
        }
      } catch (e) {
        // no need for extra handling
      }
    });
    return taskInstance;
  }

  /* Client */

  const nuxtData = (window as any).__NUXT__;
  if (!nuxtData) {
    throw Error(`Could not access  window.__NUXT__`);
  }

  const taskCache = nuxtData.vueConcurrency[key];
  if (taskCache) {
    task._instances = taskCache.instances || [];
    task._instances.forEach(reviveTaskInstance);
    delete nuxtData.vueConcurrency[key];
    return task.last as TaskInstance<T>;
  } else {
    return task.perform();
  }
}

usage.ts

    const fetchTask = useTask(function*() {
      return ajax('/some/api');
    });
    useTaskPrefetch('data', fetchTask);