feat: add $effect.active rune

.changeset/popular-mangos-rest.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+feat: add $effect.active rune

packages/svelte/src/compiler/phases/2-analyze/validation.js

@@ -504,6 +504,17 @@ function validate_call_expression(node, scope, path) {
 	}
 
 	if (rune === '$effect') {
+		const callee = node.callee;
+		if (
+			callee.type === 'MemberExpression' &&
+			callee.property.type === 'Identifier' &&
+			callee.property.name === 'active'
+		) {
+			if (node.arguments.length !== 0) {
+				error(node, 'invalid-rune-args-length', '$effect.active', [0]);
+			}
+			return;
+		}
 		if (parent.type !== 'ExpressionStatement') {
 			error(node, 'invalid-effect-location');
 		}

packages/svelte/src/compiler/phases/3-transform/client/visitors/javascript-runes.js

@@ -135,7 +135,7 @@ export const javascript_visitors_runes = {
 		for (const declarator of node.declarations) {
 			const init = declarator.init;
 			const rune = get_rune(init, state.scope);
-			if (!rune) {
+			if (!rune || rune === '$effect') {
 				if (init != null && is_hoistable_function(init)) {
 					const hoistable_function = visit(init);
 					state.hoisted.push(
@@ -291,5 +291,19 @@ export const javascript_visitors_runes = {
 		}
 
 		context.next();
+	},
+	CallExpression(node, { state, next }) {
+		const rune = get_rune(node, state.scope);
+		const callee = node.callee;
+
+		if (
+			rune === '$effect' &&
+			callee.type === 'MemberExpression' &&
+			callee.property.type === 'Identifier' &&
+			callee.property.name === 'active'
+		) {
+			return b.call('$.effect_active');
+		}
+		next();
 	}
 };

packages/svelte/src/compiler/phases/3-transform/client/visitors/template.js

@@ -35,6 +35,7 @@ import {
 	EACH_KEYED
 } from '../../../../../constants.js';
 import { regex_is_valid_identifier } from '../../../patterns.js';
+import { javascript_visitors_runes } from './javascript-runes.js';
 
 /**
  * Serializes each style directive into something like `$.style(element, style_property, value)`
@@ -2921,5 +2922,6 @@ export const template_visitors = {
 			...context.state,
 			node: b.id('$.document')
 		});
-	}
+	},
+	CallExpression: javascript_visitors_runes.CallExpression
 };

packages/svelte/src/compiler/phases/3-transform/server/transform-server.js

@@ -550,7 +550,7 @@ const javascript_visitors_runes = {
 
 		for (const declarator of node.declarations) {
 			const rune = get_rune(declarator.init, state.scope);
-			if (!rune) {
+			if (!rune || rune === '$effect') {
 				declarations.push(/** @type {import('estree').VariableDeclarator} */ (visit(declarator)));
 				continue;
 			}
@@ -604,6 +604,20 @@ const javascript_visitors_runes = {
 			}
 		}
 		context.next();
+	},
+	CallExpression(node, { state, next }) {
+		const rune = get_rune(node, state.scope);
+		const callee = node.callee;
+
+		if (
+			rune === '$effect' &&
+			callee.type === 'MemberExpression' &&
+			callee.property.type === 'Identifier' &&
+			callee.property.name === 'active'
+		) {
+			return b.literal(false);
+		}
+		next();
 	}
 };
 
@@ -1579,7 +1593,9 @@ const template_visitors = {
 				b.stmt(b.call('$.head', b.id('$$payload'), b.arrow([b.id('$$payload')], b.block(body))))
 			)
 		);
-	}
+	},
+	// @ts-ignore: need to extract this out somehow
+	CallExpression: javascript_visitors_runes.CallExpression
 };
 
 /**

packages/svelte/src/internal/client/runtime.js

@@ -1143,6 +1143,13 @@ function internal_create_effect(type, init, sync, block, schedule) {
 	return signal;
 }
 
+/**
+ * @returns {boolean}
+ */
+export function effect_active() {
+	return current_effect ? (current_effect.f & MANAGED) === 0 : false;
+}
+
 /**
  * @param {() => void | (() => void)} init
  * @returns {import('./types.js').EffectSignal}

packages/svelte/src/internal/index.js

@@ -35,7 +35,8 @@ export {
 	onDestroy,
 	pop,
 	push,
-	reactive_import
+	reactive_import,
+	effect_active
 } from './client/runtime.js';
 
 export * from './client/validate.js';

packages/svelte/tests/runtime-runes/samples/effect-active/_config.js

@@ -0,0 +1,15 @@
+import { test } from '../../test';
+
+export default test({
+	ssrHtml: `
+		<p>false</p>
+		<p>false</p>
+		<p>false</p>
+	`,
+
+	html: `
+		<p>false</p>
+		<p>true</p>
+		<p>true</p>
+	`
+});

packages/svelte/tests/runtime-runes/samples/effect-active/main.svelte

@@ -0,0 +1,11 @@
+<script>
+	const foo = $effect.active();
+	let bar = $state(false);
+	$effect.pre(() => {
+		bar = $effect.active();
+	});
+</script>
+
+<p>{foo}</p>
+<p>{bar}</p>
+<p>{$effect.active()}</p>