fix: apply class/style directives after attributes

.changeset/eighty-hornets-breathe.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: apply class/style directives after attributes

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

@@ -19,7 +19,7 @@ import {
 import * as b from '../../../../utils/builders.js';
 import { is_custom_element_node } from '../../../nodes.js';
 import { clean_nodes, determine_namespace_for_children } from '../../utils.js';
-import { build_getter, can_inline_variable } from '../utils.js';
+import { build_getter, can_inline_variable, create_derived } from '../utils.js';
 import {
 	get_attribute_name,
 	build_attribute_value,
@@ -534,6 +534,14 @@ function build_element_attribute_update_assignment(element, node_id, attribute,
 	let update;
 
 	if (name === 'class') {
+		if (attribute.metadata.expression.has_state && has_call) {
+			// ensure we're not creating a separate template effect for this so that
+			// potential class directives are added to the same effect and therefore always apply
+			const id = b.id(state.scope.generate('class_derived'));
+			state.init.push(b.const(id, create_derived(state, b.thunk(value))));
+			value = b.call('$.get', id);
+			has_call = false;
+		}
 		update = b.stmt(
 			b.call(
 				is_svg ? '$.set_svg_class' : is_mathml ? '$.set_mathml_class' : '$.set_class',
@@ -548,6 +556,14 @@ function build_element_attribute_update_assignment(element, node_id, attribute,
 	} else if (is_dom_property(name)) {
 		update = b.stmt(b.assignment('=', b.member(node_id, name), value));
 	} else {
+		if (name === 'style' && attribute.metadata.expression.has_state && has_call) {
+			// ensure we're not creating a separate template effect for this so that
+			// potential style directives are added to the same effect and therefore always apply
+			const id = b.id(state.scope.generate('style_derived'));
+			state.init.push(b.const(id, create_derived(state, b.thunk(value))));
+			value = b.call('$.get', id);
+			has_call = false;
+		}
 		const callee = name.startsWith('xlink') ? '$.set_xlink_attribute' : '$.set_attribute';
 		update = b.stmt(
 			b.call(

packages/svelte/tests/runtime-runes/samples/dynamic-attribute-and-attribute-directive-2/_config.js

@@ -0,0 +1,37 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	test({ target, logs, assert }) {
+		const [div, div2] = target.querySelectorAll('div');
+		const button = target.querySelector('button');
+
+		assert.deepEqual(logs, [
+			'updated class attribute',
+			'updated class directive',
+			'updated style attribute',
+			'updated style directive'
+		]);
+
+		assert.ok(div.classList.contains('dark'));
+		assert.ok(div.classList.contains('small'));
+
+		assert.equal(div2.getAttribute('style'), 'background: green; color: green;');
+
+		flushSync(() => button?.click());
+
+		assert.deepEqual(logs, [
+			'updated class attribute',
+			'updated class directive',
+			'updated style attribute',
+			'updated style directive',
+			'updated class attribute',
+			'updated style attribute'
+		]);
+
+		assert.ok(div.classList.contains('dark'));
+		assert.ok(div.classList.contains('big'));
+
+		assert.equal(div2.getAttribute('style'), 'background: red; color: green;');
+	}
+});

packages/svelte/tests/runtime-runes/samples/dynamic-attribute-and-attribute-directive-2/main.svelte

@@ -0,0 +1,23 @@
+<script>
+	let value = $state(0);
+	function dark(){
+		console.log('updated class directive');
+		return true;
+	}
+	function get_class(){
+		console.log('updated class attribute');
+		return value % 2 ? 'big' : 'small';
+	}
+	function color(){
+		console.log('updated style directive');
+		return "green";
+	}
+	function get_style(){
+		console.log('updated style attribute');
+		return value % 2 ? 'background: red' : 'background: green';
+	}
+</script>
+
+<div class:dark={dark()} class={get_class()}></div>
+<div style:color={color()} style={get_style()}></div>
+<button onclick={()=> value++}>switch</button>