feat: set up accordion file structure, API, and a11y

2nd-gen/packages/core/components/accordion/Accordion.base.ts

@@ -0,0 +1,208 @@
+/**
+ * Copyright 2026 Adobe. All rights reserved.
+ * This file is licensed to you under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License. You may obtain a copy
+ * of the License at http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under
+ * the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR REPRESENTATIONS
+ * OF ANY KIND, either express or implied. See the License for the specific language
+ * governing permissions and limitations under the License.
+ */
+
+import { PropertyValues } from 'lit';
+import { property } from 'lit/decorators.js';
+
+import { SpectrumElement } from '@spectrum-web-components/core/element/index.js';
+import { SizedMixin } from '@spectrum-web-components/core/mixins/index.js';
+
+import {
+  ACCORDION_VALID_SIZES,
+  type AccordionDensity,
+  type AccordionHeadingLevel,
+  type AccordionSize,
+  SWC_ACCORDION_ITEM_TOGGLE_EVENT,
+} from './Accordion.types.js';
+import { AccordionItemBase } from './AccordionItem.base.js';
+
+/**
+ * Base class for accordion components. Manages item propagation, heading
+ * level, density, and the exclusive-open constraint.
+ *
+ * @attribute {boolean} allowMultiple - Reflected as `allow-multiple`. When set,
+ *   multiple items may be open at the same time.
+ * @attribute {number} level - Heading level (2–6) applied to every item header.
+ *   Values outside that range are clamped.
+ * @attribute {AccordionSize} size - Size applied to all items.
+ * @attribute {AccordionDensity} density - Vertical spacing between items.
+ * @attribute {boolean} quiet - Renders the accordion in its quiet visual variant.
+ * @attribute {boolean} disabled - Disables all items in the accordion.
+ *
+ * @slot - One or more `swc-accordion-item` elements.
+ */
+export abstract class AccordionBase extends SizedMixin(SpectrumElement, {
+  validSizes: ACCORDION_VALID_SIZES,
+  defaultSize: 'm',
+}) {
+  // ──────────────────
+  //     PUBLIC API
+  // ──────────────────
+
+  /**
+   * When set, multiple items may be open at the same time. By default only
+   * one item can be open.
+   */
+  @property({ type: Boolean, reflect: true, attribute: 'allow-multiple' })
+  public allowMultiple: boolean = false;
+
+  /**
+   * Heading level applied to every item header (2–6). Defaults to 3.
+   * Values outside that range are clamped.
+   */
+  @property({ type: Number, reflect: true })
+  public level: AccordionHeadingLevel = 3;
+
+  /**
+   * Size applied to all items. Defaults to `m`.
+   */
+  declare public size: AccordionSize;
+
+  /**
+   * Controls vertical spacing between items.
+   *
+   * @default regular
+   */
+  @property({ type: String, reflect: true })
+  public density: AccordionDensity = 'regular';
+
+  /**
+   * Renders the accordion in its quiet (no-border) visual variant.
+   */
+  @property({ type: Boolean, reflect: true })
+  public quiet: boolean = false;
+
+  /**
+   * Disables all items in the accordion. Individual items may also be
+   * disabled independently.
+   */
+  @property({ type: Boolean, reflect: true })
+  public disabled: boolean = false;
+
+  // ──────────────────────
+  //     IMPLEMENTATION
+  // ──────────────────────
+
+  private assignedItems(): AccordionItemBase[] {
+    const slot = this.renderRoot?.querySelector('slot');
+    if (!slot) {
+      return [];
+    }
+    return slot
+      .assignedElements({ flatten: true })
+      .filter((el): el is AccordionItemBase => el instanceof AccordionItemBase);
+  }
+
+  private closeSiblingsOnOpen = (event: Event): void => {
+    if (this.disabled) {
+      event.preventDefault();
+      return;
+    }
+    if (this.allowMultiple) {
+      return;
+    }
+    const toggling = event.target;
+    if (!(toggling instanceof AccordionItemBase)) {
+      return;
+    }
+    // Defer until after dispatch returns so that a canceled toggle (where the
+    // item reverts open back to false) does not incorrectly close siblings.
+    queueMicrotask(() => {
+      if (!toggling.open) {
+        return;
+      }
+      for (const item of this.assignedItems()) {
+        if (item !== toggling) {
+          item.open = false;
+        }
+      }
+    });
+  };
+
+  protected syncAccordionItems(): void {
+    for (const item of this.assignedItems()) {
+      item.setManagedHeading(this.level);
+      item.size = this.size;
+      item.setManagedParentDisabled(this.disabled);
+    }
+  }
+
+  private enforceExclusiveOpen(): void {
+    let foundOpen = false;
+    for (const item of this.assignedItems()) {
+      if (item.open) {
+        if (foundOpen) {
+          item.open = false;
+        } else {
+          foundOpen = true;
+        }
+      }
+    }
+  }
+
+  public override connectedCallback(): void {
+    super.connectedCallback();
+    this.addEventListener(
+      SWC_ACCORDION_ITEM_TOGGLE_EVENT,
+      this.closeSiblingsOnOpen
+    );
+  }
+
+  public override disconnectedCallback(): void {
+    super.disconnectedCallback();
+    this.removeEventListener(
+      SWC_ACCORDION_ITEM_TOGGLE_EVENT,
+      this.closeSiblingsOnOpen
+    );
+  }
+
+  protected override update(changedProperties: PropertyValues): void {
+    if (changedProperties.has('level')) {
+      const clamped = Math.min(
+        6,
+        Math.max(2, this.level)
+      ) as AccordionHeadingLevel;
+      if (this.level !== clamped) {
+        this.level = clamped;
+      }
+    }
+    if (
+      changedProperties.has('level') ||
+      changedProperties.has('size') ||
+      changedProperties.has('disabled')
+    ) {
+      this.syncAccordionItems();
+    }
+    // changedProperties.get() returns the previous value; this fires only when
+    // disabled transitions from true → false (re-enable).
+    if (
+      changedProperties.has('disabled') &&
+      changedProperties.get('disabled') === true &&
+      !this.allowMultiple
+    ) {
+      this.enforceExclusiveOpen();
+    }
+    super.update(changedProperties);
+  }
+
+  protected override firstUpdated(changedProperties: PropertyValues): void {
+    super.firstUpdated(changedProperties);
+    if (window.__swc?.DEBUG && !this.hasAttribute('density')) {
+      window.__swc.warn(
+        this,
+        `<${this.localName}> should have an explicit "density" attribute set. Defaulting to "regular".`,
+        'https://opensource.adobe.com/spectrum-web-components/components/accordion/',
+        { type: 'api', level: 'low' }
+      );
+    }
+  }
+}

2nd-gen/packages/core/components/accordion/Accordion.types.ts

@@ -0,0 +1,33 @@
+/**
+ * Copyright 2026 Adobe. All rights reserved.
+ * This file is licensed to you under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License. You may obtain a copy
+ * of the License at http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under
+ * the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR REPRESENTATIONS
+ * OF ANY KIND, either express or implied. See the License for the specific language
+ * governing permissions and limitations under the License.
+ */
+
+import type { ElementSize } from '@spectrum-web-components/core/mixins/index.js';
+
+export const ACCORDION_VALID_SIZES = [
+  's',
+  'm',
+  'l',
+  'xl',
+] as const satisfies readonly ElementSize[];
+export type AccordionSize = (typeof ACCORDION_VALID_SIZES)[number];
+
+export const ACCORDION_DENSITIES = ['compact', 'regular', 'spacious'] as const;
+export type AccordionDensity = (typeof ACCORDION_DENSITIES)[number];
+
+export const ACCORDION_HEADING_LEVELS = [2, 3, 4, 5, 6] as const;
+export type AccordionHeadingLevel = (typeof ACCORDION_HEADING_LEVELS)[number];
+
+export const SWC_ACCORDION_ITEM_TOGGLE_EVENT = 'swc-accordion-item-toggle';
+export const SWC_ACCORDION_ITEM_OPEN_EVENT = 'swc-open';
+export const SWC_ACCORDION_ITEM_CLOSE_EVENT = 'swc-close';
+export const SWC_ACCORDION_ITEM_AFTER_OPEN_EVENT = 'swc-after-open';
+export const SWC_ACCORDION_ITEM_AFTER_CLOSE_EVENT = 'swc-after-close';