fix(watch): update `oldValue` before running `cb` to prevent stale value by jh-leong · Pull Request #12296 · vuejs/core

jh-leong · 2024-10-30T03:26:50Z

Summary by CodeRabbit

Bug Fixes
- Improved reactivity behavior to ensure watched values are updated correctly during synchronous watcher callbacks.
Tests
- Added a new test case to verify immediate reset functionality with synchronous watchers.

github-actions · 2024-10-30T03:27:52Z

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	101 kB	38.3 kB (-1 B)	34.5 kB (+13 B)
vue.global.prod.js	159 kB	58.5 kB	52 kB (+6 B)

Usages

Name	Size	Gzip	Brotli
createApp (CAPI only)	46.6 kB	18.2 kB	16.7 kB
createApp	54.5 kB	21.2 kB	19.4 kB (-1 B)
createSSRApp	58.8 kB	23 kB	20.9 kB (+1 B)
defineCustomElement	59.5 kB	22.8 kB (+1 B)	20.8 kB (-10 B)
overall	68.6 kB	26.4 kB	24.1 kB (-5 B)

pkg-pr-new · 2024-10-30T03:28:06Z

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@vue/compiler-sfc

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@vue/reactivity

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@vue/runtime-core

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@vue/runtime-dom

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vue

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commit: 9623e8f

coderabbitai · 2025-05-19T00:44:29Z

Walkthrough

A new test case was added to the watch test suite to verify watcher behavior when the watched value is reset synchronously. The watch implementation was modified to update the oldValue before invoking the callback, ensuring correct watcher firing in synchronous scenarios. No changes to public APIs were made.

Changes

File(s)	Change Summary
packages/reactivity/tests/watch.spec.ts	Added a test for "watch with immediate reset and sync flush" to validate watcher reset behavior.
packages/reactivity/src/watch.ts	Moved `oldValue` assignment before callback in watcher logic to ensure correct firing.

Sequence Diagram(s)

sequenceDiagram
    participant ReactiveRef as Reactive Reference
    participant Watcher as Watcher
    participant Callback as Watch Callback

    ReactiveRef->>Watcher: Value changes (e.g., to true)
    Watcher->>Watcher: Update oldValue = newValue
    Watcher->>Callback: Invoke callback (may reset value)
    Callback->>ReactiveRef: Optionally reset value (e.g., to false)
    loop If value changes again
        ReactiveRef->>Watcher: Value changes (again)
        Watcher->>Watcher: Update oldValue = newValue
        Watcher->>Callback: Invoke callback
    end

Assessment against linked issues

Objective	Addressed	Explanation
Ensure synchronous watchers fire correctly when the watched value is reset in the callback (#12294)	✅

Poem

A watcher peeks with eager eyes,
At values flipping, truth and lies.
Now old and new are set just right,
No sneaky bugs escape the night.
With tests to prove our code is sound,
The sync-watch rabbit hops around! 🐇✨

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Knowledge Base: Disabled due to data retention organization setting

📥 Commits

Reviewing files that changed from the base of the PR and between 163b365 and 6f20660.

📒 Files selected for processing (2)

packages/reactivity/__tests__/watch.spec.ts (1 hunks)
packages/reactivity/src/watch.ts (1 hunks)

🔇 Additional comments (2)

packages/reactivity/src/watch.ts (1)

267-267: Critical bugfix that prevents stale values in synchronous watch callbacks

This change ensures that the oldValue is updated before the callback is invoked, which is essential for cases where the callback might trigger additional synchronous changes to the watched value. Previously, the oldValue was updated after the callback, which could lead to incorrect behavior in watchers that immediately reset the watched value.

packages/reactivity/__tests__/watch.spec.ts (1)

281-291: Excellent test case that verifies the bugfix

This test properly validates the fix for issue #12294 by creating a scenario where a watcher callback immediately resets the watched value. The test confirms that when setting the value to true twice in succession, the final value is correctly reset to false, demonstrating that the watcher fires appropriately for each change.

The test case is well-designed:

It creates a reactive ref with an initial value of false

Sets up a watcher that resets the value to false whenever it changes

Makes multiple consecutive assignments to true

Verifies the watcher behaves correctly by resetting to false

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edison1105 · 2025-05-20T00:34:59Z

/ecosystem-ci run

vue-bot · 2025-05-20T00:42:15Z

📝 Ran ecosystem CI: Open

suite	result	latest scheduled
language-tools	✅ success	✅ success
pinia	✅ success	✅ success
nuxt	✅ success	✅ success
primevue	✅ success	✅ success
test-utils	✅ success	✅ success
quasar	✅ success	✅ success
router	✅ success	✅ success
vitepress	✅ success	✅ success
radix-vue	✅ success	✅ success
vite-plugin-vue	✅ success	✅ success
vue-i18n	✅ success	✅ success
vue-macros	✅ success	✅ success
vuetify	✅ success	✅ success
vue-simple-compiler	✅ success	✅ success
vant	✅ success	✅ success
vueuse	✅ success	✅ success

fix(watch): update oldValue before running cb to prevent stale value

9f5eb0c

edison1105 added ready to merge The PR is ready to be merged. scope: reactivity labels Oct 30, 2024

Merge branch 'main' into fix/12294

199f0a9

edison1105 approved these changes Nov 15, 2024

View reviewed changes

edison1105 added the 🔨 p3-minor-bug Priority 3: this fixes a bug, but is an edge case that only affects very specific usage. label May 19, 2025

Merge branch 'main' into fix/12294

6f20660

edison1105 approved these changes May 19, 2025

View reviewed changes

Merge branch 'main' into fix/12294

9623e8f

vuejs deleted a comment from jh-leong May 20, 2025

edison1105 merged commit c69c4bb into vuejs:main May 20, 2025
14 checks passed

coderabbitai bot mentioned this pull request Dec 26, 2025

feat(reactivity): add 'equals' option to watch for custom equality checks #14256

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fix(watch): update `oldValue` before running `cb` to prevent stale value#12296

fix(watch): update `oldValue` before running `cb` to prevent stale value#12296
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