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Allow useReducer to bail out of rendering by returning previous state #14569

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merged 4 commits into from Jan 17, 2019

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@acdlite acdlite commented Jan 11, 2019

This is conceptually similar to shouldComponentUpdate, except because there could be multiple useReducer (or useState) Hooks in a single component, we can only bail out if none of the Hooks produce a new value. We also can't bail out if any the other types of inputs — props and context — have changed.

These optimizations rely on the constraint that components are pure functions of props, state, and context.

In some cases, we can bail out without entering the render phase by eagerly computing the next state and comparing it to the current one. This only works if we are absolutely certain that the queue is empty at the time of the update. In concurrent mode, this is difficult to determine, because there could be multiple copies of the queue and we don't know which one is current without doing lots of extra work, which would defeat the purpose of the optimization. However, in our implementation, there are at most only two copies of the queue, and if both are empty then we know that the current queue must be.

Summary of changes:

  • We use the PerformedWork effect tag bit to track whether a component's props, state, or context has changed. This means we need to mark it dirty every time we detect that one of those inputs has a new value. Props and state are fairly straightforward, but context is tricky. I needed to store something on the fiber that represents whether it contains a pending context update. I didn't want to add an additional fiber field, because most fibers do not read from context. So I changed the existing firstContextDependency field to contextDependencies. Instead of pointing to the first item in the list, it points to a list object contains both first and expirationTime.
  • If a component bails out, we can remove the update priority flag from the current fiber. Normally we only mutate the current tree in the commit phase, never in the render phase. In this case it's ok to do because the update priority field only affects whether React needs to visit that fiber. If we receive new props, state, or context, then the fiber will be visited again anyway.
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@sizebot sizebot commented Jan 11, 2019

ReactDOM: size: 🔺+0.4%, gzip: 🔺+0.4%

Details of bundled changes.

Comparing: 8a12009...6c9ae30

react-dom

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react-dom-test-utils.development.js 0.0% -0.0% 44.59 KB 44.59 KB 12.24 KB 12.24 KB NODE_DEV
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@acdlite acdlite force-pushed the acdlite:hook-state-bailout branch 3 times, most recently from f3804aa to 6a1c6f8 Jan 11, 2019
@acdlite acdlite force-pushed the acdlite:hook-state-bailout branch 2 times, most recently from d076142 to 695f36f Jan 14, 2019
@acdlite acdlite force-pushed the acdlite:hook-state-bailout branch from 695f36f to 4c65d54 Jan 16, 2019
@gaearon
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@gaearon gaearon commented Jan 16, 2019

We should also change the SSR, no?

For example

function Foo(props) {
  const [name, setName] = useState('Dan');
  if (props.name === 'Andrew') {
    setName(props.name);
  }
  return name; 
}

renderToString(<Foo name="Andrew" />

would previously go into an infinite loop but I think should now stabilize.

@acdlite
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@acdlite acdlite commented Jan 16, 2019

@gaearon The eager bailout doesn't apply to render phase updates, but that's an interesting idea.

@acdlite
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@acdlite acdlite commented Jan 16, 2019

I think we should only eagerly compute render phase updates and bail out if we're sure the bailout can be relied on completely and predictably. Not sure about that. Seems safer to always require the guard around the update.

@sebmarkbage
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@sebmarkbage sebmarkbage commented Jan 16, 2019

The guard provides a natural place to other code that doesn't need to be evaluated if nothing has changed so it seems good to encourage that.

@sebmarkbage
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@sebmarkbage sebmarkbage commented Jan 16, 2019

Although, counter-argument: This optimization isn’t really meant for useState. It’s mostly for useReducer where it is difficult to know if a reducer will yield the same value or not.

acdlite added 3 commits Jan 10, 2019
This is conceptually similar to `shouldComponentUpdate`, except because
there could be multiple useReducer (or useState) Hooks in a single
component, we can only bail out if none of the Hooks produce a new
value. We also can't bail out if any the other types of inputs — state
and context — have changed.

These optimizations rely on the constraint that components are pure
functions of props, state, and context.

In some cases, we can bail out without entering the render phase by
eagerly computing the next state and comparing it to the current one.
This only works if we are absolutely certain that the queue is empty at
the time of the update. In concurrent mode, this is difficult to
determine, because there could be multiple copies of the queue and we
don't know which one is current without doing lots of extra work, which
would defeat the purpose of the optimization. However, in our
implementation, there are at most only two copies of the queue, and if
*both* are empty then we know that the current queue must be.
Should not bail out during subsequent update. (This isn't directly
related to this PR because we should have had this test, anyway.)
@acdlite acdlite force-pushed the acdlite:hook-state-bailout branch from 15ca89d to e7a99f9 Jan 17, 2019
@acdlite acdlite force-pushed the acdlite:hook-state-bailout branch from f178758 to 6c9ae30 Jan 17, 2019
@acdlite acdlite merged commit 790c8ef into facebook:master Jan 17, 2019
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8 participants