Lit Component

Lit Component Development for Common UI

This skill provides guidance for developing Lit web components within the Common UI v2 component library (packages/ui/src/v2).

When to Use This Skill

Use this skill when:

• Creating new cf- prefixed components in the UI package
• Modifying existing Common UI v2 components
• Implementing theme-aware components
• Integrating components with Cell abstractions from the runtime
• Building reactive components for pattern UIs
• Debugging component lifecycle or reactivity issues

Do NOT use this skill when authoring or styling pattern UIs that consume cf- components — that's pattern-ui's job. Patterns must never touch component internals, and pattern JSX uses theme={...}, not Lit's .theme=${...}.

Core Philosophy

Common UI is inspired by SwiftUI and emphasizes:

1. Default Configuration Works: Components should work together with minimal configuration
2. Composition Over Control: Emphasize composing components rather than granular styling
3. Adaptive to User Preferences: Respect system preferences and theme settings (theme is ambient context, not explicit props)
4. Reactive Binding Model: Integration with FRP-style Cell abstractions from the runtime
5. Progressive Enhancement: Components work with plain values but enhance with Cells for reactivity
6. Separation of Concerns: Presentation components, theme-aware inputs, Cell-aware state, runtime-integrated operations

Quick Start Pattern

1. Choose Component Category

Identify which category the component falls into:

• Layout: Arranges other components (vstack, hstack, screen)
• Visual: Displays styled content (separator, skeleton, label)
• Input: Captures user interaction (button, input, checkbox)
• Complex/Integrated: Deep runtime integration with Cells (render, code-editor, outliner)

Complexity spectrum: Components range from pure presentation (no runtime) to deeply integrated (Cell operations, pattern execution, backlink resolution). Choose the simplest pattern that meets requirements.

See references/component-patterns.md for detailed patterns for each category and references/advanced-patterns.md for complex integration patterns.

2. Create Component Files

Create the component directory structure:

packages/ui/src/v2/components/cf-component-name/
├── cf-component-name.ts    # Component implementation
├── index.ts                # Export and registration
└── styles.ts               # Optional: for complex components

3. Implement Component

Basic template:

import { css, html } from "lit";
import { BaseElement } from "../../core/base-element.ts";

export class CFComponentName extends BaseElement {
  static override styles = [
    BaseElement.baseStyles,
    css`
      :host {
        display: block;
        box-sizing: border-box;
      }

      *,
      *::before,
      *::after {
        box-sizing: inherit;
      }
    `,
  ];

  static override properties = {
    // Define reactive properties
  };

  constructor() {
    super();
    // Set defaults
  }

  override render() {
    return html`
      <!-- component template -->
    `;
  }
}

globalThis.customElements.define("cf-component-name", CFComponentName);

4. Create Index File

import { CFComponentName } from "./cf-component-name.ts";

if (!customElements.get("cf-component-name")) {
  customElements.define("cf-component-name", CFComponentName);
}

export { CFComponentName };
export type {}; /* exported types */

Both registrations are the codebase convention, not a contradiction: the component file registers unconditionally when it is imported, and the index file's guarded define is a safe no-op in that case — it only registers when the component module didn't (and prevents duplicate-registration errors during hot module replacement). Keep both.

Never setAttribute in the constructor

The custom-element spec forbids setting attributes during construction — the browser throws Failed to execute 'createElement' on 'Document': The result must not have attributes and the component silently fails to render. Set host attributes (role, exportparts, ARIA defaults) in connectedCallback instead, which is what the existing components do: cf-tabs, cf-progress, cf-tab-list and their siblings call super.connectedCallback() and then assign. Lit schedules the first update asynchronously, so attributes set immediately after the super call are still in place before rendering — only impose a specific ordering where a component actually needs one. A unit test that depends on these attributes has to connect the element (append it, or call connectedCallback directly in a narrow harness); updated() is a post-update hook and does not run the connection lifecycle.

Theme Integration

Most components should use var(--cf-theme-*) CSS variables with fallbacks (--cf-theme-* first, then --cf-* base token, then a literal). Consume cfThemeContext (with applyThemeToElement) only when JavaScript needs the theme object for runtime logic, derived values, or applying theme variables to dynamically created elements.

Theme consumption code and complete reference: See references/theme-system.md for the @consume boilerplate, all available CSS variables, and helper functions.

Cell Integration

For components that work with reactive runtime data, declare the cell as @property({ attribute: false }), subscribe with cell.sink(() => this.requestUpdate()) when the cell property changes, and read with cell.get() in render() (guarding the no-cell case). The pitfalls that matter:

• Clean up the previous subscription before subscribing to a new cell, and unsubscribe in disconnectedCallback() (memory leaks)
• Check isCell(this.cell) before subscribing
• Mutate cells through transactions, never directly

Subscription boilerplate and complete Cell patterns: See references/cell-integration.md for subscription management, nested property access with .key(), array cell manipulation, transaction-based mutations, and finding cells by equality.

Reactive Controllers

For reusable component behaviors, use reactive controllers. Example: InputTimingController for debouncing/throttling:

import { InputTimingController } from "../../core/input-timing-controller.ts";

export class CFInput extends BaseElement {
  @property()
  timingStrategy: "immediate" | "debounce" | "throttle" | "blur" = "debounce";

  @property()
  timingDelay: number = 500;

  private inputTiming = new InputTimingController(this, {
    strategy: this.timingStrategy,
    delay: this.timingDelay,
  });

  private handleInput(event: Event) {
    const value = (event.target as HTMLInputElement).value;

    this.inputTiming.schedule(() => {
      this.emit("cf-change", { value });
    });
  }
}

Common Patterns

Event Emission

Use the emit() helper from BaseElement:

private handleChange(newValue: string) {
  this.emit("cf-change", { value: newValue });
}

Events are automatically bubbles: true and composed: true.

Dynamic Classes

Use classMap for conditional classes:

import { classMap } from "lit/directives/class-map.js";

const classes = {
  button: true,
  [this.variant]: true,
  disabled: this.disabled,
};

return html`
  <button class="${classMap(classes)}">...</button>
`;

List Rendering

Use repeat directive with stable keys:

import { repeat } from "lit/directives/repeat.js";

return html`
  ${repeat(
    items,
    (item) => item.id, // stable key
    (item) =>
      html`
        <div>${item.title}</div>
      `,
  )}
`;

Testing

Colocate tests with components:

// cf-button.test.ts
import { describe, it } from "@std/testing/bdd";
import { expect } from "@std/expect";
import { CFButton } from "./cf-button.ts";

describe("CFButton", () => {
  it("should be defined", () => {
    expect(CFButton).toBeDefined();
  });

  it("should have default properties", () => {
    const element = new CFButton();
    expect(element.variant).toBe("primary");
  });
});

Run with: deno task test (includes required flags)

Package Structure

Each component is declared in its own directory under packages/ui/src/v2/components/. packages/ui/src/v2/index.ts re-exports every component, and the package root re-exports that in turn, so consumers import them from @commonfabric/ui:

// packages/ui/src/v2/index.ts
export * from "./components/cf-button/index.ts";

Reference Documentation

Load these references as needed for detailed guidance:

• references/component-patterns.md - Detailed patterns for each component category, file structure, type safety, styling conventions, event handling, and lifecycle methods
• references/theme-system.md - Theme philosophy, cf-theme provider, CFTheme interface, CSS variables, and theming patterns
• references/cell-integration.md - Comprehensive Cell integration patterns including subscriptions, mutations, array handling, and common pitfalls
• references/advanced-patterns.md - Advanced architectural patterns revealed by complex components: context provision, third-party integration, reactive controllers, path-based operations, diff-based rendering, and progressive enhancement

Key Conventions

1. Always extend BaseElement - Provides emit() helper and base CSS variables
2. Include box-sizing reset - Ensures consistent layout behavior
3. Use attribute: false for objects/arrays/Cells - Prevents serialization errors
4. Prefix custom events with cf- - Namespace convention
5. Export types separately - Use export type { ... }
6. Clean up subscriptions - Always unsubscribe in disconnectedCallback()
7. Use transactions for Cell mutations - Never mutate cells directly
8. Provide CSS variable fallbacks - Components should work without theme context
9. Document with JSDoc - Include @element, @attr, @fires, @example
10. Run tests with deno task test - Not plain deno test

Common Pitfalls to Avoid

• ❌ Forgetting to clean up Cell subscriptions (causes memory leaks)
• ❌ Mutating Cells without transactions (breaks reactivity)
• ❌ Using array index as key in repeat() (breaks reactivity)
• ❌ Missing box-sizing reset (causes layout issues)
• ❌ Not providing CSS variable fallbacks (breaks without theme)
• ❌ Using attribute: true for objects/arrays (serialization errors)
• ❌ Skipping super calls in lifecycle methods (breaks base functionality)

Architecture Patterns to Study

Study these components to understand architectural patterns:

Basic patterns:

• Simple visual: cf-separator - Minimal component, CSS parts, ARIA
• Layout: cf-vstack - Flexbox abstraction, utility classes with classMap
• Themed input: cf-button - Theme consumption, event emission, variants

Advanced patterns:

• Context provider: cf-theme - Ambient configuration with @provide, display: contents, reactive Cell subscriptions
• Runtime rendering: cf-render - Pattern loading, UI extraction, lifecycle management
• Third-party integration: cf-code-editor - CodeMirror lifecycle, Compartments, bidirectional sync, CellController
• Legacy tree editor patterns: historical outliner implementation - Path-based operations, diff-based rendering, keyboard commands, MentionController

Each component reveals deeper patterns - study them not just for API but for architectural principles.