Drupal Canvas SDC Components (Twig)

Overview

Drupal Canvas 1.0 (released December 2025) is a React-based visual page builder powered by Single Directory Components (SDC). Canvas consolidates three component types:

• SDC components from themes/modules
• Code Components written in-browser with React/JSX
• Traditional Drupal Blocks

Canvas becomes the default page builder in Drupal CMS 2.0 (January 2026).

Installation

composer require 'drupal/canvas:^1.0'
drush en canvas -y

Requirements: Drupal ^11.2

Component Discovery

SDC components live in a components/ directory within any theme or module. Drupal scans enabled themes/modules for directories containing a .component.yml file.

Component ID pattern: {provider}:{component-name} (e.g., mytheme:hero-banner)

Management UI: Appearance > Components (/admin/appearance/component)

SDC File Structure

Every SDC requires exactly two files. CSS/JS with matching names auto-attach.

theme_name/
└── components/
    ├── atoms/
    │   └── button/
    │       ├── button.component.yml   # REQUIRED
    │       ├── button.twig            # REQUIRED
    │       ├── button.css             # Auto-attached
    │       └── button.js              # Auto-attached
    └── molecules/
        └── card/
            ├── card.component.yml
            ├── card.twig
            ├── card.css
            └── thumbnail.png          # Optional preview

Naming Rules:

• Use kebab-case for directories and files
• All files share the same base name as the directory
• Templates use .twig extension (NOT .html.twig)
• Organizational folders (atoms/, molecules/) don't affect component IDs

component.yml Schema

$schema: https://git.drupalcode.org/project/drupal/-/raw/HEAD/core/assets/schemas/v1/metadata.schema.json
name: Hero Banner
description: Full-width hero with image background and CTA
status: stable  # experimental, stable, deprecated, obsolete
group: Content

props:
  type: object
  required:
    - heading
  properties:
    heading:
      type: string
      title: Heading
      examples: ['Welcome to our site']  # REQUIRED for Canvas preview
    background_image:
      type: object
      $ref: json-schema-definitions://canvas.module/image
    cta_text:
      type: string
      title: Button Text
      default: Learn More
    size:
      type: string
      title: Size
      enum: ['small', 'medium', 'large']
      default: medium
      meta:enum:
        small: "Compact"
        medium: "Standard"
        large: "Full Screen"

slots:
  content:
    title: Body Content
    description: Main content area below the heading

libraryOverrides:
  dependencies:
    - core/drupal
    - core/once

CRITICAL: The examples key is required for Canvas to display helpful placeholders.

Prop Types and Canvas Widgets

Rich Text Props

content:
  type: string
  title: Rich Text Content
  contentMediaType: text/html
  x-formatting-context: block  # or 'inline'
  examples: ['<p>Formatted <strong>content</strong></p>']

Enum Labels (Human-Readable Dropdowns)

color:
  type: string
  enum: ['primary', 'secondary', 'danger']
  meta:enum:
    primary: "Primary Brand Color"
    secondary: "Secondary Accent"
    danger: "Warning/Error State"

Validation Patterns

Required Properties

props:
  type: object
  required:
    - title
    - url
  properties:
    title:
      type: string

Numeric Constraints

spacing:
  type: integer
  minimum: 0
  maximum: 100

heading_level:
  type: integer
  enum: [2, 3, 4, 5, 6]

Nullable Values

subtitle:
  type: ['string', 'null']
  title: Optional Subtitle

Array Constraints

tags:
  type: array
  items:
    type: string
  minItems: 1
  maxItems: 5

Default Values in Twig

IMPORTANT: Schema default values are documentation only. Always use |default() in Twig:

{% set heading_level = heading_level|default(2) %}
{% set color = color|default('primary') %}

Slots for Composition

Slots accept arbitrary markup or nested components. Props handle typed data; slots handle content.

Define Slots

slots:
  header:
    title: Header Content
    description: Optional header area
  body:
    title: Body Content
  footer: {}  # Minimal definition

Render Slots in Twig

<article class="card">
  {% if header %}
    <header class="card__header">{{ header }}</header>
  {% endif %}
  <div class="card__body">{{ body }}</div>
  {% if footer %}
    <footer class="card__footer">{{ footer }}</footer>
  {% endif %}
</article>

Populate Slots with Embed

{% embed 'my_theme:card' with { heading: 'Featured Article' } only %}
  {% block body %}
    {{ content.field_image }}
    <p>{{ content.field_summary }}</p>
    {{ include('my_theme:button', { text: 'Read More', url: url }) }}
  {% endblock %}
{% endembed %}

Populate Slots from PHP

$build = [
  '#type' => 'component',
  '#component' => 'my_theme:card',
  '#props' => ['heading' => $title],
  '#slots' => [
    'body' => ['#markup' => $content],
    'footer' => $footer_render_array,
  ],
];

Atomic Design Principles

• Atoms: Buttons, icons, labels, form inputs
• Molecules: Cards, media objects, search forms
• Organisms: Headers, footers, article teasers
• Templates: Page layouts with slots for organisms

Single Responsibility Pattern

# ✅ Good: Single component with variants
name: Button
props:
  type: object
  properties:
    variant:
      type: string
      enum: ['primary', 'secondary', 'outline', 'ghost']
    size:
      type: string
      enum: ['small', 'medium', 'large']

# ❌ Avoid: Separate components per variant
# button-primary.component.yml
# button-secondary.component.yml

CSS Styling with BEM

Component-Scoped CSS (CRITICAL): NEVER scope styles to route/path body classes (e.g., .alias--masterclasses, .path-some-page). Instead, scope styles to the component's own classes (e.g., .view-masterclasses, .block-views-blockmasterclasses-block-2). The Canvas editor renders page previews in an iframe without the page's body classes, so route-scoped styles won't appear there. All styles must be componentized and portable -- they should look correct regardless of what route or context they render in.

SDC auto-attaches CSS when file matches component name. Use BEM for scoped styles:

/* card.css */
.card {
  --card-padding: 1.5rem;
  --card-radius: 0.5rem;
  --card-shadow: 0 2px 8px rgba(0,0,0,0.1);

  display: flex;
  flex-direction: column;
  padding: var(--card-padding);
  border-radius: var(--card-radius);
  box-shadow: var(--card-shadow);
}

.card__header {
  margin-bottom: 1rem;
  font-weight: 700;
}

.card__body {
  flex: 1;
}

.card--featured {
  --card-shadow: 0 4px 16px rgba(0,0,0,0.15);
  border: 2px solid var(--color-primary);
}

.card--compact {
  --card-padding: 1rem;
}

External Dependencies

libraryOverrides:
  css:
    component:
      card.css: {}
      additional-styles.css: {}
  dependencies:
    - core/normalize

JavaScript Behaviors

Use Drupal's behavior pattern with once():

// accordion.js
((Drupal, once) => {
  Drupal.behaviors.accordion = {
    attach(context) {
      once('accordion', '.accordion__trigger', context).forEach((trigger) => {
        trigger.addEventListener('click', (e) => {
          const panel = document.getElementById(trigger.getAttribute('aria-controls'));
          const expanded = trigger.getAttribute('aria-expanded') === 'true';

          trigger.setAttribute('aria-expanded', !expanded);
          panel.hidden = expanded;
        });
      });
    },
    detach(context, settings, trigger) {
      if (trigger === 'unload') {
        once.remove('accordion', '.accordion__trigger', context);
      }
    }
  };
})(Drupal, once);

Key Patterns:

• once() prevents duplicate event binding
• context scopes to newly added DOM
• attach() runs on page load and AJAX updates
• detach() handles cleanup

Accessibility (WCAG 2.2 AA)

Semantic HTML First

<nav aria-label="Main navigation">
  <ul>
    <li><a href="{{ url }}">{{ label }}</a></li>
  </ul>
</nav>

{# Accessible button with states #}
<button
  type="button"
  aria-expanded="{{ expanded ? 'true' : 'false' }}"
  aria-controls="panel-{{ id }}"
>
  {{ trigger_text }}
</button>

{# Modal dialog #}
<div
  role="dialog"
  aria-modal="true"
  aria-labelledby="dialog-title-{{ id }}"
>
  <h2 id="dialog-title-{{ id }}">{{ title }}</h2>
  {{ content }}
</div>

{# Live region for dynamic updates #}
<div aria-live="polite" class="visually-hidden">
  {{ status_message }}
</div>

Keyboard Navigation Checklist

• All interactive elements focusable via Tab
• Visible focus indicators (never outline: none without alternative)
• Enter/Space activate buttons and links
• Escape closes modals and dropdowns
• Arrow keys navigate within composite widgets

Dynamic Announcements

Drupal.announce('Form submitted successfully');
Drupal.announce('Error: Please fix the highlighted fields', 'assertive');

Canvas Preview for Traditional Blocks

CRITICAL: Canvas renders block previews in iframes that only include CSS from explicitly attached libraries.

Problem

Traditional Drupal blocks relying on theme global SCSS won't show styles in Canvas previews.

Solution Pattern

1. Create standalone CSS file in module's css/ directory
2. Add library to module's .libraries.yml
3. Attach library via #attached in block's build() method

// In BlockClass.php build() method
public function build() {
  return [
    '#theme' => 'my_block_template',
    '#variables' => $variables,
    '#attached' => [
      'library' => [
        'my_module/my_block_styles',
      ],
    ],
  ];
}

# my_module.libraries.yml
my_block_styles:
  version: VERSION
  css:
    theme:
      css/my-block.css: {}

CSS Compilation for Standalone Files

When extracting from theme SCSS, compile variables to hardcoded values:

/* Compiled from SCSS - replace variables */
.my-component {
  max-width: 1440px;  /* was $max-content-width */
  color: #7d11ff;     /* was $primary-color */
}

Blocks That Return Empty

Blocks returning empty arrays [] won't preview in Canvas. Always render the template structure:

public function build() {
  // Always return template for Canvas preview support
  return [
    '#theme' => 'my_block_template',
    '#content' => $content ?? NULL,
    '#attached' => [
      'library' => ['my_module/my_block'],
    ],
    '#cache' => ['contexts' => ['user']],
  ];
}

SDC vs Traditional Blocks Comparison

Testing SDC Components

PHPUnit Kernel Tests

namespace Drupal\Tests\my_module\Kernel;

use Drupal\KernelTests\KernelTestBase;

class CardComponentTest extends KernelTestBase {
  protected static $modules = ['system', 'sdc'];

  public function testCardRendering() {
    $build = [
      '#type' => 'component',
      '#component' => 'my_theme:card',
      '#props' => ['heading' => 'Test'],
    ];
    $output = \Drupal::service('renderer')->renderRoot($build);
    $this->assertStringContainsString('Test', (string) $output);
  }
}

Storybook Integration

// card.stories.js
export default {
  title: 'Molecules/Card',
  component: 'my_theme:card',
};

export const Default = {
  args: {
    heading: 'Card Title',
    body: '<p>Card content goes here</p>',
  },
};

export const Featured = {
  args: {
    heading: 'Featured Card',
    variant: 'featured',
  },
};

Canvas API Stability

Canvas Development & Migration Tools

Canvas Submodules

canvas_dev_mode details:

• Removes Choice constraint on ComponentSource plugins, allowing unstable plugin types
• Shows the extensions toolbar in the Canvas editor UI
• Enable: ddev drush en canvas_dev_mode -y

canvas_vite details:

• Connects to Vite dev server at localhost:5173
• Disables CSS/JS preprocessing so changes are instant (no cache clearing for CSS/JS)
• Injects React Refresh runtime for live component updates
• Set VITE_SERVER_ORIGIN env var to customize the Vite server URL
• Enable: ddev drush en canvas_vite -y then start Vite: cd docroot/modules/contrib/canvas/ui && npm run dev
• Disable when done: ddev drush pm:uninstall canvas_vite -y

Canvas Admin UI Pages

Use these during migration and development:

The audit page is the most useful during migration - it shows every canvas page, content template, pattern, and region that uses a given component.

Canvas API Endpoints (JSON)

Useful for scripting or programmatic migration:

GET /canvas/api/v0/usage/component                          # List all component usage (paginated)
GET /canvas/api/v0/usage/component/{component}              # Usage summary for one component
GET /canvas/api/v0/usage/component/{component}/details      # Detailed usage breakdown
GET /canvas/api/v0/config/{type}                            # List all components/content-templates/patterns
GET /canvas/api/v0/config/{type}/{id}                       # Get specific config entity
GET /canvas/api/v0/layout/{entity_type}/{entity}            # Get entity's component tree layout
GET /canvas/api/v0/auto-saves/pending                       # Get pending auto-saves
POST /canvas/api/v0/log-error                               # JS error logging from Canvas UI

Canvas PHP Services (Dependency Injection)

Key services available for custom code:

ComponentAudit Service (Migration Key Tool)

$audit = \Drupal::service(Drupal\canvas\Audit\ComponentAudit::class);

// Find all content using a component
$audit->getContentRevisionsUsingComponent($component);

// Find config entities (templates, patterns, regions) using it
$audit->getConfigEntityDependenciesUsingComponent($component);

// Quick boolean check
$audit->hasUsages($component);

Canvas Component Versioning (CRITICAL)

Understanding component versioning is essential when modifying SDC components.

How Versioning Works

Canvas uses a version-pinning system for backward compatibility:

1. Each component has an active_version (an xxh64 hash)
2. Component instances are pinned to the version they were created with
3. When you add/change props, Canvas creates a new active_version
4. Existing instances stay on their old version - they won't show new fields

The Problem

When you add a new prop to a component (e.g., adding gap to section):

• New component instances will have the new field
• Existing instances created before the change will NOT show the new field

This is intentional - Canvas preserves backward compatibility so existing pages don't break.

Solution: Migrate Instances to Active Version

IMPORTANT: After modifying component props, always run the upgrade command to migrate existing instances.

Use the custom drush commands to upgrade existing instances:

# List components with outdated instances
ddev drush canvas:upgrade-instances

# Preview what would be migrated (dry run)
ddev drush canvas:upgrade-instances sdc.mytheme.section --dry-run

# Migrate a specific component's instances
ddev drush canvas:upgrade-instances sdc.mytheme.section

# Migrate ALL outdated instances at once
ddev drush canvas:upgrade-instances --all

# Show detailed version info for a component
ddev drush canvas:component-info sdc.mytheme.section

Development Workflow for Component Prop Changes

When modifying SDC component props during development:

1. Modify the component (component.yml, .twig, .css)
2. Clear cache: ddev drush cr
3. Check for outdated instances: ddev drush canvas:upgrade-instances
4. Migrate if needed: ddev drush canvas:upgrade-instances <component_id>

canvas_styling_traits Schema Changes

When modifying canvas_styling_traits/schema.json (adding/removing/renaming enum values):

1. Update schema.json with new enum values
2. Clear cache: ddev drush cr
3. Upgrade instances: ddev drush canvas:upgrade-instances --all -y

CRITICAL: If you remove an enum value (e.g., removing "none" from spacing enums), existing component instances that have that value stored will fail validation at render time with Does not have a value in the enumeration. You must also clean the stored data. The canvas:upgrade-instances command migrates instances to the new component version but does NOT update stored field values that are no longer valid.

To fix invalid stored values, use a drush script or hook_update_N():

$database = \Drupal::database();
$tables = ['canvas_page__components', 'canvas_page_revision__components'];
foreach ($tables as $table) {
  $results = $database->select($table, 'c')
    ->fields('c')
    ->condition('c.components_component_id', 'sdc.mytheme.page-header')
    ->execute();
  foreach ($results as $row) {
    $inputs = json_decode($row->components_inputs, TRUE);
    $changed = FALSE;
    foreach (['margin_top', 'margin_bottom'] as $field) {
      if (isset($inputs[$field]) && $inputs[$field] === 'none') {
        unset($inputs[$field]); // Or set to a valid value
        $changed = TRUE;
      }
    }
    if ($changed) {
      $database->update($table)
        ->fields(['components_inputs' => json_encode($inputs)])
        ->condition('entity_id', $row->entity_id)
        ->condition('revision_id', $row->revision_id)
        ->condition('delta', $row->delta)
        ->execute();
    }
  }
}
\Drupal::cache('entity')->deleteAll();
\Drupal::cache('render')->deleteAll();

Summary of enum semantics for styling traits:

• "- None -" (Drupal's built-in for non-required list_string) = unset, no CSS class applied, element keeps its default styling
• "zero" (explicit enum value) = margin: 0 / padding: 0, actively removes spacing via CSS class cst-mt-zero etc.

Production Deployment Workflow

For production deployments where you change component props:

• Write a hook_update_N() to migrate instances (see my_module_update_9015 for example)
• Or run drush canvas:upgrade-instances --all as part of deployment scripts

Canvas Config Entity Structure

Component config entities store versions in config/default/canvas.component.*.yml:

active_version: c533daa1ccfd0fba  # Current version hash
versioned_properties:
  active:
    settings:
      prop_field_definitions:
        gap:  # New prop - only in active version
          field_type: list_string
          # ...
  2076209c0228c2bb:  # Old version - no gap prop
    settings:
      prop_field_definitions:
        # ...

Instance Storage

Component instances live in the canvas_page__components table:

• components_component_id - which component (e.g., sdc.mytheme.section)
• components_component_version - pinned version hash
• components_inputs - JSON blob of prop values
• components_uuid - unique instance identifier
• Revision data in canvas_page_revision__components

Drush Commands

# Clear cache after component changes
ddev drush cr

# List all SDC components
ddev drush ev "print_r(array_keys(\Drupal::service('plugin.manager.sdc')->getDefinitions()));"

# Canvas component version management (your project custom)
ddev drush canvas:upgrade-instances          # List outdated
ddev drush canvas:upgrade-instances --all    # Migrate all
ddev drush canvas:component-info             # Show all components
ddev drush canvas:component-info <id>        # Show specific component

Troubleshooting: Component Version Mismatches (500 Errors)

Symptom

Canvas editor throws 500: 'propName' is not a prop on this version of the Component 'Code component: ComponentName'

Root Cause

The Canvas component entity's active_version was generated from an older build of the component that didn't include the prop. The source component.yml has the prop, but the deployed Canvas entity doesn't.

Key distinction by component type:

• SDC components (sdc.*): Canvas regenerates versions on drush cr from the source component.yml
• JS code components (js.*): Canvas versions are set when the component is uploaded via CLI. drush cr alone does NOT update them.

Recovery Steps

For JS Code Components (js.*)

# 1. Re-upload the component to generate a new version from source component.yml
npx canvas upload --components container -y

# 2. Upgrade existing instances to the new active version
ddev drush canvas:upgrade-instances js.container -y

# 3. Verify
ddev drush canvas:component-info js.container

The upload command is configured via .env in the canvas-components project:

• CANVAS_SITE_URL=http://example.ddev.site
• CANVAS_CLIENT_ID=canvas_cli
• CANVAS_CLIENT_SECRET=canvas_cli_secret

For SDC Components (sdc.*)

# 1. Clear cache to trigger re-discovery from source
ddev drush cr

# 2. Upgrade existing instances
ddev drush canvas:upgrade-instances sdc.mytheme.component-name -y

# 3. Export updated config entity
ddev drush cex -y

Diagnosis Commands

# Check what props the active version has
ddev drush ev "\$c = \Drupal::entityTypeManager()->getStorage('component')->load('js.container'); echo implode(', ', array_keys(\$c->get('versioned_properties')['active']['settings']['prop_field_definitions'] ?? []));"

# Check what version/inputs an instance has on a specific page
ddev drush ev "\$p = \Drupal::entityTypeManager()->getStorage('canvas_page')->load(6); foreach (\$p->toArray()['components'] ?? [] as \$c) { if (\$c['component_id'] === 'js.container') echo \$c['component_version'] . ': ' . \$c['inputs']; }"

# Compare: does the instance version match the active version?
ddev drush canvas:upgrade-instances  # Lists all mismatches

Stale Auto-Saves

Canvas auto-saves can reference old component versions. If a 500 persists after upgrading instances, check for and delete stale auto-saves:

# Check pending auto-saves
curl -s -b /tmp/cookies.txt "https://example.ddev.site/canvas/api/v0/auto-saves/pending"

# Delete a stale auto-save (need CSRF token)
CSRF=$(curl -s -b /tmp/cookies.txt "https://example.ddev.site/session/token")
curl -s -b /tmp/cookies.txt -X DELETE -H "X-CSRF-Token: $CSRF" \
  "https://example.ddev.site/canvas/api/v0/auto-saves/canvas_page/{PAGE_ID}"

Prevention

After modifying any component's component.yml props:

1. JS components: npx canvas upload --components <name> -y
2. SDC components: ddev drush cr
3. Always: ddev drush canvas:upgrade-instances to check for stale instances
4. Always: Test the Canvas editor page before pushing

Resources

• Canvas project: https://www.drupal.org/project/canvas
• Canvas docs: https://project.pages.drupalcode.org/canvas/
• SDC docs: https://www.drupal.org/docs/develop/theming-drupal/using-single-directory-components
• Canvas SDC Starterkit: https://www.drupal.org/project/canvas_sdc_starterkit
• SDC Examples: https://www.drupal.org/project/sdc_examples

Apply to Files

• **/components/**/*.component.yml
• **/components/**/*.twig
• **/components/**/*.css
• **/components/**/*.js
• docroot/themes/custom/mytheme/components/**/*
• docroot/modules/custom/*/components/**/*