Maybe should pre-trash Promises if registerPromise called after unmount?

[glasser]

I think it makes sense that if registerPromise is called after unmount, that it should return something like new Promise(() => {}), ie a Promise that never will resolve.

I ran into this in this situation: I had forgotten to register one particular Promise, and so the component unmounted (because of a client-side redirect that occurred on page load) before most of my Promises even ran, and so the fact that I registered later Promises was irrelevant.

Here's how I'm using this by the way:

  componentDidMount() {
    this.pollInterval = setInterval(() => this.poll(), 30 * 1000);
    this.poll();
  }
  componentWillUnmount() {
    if (this.pollInterval) {
      clearInterval(this.pollInterval);
      this.pollInterval = null;
    }
  }
  poll() {
    this.props.registerPromise(Promise.resolve())
      .then(() => this.props.registerPromise(this.checkStatus()))
      .then(details => details || this.props.registerPromise(this.checkIncident()))
      .then(details => details || this.props.registerPromise(this.checkMaintenance()))
      .then(details => this.setState({details}));
  }

When I was missing the registerPromise around the initial Promise.resolve() I would trigger the issue described here every time I loaded the page that did a redirect (and thus a component unmount) on startup.

Arguably this was me mis-using the API by forgetting a registration, but it still seems like this should have been able to work.

[hjylewis]

Hmm, interesting. So I don't want to encourage calling registerPromise after unmounting since that references this and prevents the React object from getting GC'd.

But on the other hand, what happens now when you don't have the registerPromise around the initial Promise.resolve()? Do to promises go on to be called? 😬

[glasser]

Yep!

So arguably maybe the (or "a") problem is that I'm registering the promise asynchronously, and I should only ever be calling registerPromise at a time that is "known untrashed"?

Honestly I kinda just want to have to call registerPromise once and have it be "viral".

ie, maybe sooooomehow, the way that base trashable should work is that if you trash a Promise, you automatically trash any Promise made from a chain of then or catch on it too. Is that too awful?

[hjylewis]

Yeah, I think that would be the goal. It IS annoying to have to wrap all those chained promises.

If we were able to create an actual TrashablePromise as proposed here in hjylewis/trashable#6, we might be able to wrap the returned values of the handles to chain together Trashable Promises so you could write:

    this.props.registerPromise(Promise.resolve())
      .then(() => this.firstPromise())
      .then(() => this.secondPromise())
      .then(() => this.thirdPromise());

instead of:

    this.props.registerPromise(Promise.resolve())
      .then(() => this.props.registerPromise(this.firstPromise()))
      .then(() => this.props.registerPromise(this.secondPromise()))
      .then(() => this.props.registerPromise(this.thirdPromise()));

Which would be great. Not sure if possible though...