Feature request - New attribute @runOnce - for tasks that should run only once

[ondrejsevcik]

Hi,

in our app, we use following pattern where we load data only once quite a lot.

Here is an example

// services/user-session.js
loadData: task(...).drop(),

async loadOnce() {
  if (this.loadData.last || this.loadData.isRunning) {
    await this.loadData.last;
    return;
  }
  await this.loadData.perform();
  return;
}

and then in our components and routes, we can call await userSession.loadOnce() and do not worry to make accidentally multiple requests against our server.

---

It would be nice to have this baked-in into this library so that we can just add this modifier directly on the task itself.

loadData: task(...).runOnce(); // could be also called .onlyOnce()

What do you think?

[maxfierke]

One thing I've seen before is adding the check within the task, something like:

@dropTask *myTask() {
  if (this.myTask.lastSuccessful?.value) {
    return this.myTask.lastSuccessful?.value;
  }
  
  // ... other stuff
}

This will basically ensure the same result is carried forward every time the task runs. If you were to run cancelAll({ resetState: true }) on the task, you would then clear out the lastSuccessful state and be able to recompute it, if that's useful too.

I'm working on an RFC for user-definable modifiers, and this seems like maybe a good space for that, rather than inclusion within core ember-concurrency, as I'm not sure how broadly applicable it is, other than the one-time data loading case. (I do think it's useful though!)