Scheduling in React

Re:Scheduling in React by gambler on Thursday March 07, 2019 @04:10PM (#19332478)

You can significantly improve on the baseline performance simply by doing:

  handleChange = event => {
    const value = event.target.value;
    this.setState({ inputValue: value });

    setTimeout(() => {
      this.props.onChange(value);
    }, 1);

    setTimeout(() => {
      sendAnalyticsPing(value);
    }, 1);
  };

And wrapping mining function inside Name():

  setTimeout(() => {
    miningBitcoin(2);
  }, 1);

In fact, to me this pretty much seems as fast as their fancy solution. But what do I know, I'm just an idiot full-stack developer who doesn't use JS frameworks.

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Re:Scheduling in React by acemarke on Thursday March 07, 2019 @04:15PM (#19332536)

How does this "improve" at all?

In fact, that example will eliminate any chance React might have of batching multiple updates into a single render pass.

React wraps all event handlers in a call to `unstable_batchedUpdates()`. All updates that are queued in that event handler will be batched together.

Splitting updates into multiple ticks will keep them from being batched.

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Re:Scheduling in React by gambler on Thursday March 07, 2019 @04:24PM (#19332615)

Try it instead of theorizing:

https://codesandbox.io/s/07293kpnn

The UI is nearly as responsive as their improved example.

I'm not a React expert, but I do know that browsers defer timeouts until they have time to process them. This can be used to take heavy-hitting functions out of events that block UI interactions. It can also be used for self-throttling recursive IIFEs.

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Re:Scheduling in React by philipp-spiess on Thursday March 07, 2019 @04:40PM (#19332761)

If you delay the artificial slowness that I added in the Text component after the rendering, it sure will be faster. The point is to simulate an expensive component tree with this.

If we keep that in (https://codesandbox.io/s/93yv4j129p), my previous comment holds true: https://news.ycombinator.com/item?id=19332577

Here is by the way a version with all artificial slowness removed: https://codesandbox.io/s/9859x0wm9p You can see that this example does not need any optimizations out of the box. This is why I added the slowness to simulate a more complex app.

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Re:Scheduling in React by danabramov on Thursday March 07, 2019 @05:36PM (#19333298)

The important part btw is that in Concurrent Mode we can interrupt a render in the middle if user does an interaction, and handle that interaction first. Without blocking the thread.

This is something setTimeout is incapable of helping with. Because even if you delay the work, at some point it’s still gonna block.

In either case this example is very simplified and doesn’t illustrate the subtle difference as much. We’ll prepare our own examples when the features are stable.

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Re:Scheduling in React by gambler on Thursday March 07, 2019 @06:18PM (#19333653)

>in Concurrent Mode we can interrupt a render in the middle if user does an interaction

Can you interrupt an arbitrary user-written function that's called inside the rendering stack?

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Re:Scheduling in React by philipp-spiess on Thursday March 07, 2019 @06:24PM (#19333689)

No, that's not possible in JavaScript. The important part is that React components are lazily evaluated[1] and are not rendered as a stack. So React can render a few components and then pause for higher priority work.

As a user of the framework, you don't need to care about this though. So your mental model can be that of an interruptable function.

[1]: https://overreacted.io/react-as-a-ui-runtime/#lazy-evaluatio...

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Re:Scheduling in React by ivanwonder on Friday March 08, 2019 @01:24AM (#19335549)

I think to interrupt a render means interrupt the process of reconciliation. you don't have to response the user input until the difference of whole React element has been found. https://overreacted.io/react-as-a-ui-runtime/#consistency

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Re:Scheduling in React by acemarke on Thursday March 07, 2019 @04:35PM (#19332718)

And I would say I reasonably qualify as a "React expert", and was having a discussion on this topic yesterday:

https://twitter.com/acemarke/status/1103373148169347072

There's a lot of nuance involved here. Moving some behavior outside the current tick may speed certain things up. Keeping multiple React updates in the same tick may _also_ speed other things up. It depends on what work is being done in these additional callbacks, and where/when they're being executed.

In this specific case: `this.props.onChange()` calls the parent component's `setState()`. That should ideally be kept in the same tick as `this.setState()`, so they can be batched.

`sendAnalyticsPing()` does _not_ cause a React update. That should indeed be moved out to a separate tick, so that it doesn't slow down this update.

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Re:Scheduling in React by philipp-spiess on Thursday March 07, 2019 @04:19PM (#19332577)

It feels a bit faster, yes. The reason is that the input value is updated earlier.

The problem is, that the UI is still unresponsive when the timeouts fire which you feel if you type fast or look at the animation/fps meter.

Instead of a setTimeout, you could also debounce the two expensive updates (as I've noted in the post). This would at least make sure that they are batched so if you type fast you only need to re-render the list once.

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Re:Scheduling in React by exogen on Thursday March 07, 2019 @07:20PM (#19334002)

In addition to everyone else's comments, there's another big problem: browsers treat setTimeout delays as very loose suggestions.

If you're scrolling when one of these timeouts is set, or one starts and then you start scrolling, it's possible for the browser to delay the callback for a super noticeable amount of time, on the order of 3–10 seconds (about a thousand times slower than you intended).

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