mirror of https://github.com/sveltejs/svelte
fix: re-run non-render-bound deriveds on the server (#17674)
I tried to bring #14977 up-to-date but it's slipped too far. Also, I wanted to try a slightly different approach. In this PR, deriveds are memoized if they're created during render — in other words if you have something like this... ```svelte <script> let thing = $derived(expensivelyComputeThing()); </script> ``` ...`thing` will only be computed once. This seems correct since the inputs should never change during render. For deriveds created _outside_ render, we re-run the derived each time it is accessed, which fixes #14954. This way, there's still _some_ overhead compared to how deriveds work in the browser (where they only recompute when their dependencies have changed), but only in the rare places where it is necessary. There is one wrinkle: writable deriveds. On `main` these are just regular old variables, which means they can be written to during render. This PR currently preserves that behaviour, but I'm not sure it's desirable. It prevents the values of non-render-bound deriveds from ever updating, and makes no sense in the context of render-bound deriveds since they shouldn't be changing during render _anyway_. So my preference would be to disallow writes to deriveds on the server, but I'm not sure if we would need to consider that a breaking change. Draft because of that question, and also because I think we might be able to tidy up some stuff around class fields. - [x] figure out if we can delete some existing code around derived class fields - [x] figure out what to do about writable deriveds - [x] add a test ### Before submitting the PR, please make sure you do the following - [x] It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs - [x] Prefix your PR title with `feat:`, `fix:`, `chore:`, or `docs:`. - [x] This message body should clearly illustrate what problems it solves. - [x] Ideally, include a test that fails without this PR but passes with it. - [x] If this PR changes code within `packages/svelte/src`, add a changeset (`npx changeset`). ### Tests and linting - [x] Run the tests with `pnpm test` and lint the project with `pnpm lint`pull/17742/head
Rich Harris
6 months ago
committed by
GitHub
parent
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---
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'svelte': patch
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---
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fix: re-run non-render-bound deriveds on the server
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import { test } from '../../test';
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export default test({
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test_ssr({ assert, logs }) {
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assert.deepEqual(logs, [0, 2, { count: 2 }, 0, 0, { local_count: 1 }]);
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},
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test({ assert, logs }) {
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assert.deepEqual(logs, [0, 2, { count: 2 }, 0, 0, { local_count: 1 }]);
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}
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});
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<script>
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import { reset, increment, get, count } from './state.svelte.js';
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reset();
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console.log(get());
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increment();
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console.log(get());
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console.log({ count });
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let local_count = 0;
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let s = $state(0);
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let d = $derived.by(() => {
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local_count += 1;
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return s * 2;
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});
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console.log(d);
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console.log(d);
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console.log({ local_count });
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</script>
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let s = $state(0);
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let d = $derived.by(() => {
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count += 1;
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return s * 2;
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});
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export let count = 0;
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export function reset() {
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count = 0;
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s = 0;
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}
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export function increment() {
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s += 1;
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}
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export function get() {
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return d;
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}
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