# File Drop

> A file input you can also drop files onto. One list of files, either way.

`<slab-file-drop>` · Form · built on `<input type=file>`

## Install

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

## Usage

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

<slab-file-drop label="Attachments" name="attachments" multiple></slab-file-drop>
```

## Example

Here's a styled version, `<slab-styled-file-drop>`. 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-file-drop label="Site photos" accept="image/*" multiple>
  <span slot="description">Photos of the forms and the rebar, for the inspector.</span>
</slab-styled-file-drop>
```

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.

## Keyboard

| Key          | What it does                                         |
| ------------ | ---------------------------------------------------- |
| Enter, Space | Opens the browser's file picker, as on a file input. |

## Behavior

A file input with no visual styles that files can also be dropped onto, around a native `<input type="file">`, so it is picked from, focused, and read the way one is.

Files dropped anywhere on the host go into the input's own `files`, the same list the picker fills, so there is one list and one source of truth. A drop dispatches "input" and "change" the way picking does.

Validity is the native input's, as with Text Field: `required` is passed through and `valueMissing` is mirrored onto the host.

A drop is held to what the picker would allow. Dropped files that don't match `accept` are left out, and without `multiple`, a drop of more than one file is ignored, as a browser ignores it on a native file input.

With no files, it submits one empty file under its `name`, as a native file input does.

Style it through its parts and through `:state(disabled)`, `:state(drag-over)`, `:state(error)` and `:state(required)`. What the drop target looks like, and how big it is, is left to the styles around it.

## API

### `<slab-file-drop>`

#### Attributes

| Name | Type | Default | Description |
| --- | --- | --- | --- |
| `label` (required) | `string` |  |  |
| `accept` | `string` |  | A hint to the picker. Dropped files aren't filtered by it. |
| `disabled` | `boolean` | `false` |  |
| `hide-label` | `boolean` | `false` |  |
| `multiple` | `boolean` | `false` | Without it, only the first of several dropped files is kept. |
| `name` | `string` | `''` |  |
| `required` | `boolean` | `false` |  |
| `version` | `string` |  |  |

#### Properties

| Name | Type | Description |
| --- | --- | --- |
| `files` (read-only) | `FileList \| null` |  |
| `form` (read-only) | `HTMLFormElement \| null` |  |
| `validity` (read-only) | `ValidityState` |  |

#### Methods

| Signature | Description |
| --- | --- |
| `checkValidity(): boolean` |  |
| `click(): void` |  |
| `focus(options?: FocusOptions): void` |  |
| `reportValidity(): boolean` |  |
| `resetValidityFeedback(): void` | Clears the validity feedback without changing `validity`. |
| `setCustomValidity(message: string): void` |  |
| `setValidity(flags?: ValidityStateFlags, message?: string): void` |  |

#### Events

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

#### Slots

| Name | Type | Description |
| --- | --- | --- |
| `description` | `Element \| string` |  |

#### CSS parts

| Name | Description |
| --- | --- |
| `description` | Text about the field, and its validity message when it has one |
| `input` |  |
| `label` | The label |
| `validity-message` |  |

## Copy and own it

`<slab-styled-file-drop>` 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.

### file-drop.ts

```ts
import { customElement } from 'lit/decorators.js';
import FileDrop from 'slab-elements/file-drop.js';
import textInput from './text-input.styles.js';
import styles from './file-drop.styles.js';
import tokens from './tokens.styles.js';

declare global {
  interface HTMLElementTagNameMap {
    'slab-styled-file-drop': StyledFileDrop;
  }
}

/**
 * File Drop, styled: a dashed drop zone with a button for the picker, which
 * turns ember while files are dragged over it.
 */
@customElement('slab-styled-file-drop')
export default class StyledFileDrop extends FileDrop {
  static override styles = [tokens, textInput, styles];
}
```

### text-input.styles.ts

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

/*
  The look shared by everything typed into: Text Field, Textarea, Number, Date
  Field and Combobox, whose fields are "::part(input)", or "::part(textarea)"
  for Textarea. File Drop starts from it too, for its label and description.
*/
export default css`
  :host {
    color: var(--ink);
    display: grid;
    font-family: var(--font-body);
  }

  [part~='label'] {
    font-family: var(--font-display);
    font-size: 1.0625rem;
    font-weight: 600;
    margin-block-end: 0.375rem;
  }

  :host(:state(required)) [part~='label']::after {
    color: var(--ember-deep);
    content: ' *' / '';
  }

  [part~='input'],
  [part~='textarea'] {
    background-color: var(--paper);
    block-size: 3rem;
    border: var(--border);
    border-radius: 0;
    box-sizing: border-box;
    color: inherit;
    font: inherit;

    /* A fixed height, shared with Select, so fields line up in a row. */
    padding: 0 0.75rem;
    transition:
      background-color 100ms,
      border-color 100ms,
      box-shadow 100ms,
      translate 100ms;

    &::placeholder {
      color: var(--ash);
    }

    /*
      The outline is transparent rather than gone, so it still shows in forced
      colors, where box shadows don't.
    */
    &:focus-visible {
      box-shadow: 4px 4px 0 var(--ember);
      outline: 2px solid transparent;
      translate: -2px -2px;
    }

    &:disabled {
      background-color: var(--chalk);
      border-style: dashed;
      color: var(--ash);
      cursor: not-allowed;
    }
  }

  [part~='textarea'] {
    block-size: auto;
    min-block-size: 7rem;
    padding: 0.625rem 0.75rem;
    resize: vertical;
  }

  :host(:state(readonly)) [part~='input'],
  :host(:state(readonly)) [part~='textarea'] {
    background-color: var(--chalk);
  }

  :host(:state(error)) [part~='input'],
  :host(:state(error)) [part~='textarea'] {
    border-color: var(--ember-deep);
  }

  /*
    Spaced by what's in it rather than by a gap, so a description with
    nothing in it takes no room, and a control beside the field lines up
    with its bottom edge.
  */
  [part~='description'] {
    color: var(--ash);
    display: grid;
    font-size: 0.875rem;
  }

  ::slotted([slot='description']) {
    margin-block-start: 0.375rem;
  }

  [part~='validity-message'] {
    color: var(--ember-deep);
    font-weight: 600;
    margin-block-start: 0.375rem;

    &::before {
      content: '■ ' / '';
    }
  }

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

### file-drop.styles.ts

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

/*
  Text Field's label and description, with the file input as a drop zone
  rather than a field. The input is the zone, so a drop anywhere on it lands,
  and its picker button is styled as a Button.
*/
export default css`
  [part~='input'] {
    background-color: var(--paper);
    block-size: auto;
    border: 2px dashed var(--ink);
    cursor: pointer;
    padding: 1.5rem;
    transition:
      background-color 100ms,
      border-color 100ms;

    &::file-selector-button {
      background-color: var(--ink);
      border: 0;
      box-shadow: 3px 3px 0 var(--ember);
      color: var(--paper);
      cursor: pointer;
      font-family: var(--font-display);
      font-size: 0.9375rem;
      font-weight: 600;
      letter-spacing: 0.04em;
      margin-inline-end: 1rem;
      padding: 0.5rem 1rem;
      text-transform: uppercase;
    }
  }

  :host(:state(drag-over)) [part~='input'] {
    background-color: var(--chalk);
    border-color: var(--ember);
    border-style: solid;
  }

  @media (prefers-reduced-motion: reduce) {
    [part~='input'] {
      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);
  }
`;
```