# Splitter

> Two panes with a separator between them that resizes them, dragged or moved with the keyboard.

`<slab-splitter>` · Disclosure · built on `[role=separator]`

## Install

```sh
pnpm add slab-elements lit
```

## Usage

```html
<script type="module">
  import 'slab-elements/splitter.js';
</script>

<slab-splitter label="Sidebar" min="15" max="50" value="25">
  <nav slot="primary">…</nav>
  <main slot="secondary">…</main>
</slab-splitter>
```

## Example

Here's a styled version, `<slab-styled-splitter>`. It's not in the package — it's an example of what you can build with the headless element. Copy it and make it yours at the bottom of this page.

```html
<slab-styled-splitter label="Mixes" min="20" max="80" value="35" style="block-size: 14rem">
  <div slot="primary">
    <strong>Mixes</strong>
    <p>Footing, slab, wall.</p>
  </div>
  <div slot="secondary">
    <strong>Slab</strong>
    <p>4,000 psi, a 4-inch slump, broom finish. Cure for seven days before it takes a load.</p>
  </div>
</slab-styled-splitter>
```

Swap `slab-styled-` for `slab-` in that markup and you've got the same example as it ships: the headless elements with no CSS at all. That's your starting point.

The separator is named by `label`, which should name the primary pane, since that's the one `value` sizes. Give the splitter a size, such as a `block-size`, or it's as tall as its tallest pane.

## Keyboard

| Key         | What it does                                                                                                                                                        |
| ----------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| Left, Right | Moves the separator by `step`, the way the arrow points, so Left shrinks the primary pane unless it's on the right. Up and Down when `orientation` is "horizontal". |
| Home, End   | Moves the separator to `min` or `max`.                                                                                                                              |
| Enter       | Collapses the primary pane to `min`, or restores it to where it was.                                                                                                |

## Behavior

A window splitter with no visual styles: two panes and a `[role=separator]` between them that's dragged, or moved with the keyboard, to resize them.

`value` is how much of the splitter the primary pane takes up, as a percentage, and the secondary pane takes the rest. The separator keeps its own size, whatever your styles make it, and the panes share what's left. `min` and `max` bound it, so a sidebar can't be squeezed to nothing or take over.

The separator is named by `label`, which should name the primary pane, and reports `value` as its `aria-valuenow`. Left and Right move it, or Up and Down when `orientation` is `"horizontal"`, by `step`. Home and End move it to `min` and `max`. Enter collapses the primary pane to `min`, and restores it to where it was before.

Laying out the panes and sizing them is what a splitter does, so that's functional and done here: the host is a grid of the two panes and the separator. Give the host a size, or it'll be as tall as its tallest pane. Style the rest through its parts and through `:state(collapsed)`, `:state(disabled)` and `:state(dragging)`.

## API

### `<slab-splitter>`

#### Attributes

| Name | Type | Default | Description |
| --- | --- | --- | --- |
| `label` (required) | `string` |  | Names the separator. Name it after the primary pane, as the separator resizes it. |
| `disabled` | `boolean` | `false` |  |
| `max` | `number` | `100` | The most of the splitter the primary pane can take up, as a percentage. |
| `min` | `number` | `0` | The least of the splitter the primary pane can take up, as a percentage. Enter collapses the primary pane to it. |
| `orientation` | `'horizontal' \| 'vertical'` | `'vertical'` | The separator's. A vertical one sits between panes side by side, and a horizontal one between panes stacked on top of each other. |
| `step` | `number` | `5` | How far each arrow key moves the separator, as a percentage. |
| `value` | `number` | `50` | How much of the splitter the primary pane takes up, as a percentage. It's always between `min` and `max`. |
| `version` | `string` |  |  |

#### Methods

| Signature | Description |
| --- | --- |
| `focus(options?: FocusOptions): void` |  |

#### Events

| Name | Type | Description |
| --- | --- | --- |
| `input` | `Event` |  |
| `change` | `Event` |  |

#### Slots

| Name | Type | Description |
| --- | --- | --- |
| `primary` | `Element` |  |
| `secondary` | `Element` |  |

#### CSS parts

| Name | Description |
| --- | --- |
| `container` | The panes and the separator |
| `primary` | The primary pane |
| `secondary` | The secondary pane |
| `separator` | The separator between the panes |

## Copy and own it

`<slab-styled-splitter>` isn't in the package. It's an example — copy it into your project and change whatever you want. Under the hood, it's just the headless element, subclassed, with `styles` set. That's it. Keyboard, focus, ARIA, forms, and events all come from the headless element, and the styles only reach in through `::part()` and `:state()`, same as yours would.

To use it, copy these files, swap the tag in `@customElement()` for one of your own, and import it. `tokens.styles.ts` holds the colors and fonts every styled example shares, so you only need it once.

### TypeScript

These files are TypeScript, and `@customElement()` is one of Lit's decorators (TypeScript's experimental flavor). You'll need two options in your tsconfig, per [Lit's docs](https://lit.dev/docs/components/decorators/#decorators-typescript): `experimentalDecorators` to compile them, and `useDefineForClassFields` turned off so class fields don't stomp on the accessors Lit sets up.

```json
{
  "compilerOptions": {
    "experimentalDecorators": true,
    "useDefineForClassFields": false
  }
}
```

Using plain JavaScript? Drop the types and register the class with `customElements.define()` instead of the decorator.

### splitter.ts

```ts
import { customElement } from 'lit/decorators.js';
import Splitter from 'slab-elements/splitter.js';
import styles from './splitter.styles.js';
import tokens from './tokens.styles.js';

declare global {
  interface HTMLElementTagNameMap {
    'slab-styled-splitter': StyledSplitter;
  }
}

/**
 * Splitter, styled: two panes in a raised box, split by an ink bar that turns
 * ember while it's hovered, focused or dragged.
 */
@customElement('slab-styled-splitter')
export default class StyledSplitter extends Splitter {
  static override styles = [tokens, styles];
}
```

### splitter.styles.ts

```ts
import { css } from 'lit';

/*
  The separator is an ink bar with a grip at its middle, drawn with a
  pseudo-element so it needs no markup. Its hit area is wider than the bar,
  so it's easy to grab.
*/
export default css`
  :host {
    background-color: var(--paper);
    border: var(--border);
    box-shadow: var(--shadow);
    color: var(--ink);
    font-family: var(--font-body);
  }

  [part~='primary'],
  [part~='secondary'] {
    padding: 1rem;
  }

  [part~='separator'] {
    align-items: center;
    background-color: var(--ink);
    cursor: col-resize;
    display: flex;
    inline-size: 0.5rem;
    justify-content: center;
    transition: background-color 100ms;

    &::before {
      background-color: var(--paper);
      block-size: 2rem;
      content: '';
      inline-size: 2px;
    }

    &:hover {
      background-color: var(--ember);
    }

    &:focus-visible {
      background-color: var(--ember);
      outline: 3px solid var(--ember);
      outline-offset: 2px;
    }
  }

  :host([orientation='horizontal']) [part~='separator'] {
    block-size: 0.5rem;
    cursor: row-resize;
    inline-size: auto;

    &::before {
      block-size: 2px;
      inline-size: 2rem;
    }
  }

  :host(:state(dragging)) [part~='separator'] {
    background-color: var(--ember);
  }

  :host(:state(disabled)) [part~='separator'] {
    background-color: var(--ash);
    cursor: not-allowed;
  }

  @media (prefers-reduced-motion: reduce) {
    [part~='separator'] {
      transition: none;
    }
  }
`;
```

### tokens.styles.ts

```ts
import { css } from 'lit';

/*
  The styled components' palette and type, taken from the logo: ink for the
  courses, ember for the base, paper behind them.

  Each is a short alias for a "--slab-styled-*" property, with the default as
  its fallback. Retheming is "--slab-styled-ember" and the rest on ":root",
  which every shadow root inherits. Were the defaults declared under those
  names here instead, each host's own declaration would shadow the page's.

  Each default is "light-dark()", so the components follow the page's
  "color-scheme": "dark" for dark, "light" for light, and "light dark" to
  follow the operating system. Ink and paper trade places in the dark, so
  ink is always the text and paper always what's behind it. Contrast is given
  as light, then dark.
*/
export default css`
  :host {
    /* Secondary text. 6.8:1 and 6.9:1 on paper. */
    --ash: var(--slab-styled-ash, light-dark(#5e5a55, #a8a19a));
    --border: 2px solid var(--ink);

    /* Behind disabled and read-only controls, and hovered rows. */
    --chalk: var(--slab-styled-chalk, light-dark(#f4f1ec, #2b2926));

    /*
      The logo's orange, brighter in the dark. 4.25:1 and 5.2:1 on paper is
      enough for borders, fills and large text, but not always for small text,
      which takes "--ember-deep".
    */
    --ember: var(--slab-styled-ember, light-dark(#d9492a, #ec5f3b));

    /*
      Orange for small text on paper, and for paper text on it. 5.7:1 and
      7.4:1.
    */
    --ember-deep: var(--slab-styled-ember-deep, light-dark(#b3381a, #ff8566));
    --font-body: var(
      --slab-styled-font-body,
      'IBM Plex Sans',
      system-ui,
      sans-serif
    );
    --font-display: var(
      --slab-styled-font-display,
      'Zilla Slab',
      rockwell,
      'Roboto Slab',
      serif
    );
    --font-mono: var(
      --slab-styled-font-mono,
      'IBM Plex Mono',
      ui-monospace,
      monospace
    );
    --ink: var(--slab-styled-ink, light-dark(#1c1c1c, #f4f1ec));
    --paper: var(--slab-styled-paper, light-dark(#fff, #1a1918));

    /* The hard, offset shadow a raised surface casts. */
    --shadow: 4px 4px 0 var(--ink);
  }
`;
```