[labs/signals] watch() no longer triggers update; adds effect #4941

sorvell · 2025-02-27T17:37:59Z

watch no longer triggers a reactive update on a SignalWatcher element
watch can be used without SignalWatcher
adds effect(callback, options) which runs the provided function whenever any signals it accesses are changed. If an options object is provided, the effect is coordinated with the given element and is called by default after rendering updates but can optionally include an options beforeUpdate: true to process the effect before any rendering updates.

changeset-bot · 2025-02-27T17:38:03Z

🦋 Changeset detected

Latest commit: 3613282

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@lit-labs/signals	Minor

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Click here if you're a maintainer who wants to add another changeset to this PR

github-actions · 2025-02-27T17:38:14Z

📊 Tachometer Benchmark Results

Summary

nop-update

this-change, tip-of-tree, previous-release: unsure 🔍 -10% - +5% (-1.24ms - +0.70ms)
^{this-change vs tip-of-tree}

render

this-change: 46.57ms - 58.47ms
this-change, tip-of-tree, previous-release: unsure 🔍 -5% - +2% (-1.06ms - +0.40ms)
^{this-change vs tip-of-tree}
this-change, tip-of-tree, previous-release: unsure 🔍 -1% - +3% (-0.40ms - +0.97ms)
^{this-change vs tip-of-tree}
this-change, tip-of-tree, previous-release: unsure 🔍 -4% - +2% (-4.35ms - +2.29ms)
^{this-change vs tip-of-tree}

update

this-change: 502.73ms - 510.18ms
this-change, tip-of-tree, previous-release: unsure 🔍 -7% - +2% (-2.91ms - +0.78ms)
^{this-change vs tip-of-tree}
this-change, tip-of-tree, previous-release: unsure 🔍 -1% - +2% (-1.14ms - +1.74ms)
^{this-change vs tip-of-tree}
this-change, tip-of-tree, previous-release: unsure 🔍 -0% - +2% (-0.71ms - +9.79ms)
^{this-change vs tip-of-tree}

update-reflect

this-change: 486.70ms - 491.28ms
this-change, tip-of-tree, previous-release: unsure 🔍 -2% - +1% (-7.92ms - +6.38ms)
^{this-change vs tip-of-tree}

Results

this-change

Browser: chrome-headless 142.0.0.0
Sample size: 50
Built by: Benchmarks #7029
Commit: 3613282

render

Version	Avg time	vs
	46.57ms - 58.47ms	-

update

Version	Avg time	vs
	502.73ms - 510.18ms	-

update-reflect

Version	Avg time	vs
	486.70ms - 491.28ms	-

this-change, tip-of-tree, previous-release

Browser: chrome-headless 142.0.0.0
Sample size: 30
Built by: Benchmarks #7029
Commit: 3613282

render

Version	Avg time	vs this-change	vs tip-of-tree tip-of-tree	vs previous-release previous-release
this-change	19.03ms - 20.01ms	-	unsure 🔍 -5% - +2% -1.06ms - +0.40ms	unsure 🔍 -8% - +0% -1.71ms - +0.11ms
tip-of-tree tip-of-tree	19.32ms - 20.39ms	unsure 🔍 -2% - +5% -0.40ms - +1.06ms	-	unsure 🔍 -7% - +2% -1.41ms - +0.47ms
previous-release previous-release	19.55ms - 21.09ms	unsure 🔍 -1% - +9% -0.11ms - +1.71ms	unsure 🔍 -2% - +7% -0.47ms - +1.41ms	-

update

Version	Avg time	vs this-change	vs tip-of-tree tip-of-tree	vs previous-release previous-release
this-change	38.34ms - 41.33ms	-	unsure 🔍 -7% - +2% -2.91ms - +0.78ms	unsure 🔍 -8% - +2% -3.35ms - +0.91ms
tip-of-tree tip-of-tree	39.82ms - 41.98ms	unsure 🔍 -2% - +7% -0.78ms - +2.91ms	-	unsure 🔍 -5% - +4% -2.01ms - +1.70ms
previous-release previous-release	39.54ms - 42.57ms	unsure 🔍 -2% - +8% -0.91ms - +3.35ms	unsure 🔍 -4% - +5% -1.70ms - +2.01ms	-

nop-update

Version	Avg time	vs this-change	vs tip-of-tree tip-of-tree	vs previous-release previous-release
this-change	11.74ms - 12.93ms	-	unsure 🔍 -10% - +5% -1.24ms - +0.70ms	faster ✔ 1% - 14% 0.03ms - 1.93ms
tip-of-tree tip-of-tree	11.83ms - 13.37ms	unsure 🔍 -6% - +10% -0.70ms - +1.24ms	-	unsure 🔍 -13% - +2% -1.78ms - +0.35ms
previous-release previous-release	12.57ms - 14.05ms	slower ❌ 0% - 16% 0.03ms - 1.93ms	unsure 🔍 -3% - +14% -0.35ms - +1.78ms	-

this-change, tip-of-tree, previous-release

Browser: chrome-headless 142.0.0.0
Sample size: 50
Built by: Benchmarks #7029
Commit: 3613282

render

Version	Avg time	vs this-change	vs tip-of-tree tip-of-tree	vs previous-release previous-release
this-change	35.78ms - 36.84ms	-	unsure 🔍 -1% - +3% -0.40ms - +0.97ms	unsure 🔍 -1% - +3% -0.38ms - +0.97ms
tip-of-tree tip-of-tree	35.59ms - 36.47ms	unsure 🔍 -3% - +1% -0.97ms - +0.40ms	-	unsure 🔍 -2% - +2% -0.60ms - +0.62ms
previous-release previous-release	35.60ms - 36.43ms	unsure 🔍 -3% - +1% -0.97ms - +0.38ms	unsure 🔍 -2% - +2% -0.62ms - +0.60ms	-

update

Version	Avg time	vs this-change	vs tip-of-tree tip-of-tree	vs previous-release previous-release
this-change	77.34ms - 79.24ms	-	unsure 🔍 -1% - +2% -1.14ms - +1.74ms	unsure 🔍 -2% - +2% -1.55ms - +1.42ms
tip-of-tree tip-of-tree	76.92ms - 79.07ms	unsure 🔍 -2% - +1% -1.74ms - +1.14ms	-	unsure 🔍 -2% - +2% -1.93ms - +1.20ms
previous-release previous-release	77.22ms - 79.50ms	unsure 🔍 -2% - +2% -1.42ms - +1.55ms	unsure 🔍 -2% - +2% -1.20ms - +1.93ms	-

this-change, tip-of-tree, previous-release

Browser: chrome-headless 142.0.0.0
Sample size: 50
Built by: Benchmarks #7029
Commit: 3613282

render

Version	Avg time	vs this-change	vs tip-of-tree tip-of-tree	vs previous-release previous-release
this-change	104.90ms - 111.23ms	-	unsure 🔍 -4% - +2% -4.35ms - +2.29ms	unsure 🔍 -5% - +1% -5.52ms - +1.10ms
tip-of-tree tip-of-tree	108.10ms - 110.09ms	unsure 🔍 -2% - +4% -2.29ms - +4.35ms	-	unsure 🔍 -2% - +0% -2.57ms - +0.22ms
previous-release previous-release	109.29ms - 111.24ms	unsure 🔍 -1% - +5% -1.10ms - +5.52ms	unsure 🔍 -0% - +2% -0.22ms - +2.57ms	-

update

Version	Avg time	vs this-change	vs tip-of-tree tip-of-tree	vs previous-release previous-release
this-change	511.22ms - 518.17ms	-	unsure 🔍 -0% - +2% -0.71ms - +9.79ms	unsure 🔍 -1% - +1% -5.99ms - +4.84ms
tip-of-tree tip-of-tree	506.23ms - 514.09ms	unsure 🔍 -2% - +0% -9.79ms - +0.71ms	-	unsure 🔍 -2% - +0% -10.83ms - +0.61ms
previous-release previous-release	511.11ms - 519.42ms	unsure 🔍 -1% - +1% -4.84ms - +5.99ms	unsure 🔍 -0% - +2% -0.61ms - +10.83ms	-

update-reflect

Version	Avg time	vs this-change	vs tip-of-tree tip-of-tree	vs previous-release previous-release
this-change	517.73ms - 526.16ms	-	unsure 🔍 -2% - +1% -7.92ms - +6.38ms	unsure 🔍 -1% - +1% -3.85ms - +6.30ms
tip-of-tree tip-of-tree	516.94ms - 528.49ms	unsure 🔍 -1% - +2% -6.38ms - +7.92ms	-	unsure 🔍 -1% - +2% -4.44ms - +8.42ms
previous-release previous-release	517.89ms - 523.55ms	unsure 🔍 -1% - +1% -6.30ms - +3.85ms	unsure 🔍 -2% - +1% -8.42ms - +4.44ms	-

_{tachometer-reporter-action v2 for Benchmarks}

github-actions · 2025-02-27T17:40:08Z

The size of lit-html.js and lit-core.min.js are as expected.

packages/labs/signals/src/lib/signal-watcher.ts

justinfagnani · 2025-07-17T16:33:43Z

packages/labs/signals/src/lib/signal-watcher.ts

+}
+
+export interface ElementEffectOptions extends EffectOptions {
+  element?: SignalWatcher & SignalWatcherApi;


JSDocs here?

Removed, no longer needed.

packages/labs/signals/src/lib/signal-watcher.ts

justinfagnani · 2025-10-05T23:20:08Z

packages/labs/signals/src/lib/signal-watcher.ts

+  manualDispose?: boolean;
+}
+
+export interface ElementEffectOptions extends EffectOptions {


Is this interface used anywhere?

Removed, no longer needed.

justinfagnani

Awesome!

Two small comments left

Westbrook · 2025-11-04T05:08:20Z

Getting into some finer testing of this change as we await it landing and wondered which of these is "correct".

In https://lit.dev/playground/#gist=5cfb3e893dbc5412ed1468483be5fa80 we get the render lifecycle on all three buttons, but only the Signal bindings on the first and last (signal powered) element.

In https://lit.dev/playground/#gist=3b14cb4a58de364ec7686f230a88435d we do not get the render lifecycle in either of the Signal bindings.

Maybe there's changes in this PR before I lifted the code, but wanted to confirm as we're working out how we want to consumer these capabilities in my team right now.

sorvell · 2025-11-04T23:34:40Z

Forwarded from discord

This should be removed: https://lit.dev/docs/data/signals/#:~:text=For%20coordination%20purposes,template%20are%20skipped
We changed this since signals are not part of the element's reactive properties so if they triggered an update it would be an update without any visible changes to properties.
We've added updateEffect which lets you use signal values when signals change. This method is coordinated with the rendering impact that any of those signals have on the element and can be either before or after them, based on options e.g. updateEffect(() => console.log(mySIgnal.get()), {beforeUpdate: true})

Steve Orvell added 3 commits February 3, 2025 16:18

watch commits independent of update and adds effect

d6b35cb

Merge branch 'main' into signals-watch-wo-update

5b8704f

adds changeset

00de5b2

sorvell requested a review from justinfagnani February 27, 2025 17:38

Steve Orvell added 5 commits March 2, 2025 05:40

update readme

195d5f4

Merge branch 'main' into signals-watch-wo-update

d6c6386

address feedback: simplify to use a single watcher

e0e6172

remove any cast

a48ccd0

Address feedback: effect is a standalone API

9d46508

sorvell marked this pull request as ready for review March 11, 2025 23:35

sorvell requested a review from kevinpschaaf as a code owner March 11, 2025 23:35

sorvell changed the title ~~watch no longer triggers update; adds SignalWatcher.effect~~ watch no longer triggers update; adds effect Mar 13, 2025

justinfagnani changed the title ~~watch no longer triggers update; adds effect~~ [labs/signals] watchI() no longer triggers update; adds effect Mar 13, 2025

justinfagnani changed the title ~~[labs/signals] watchI() no longer triggers update; adds effect~~ [labs/signals] watch() no longer triggers update; adds effect Jul 17, 2025

justinfagnani reviewed Jul 17, 2025

View reviewed changes

Steve Orvell added 2 commits July 17, 2025 12:48

Merge branch 'main' into signals-watch-wo-update

89f1ed0

address feedback

5c473d3

sorvell requested a review from justinfagnani July 17, 2025 21:05

address feedback

dce8581

justinfagnani reviewed Oct 5, 2025

View reviewed changes

justinfagnani approved these changes Oct 31, 2025

View reviewed changes

Steve Orvell added 2 commits November 2, 2025 07:56

Address feedback

99191bf

Address review feedback; remove unused interface.

56de871

Steve Orvell added 2 commits November 6, 2025 17:17

format

0dc2832

fix build

3613282

sorvell merged commit 5ee7c1f into main Nov 7, 2025
10 checks passed

sorvell deleted the signals-watch-wo-update branch November 7, 2025 12:57

lit-robot mentioned this pull request Dec 18, 2025

Version Packages #5183

Open

[labs/signals] watch() no longer triggers update; adds effect #4941

[labs/signals] watch() no longer triggers update; adds effect #4941

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📊 Tachometer Benchmark Results

Summary

nop-update

render

update

update-reflect

Results

render

update

update-reflect

render

update

nop-update

render

update

render

update

update-reflect

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Westbrook commented Nov 4, 2025

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sorvell commented Nov 4, 2025

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sorvell commented Feb 27, 2025 •

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