Add better docs for density (#1705)

packages/ui/src/Density/index.tsx

@@ -8,6 +8,71 @@ export type DensityProps<SelectedDensity extends TDensity> = {
   ? { sparse: true; compact?: never }
   : { compact: true; sparse?: never });
 
+/**
+ * Use the `<Density />` component to set the density for all descendants in the
+ * component tree that support density variants.
+ *
+ * In Penumbra UI, density is never set as a prop directly on a component.
+ * Instead, it's set indirectly via a React context, so that entire regions of a
+ * UI can have a matching density.
+ *
+ * For example, imagine you have a `<Table />` (which supports density
+ * variants), which contain a bunch of `<ValueViewComponent />`s (which also
+ * support density variants). You may also have other components in the table
+ * which contain nested components with density variants. If we used a `density`
+ * prop, you'd need to set that prop on every single component in that tree.
+ *
+ * Instead, you can simply wrap the entire `<Table />` with `<Density sparse />`
+ * or `<Density compact />`, and it will set a density context value for all
+ * descendant components:
+ *
+ * ```tsx
+ * <Density compact>
+ *   <Table>
+ *     <Table.Tbody>
+ *       <Table.Tr>
+ *         <Table.Td>This will be rendered with compact spacing.</Table.Td>
+ *       </Table.Tr>
+ *     </Table.Tbody>
+ *   </Table>
+ * </Density>
+ * ```
+ *
+ * Components that support density variants are recognizable because the use the
+ * `useDensity()` hook, and then style their elements based on the value they
+ * receive from that hook:
+ *
+ * ```tsx
+ * const SomeStyledComponent = styled.div<{ $density: Density }>`
+ *   padding: ${props => props.theme.spacing(props.$density === 'sparse' ? 4 : 2)};
+ * `
+ *
+ * const MyComponent = () => {
+ *   const density = useDensity();
+ *
+ *   return <SomeStyledComponent $density={density} />
+ * }
+ * ```
+ *
+ * In some specific situations, you may want to make sure that a given component
+ * that supports density variants always is rendered at a specific density. For
+ * example, let's say you have an icon-only button as the `startAdornment` for a
+ * `<TextInput />`, and you want to make sure that icon-only button always
+ * renders as `compact` density. In that case, simply wrap the button in
+ * `<Density compact />`. Then, it will always be compact, even if there's a
+ * higher-up `<Density sparse />`:
+ *
+ * ```tsx
+ * <TextInput
+ *   // ...
+ *   startAdornment={
+ *     <Density compact>
+ *       <Button iconOnly icon={Search}>Search</Button>
+ *     </Density>
+ *   }
+ * />
+ * ```
+ */
 export const Density = <SelectedDensity extends TDensity>({
   children,
   sparse,

packages/ui/src/hooks/useDensity/index.ts

@@ -8,8 +8,13 @@ import { DensityContext } from '../../utils/DensityContext';
  *
  * @example
  * ```tsx
+ * const SomeStyledComponent = styled.div<{ $density: Density }>`
+ *   padding: ${props => props.theme.spacing(props.$density === 'sparse' ? 4 : 2)};
+ * `
+ *
  * const MyComponent = () => {
  *   const density = useDensity();
+ *
  *   return <SomeStyledComponent $density={density} />
  * }
  * ```