Add format prop to RAC Slider to support arbitrary aria-valuetext.

packages/@react-aria/i18n/src/useNumberFormatter.ts

@@ -14,12 +14,24 @@ import {NumberFormatOptions, NumberFormatter} from '@internationalized/number';
 import {useLocale} from './context';
 import {useMemo} from 'react';
 
+export type NumberFormatFunction = (value: number, locale: string) => string;
+
+export type CustomNumberFormat = {format(value: number): string};
+
 /**
  * Provides localized number formatting for the current locale. Automatically updates when the locale changes,
  * and handles caching of the number formatter for performance.
- * @param options - Formatting options.
+ * @param format - Formatting options or function.
  */
-export function useNumberFormatter(options: NumberFormatOptions = {}): Intl.NumberFormat {
+export function useNumberFormatter(format?: NumberFormatOptions): Intl.NumberFormat
+export function useNumberFormatter(format?: NumberFormatOptions | NumberFormatFunction): CustomNumberFormat
+export function useNumberFormatter(format: NumberFormatOptions | NumberFormatFunction = {}): CustomNumberFormat {
   let {locale} = useLocale();
-  return useMemo(() => new NumberFormatter(locale, options), [locale, options]);
+  return useMemo(
+    () =>
+      typeof format === 'function'
+        ? {format: (value) => format(value, locale)}
+        : new NumberFormatter(locale, format),
+    [locale, format]
+  );
 }

packages/@react-stately/slider/src/useSliderState.ts

@@ -150,7 +150,7 @@ const DEFAULT_MAX_VALUE = 100;
 const DEFAULT_STEP_VALUE = 1;
 
 export interface SliderStateOptions<T> extends SliderProps<T> {
-  numberFormatter: Intl.NumberFormat
+  numberFormatter: {format(value: number): string}
 }
 
 /**

packages/react-aria-components/src/Slider.tsx

@@ -22,6 +22,11 @@ export interface SliderProps<T = number | number[]> extends Omit<AriaSliderProps
   /**
    * The display format of the value label.
    */
+  format?: Intl.NumberFormatOptions | ((value: number, locale: string) => string),
+  /**
+   * The display format of the value label.
+   * @deprecated Use `format` instead.
+   */
   formatOptions?: Intl.NumberFormatOptions
 }
 
@@ -50,7 +55,7 @@ export interface SliderRenderProps {
 function Slider<T extends number | number[]>(props: SliderProps<T>, ref: ForwardedRef<HTMLDivElement>) {
   [props, ref] = useContextProps(props, ref, SliderContext);
   let trackRef = useRef<HTMLDivElement>(null);
-  let numberFormatter = useNumberFormatter(props.formatOptions);
+  let numberFormatter = useNumberFormatter(props.format ?? props.formatOptions);
   let state = useSliderState({...props, numberFormatter});
   let [labelRef, label] = useSlot();
   let {

packages/react-aria-components/stories/Slider.stories.tsx

@@ -95,6 +95,55 @@ SliderCSS.argTypes = {
   }
 };
 
+interface SliderCustomFormatProps<T = number | number[]> extends SliderProps<T> {
+  fahrenheit: boolean,
+  showAlternative: boolean
+}
+
+function temperatureUnitFormat(fahrenheit: boolean): Intl.NumberFormatOptions {
+  return {
+    style: 'unit',
+    unit: fahrenheit ? 'fahrenheit' : 'celsius',
+    maximumFractionDigits: 1
+  };
+}
+
+export const SliderCustomFormat = ({fahrenheit, showAlternative, ...props}: SliderCustomFormatProps) => {
+  const mainFormat = temperatureUnitFormat(fahrenheit);
+  let format: Intl.NumberFormatOptions | ((value: number, locale: string) => string) = mainFormat;
+
+  if (showAlternative) {
+    format = (value, locale) => {
+      const formatter = new Intl.NumberFormat(locale, mainFormat);
+      const altValue = fahrenheit ? value * 9 / 5 + 32 : (value - 32) * 5 / 9;
+      const altFormat = temperatureUnitFormat(!fahrenheit);
+      const altFormatter = new Intl.NumberFormat(locale, altFormat);
+      return `${formatter.format(value)} (${altFormatter.format(altValue)})`;
+    };
+  }
+
+  return (
+    <Slider {...props} defaultValue={32} className={styles.slider} format={format}>
+      <div className={styles.label}>
+        <Label>Temperature</Label>
+        <SliderOutput />
+      </div>
+      <SliderTrack className={styles.track}>
+        <SliderThumb className={styles.thumb} />
+      </SliderTrack>
+    </Slider>
+  );
+};
+
+SliderCustomFormat.args = {
+  fahrenheit: false,
+  showAlternative: false,
+  isDisabled: false,
+  minValue: 0,
+  maxValue: 100,
+  step: 1
+};
+
 const CustomThumb = ({index, children}: {index: number, children: React.ReactNode}) => {
   return (
     <SliderThumb