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fix: If newData is deeply to the latest state, broadcast the latest state #1697

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Merged
merged 1 commit into from
Dec 9, 2021
Merged

fix: If newData is deeply to the latest state, broadcast the latest state #1697

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6 changes: 6 additions & 0 deletions src/use-swr.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -263,6 +263,12 @@ export const useSWRHandler = <Data = any, Error = any>(
// For local state, compare and assign.
if (!compare(stateRef.current.data, newData)) {
newState.data = newData
} else {
// data and newData are deeply equal
// it should be safe to broadcast the stale data
newState.data = stateRef.current.data
// At the end of this function, `brocastState` invokes the `onStateUpdate` function,
// which takes care of avoiding the re-render
}
Comment on lines 264 to 272
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This is a good fix! And I just took a deeper look and it seems the problem is caused by here:

swr/src/use-swr.ts

Line 372 in 4663154

compare(stateRef.current.data, updatedData)

When they're not equal, we broadcast the new data even if the new data is not defined. So another way to fix it is adding:

isUndefined(updatedData) ||

to that line of code.

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@icyJoseph icyJoseph Dec 8, 2021 •
edited
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Do you have any preference? I guess one if branch vs if-else is more terse?

Also, I commented heavily here, but perhaps I should revert to the original comment?

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I may have misunderstood but

if (isUndefined(newData) || !compare(stateRef.current.data, newData)) {
          newState.data = newData
 }

Still requires the else block right (the test still fails otherwise)? We need to bind newState.data to stateRef.current.data.

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Oh sorry I meant adding it here:

      setState(
        mergeObjects(
          {
            error: updatedError,
            isValidating: updatedIsValidating
          },
          // Since `setState` only shallowly compares states, we do a deep
          // comparison here.
+         isUndefined(newData) || compare(stateRef.current.data, updatedData)
            ? UNDEFINED
            : {
                data: updatedData
              }
        )
      )

I prefer this for now because it is more universal, so we have a way to call broadCastState() but skip updating the data, if it's undefined.

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I tried that too, and it caused a couple other tests to failed, iirc.

Not on my laptop now, but I'll retry first thing in the morning if that's ok.

I also agree that placing the check there seems more correct.

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Yeah, if that doesn’t work we can publish your original fix for sure 👍

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@icyJoseph icyJoseph Dec 9, 2021 •
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The test cases that stop working are:

● useSWR - local mutation › should be able to mutate data to undefined
● useSWR - local mutation › should be able to mutate data to undefined asynchronously

The test description kinda makes it clear why, because when mutating to undefined, with the proposed changed we'd be broadcasting the stale state instead.

Since the proposal didn't work out, I also tried:

 isUndefined(updatedData)
            ? { data: UNDEFINED }
            : compare(stateRef.current.data, updatedData)
            ? UNDEFINED
            : {
                data: updatedData
              }

But this breaks two tests 😅 the first one is introduced on this MR, it implicitly reintroduces the initial issue.

● useSWR - loading › should avoid extra rerenders is the data is the `same` 
● useSWR - configs › should read the config fallback from the context

I think it'd be an ok trade-off to patch this now, and investigate further.

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Thank you, yeah that makes total sense!


// For global state, it's possible that the key has changed.
Expand Down
47 changes: 46 additions & 1 deletion test/use-swr-loading.test.tsx
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
import { act, screen, fireEvent } from '@testing-library/react'
import React from 'react'
import React, { useEffect } from 'react'
import useSWR from 'swr'
import {
createResponse,
Expand Down Expand Up @@ -77,6 +77,51 @@ describe('useSWR - loading', () => {
expect(dataLoaded).toEqual(true)
})

it('should avoid extra rerenders is the data is the `same`', async () => {
let renderCount = 0,
initialDataLoaded = false,
mutationDataLoaded = false

const key = createKey()
function Page() {
const { data, mutate } = useSWR(
key,
async () => {
const res = await createResponse({ greeting: 'hello' })
initialDataLoaded = true
return res
},
{ fallbackData: { greeting: 'hello' } }
)

useEffect(() => {
const timeout = setTimeout(
() =>
mutate(async () => {
const res = await createResponse({ greeting: 'hello' })
mutationDataLoaded = true
return res
}),
200
)

return () => clearTimeout(timeout)
}, [mutate])

renderCount++
return <div>{data?.greeting}</div>
}

renderWithConfig(<Page />)
screen.getByText('hello')

await act(() => sleep(1000)) // wait
// it doesn't re-render, but fetch was triggered
expect(initialDataLoaded).toEqual(true)
expect(mutationDataLoaded).toEqual(true)
expect(renderCount).toEqual(1)
})

it('should return enumerable object', async () => {
// If the returned object is enumerable, we can use the spread operator
// to deconstruct all the keys.
Expand Down