import { Descriptor } from 'hybrids';
/**
 * An observer executing each passed observer in sequence
 * ```
 * define<MyElement>({
 *   color: {
 *     value: 'red',
 *     observe: forEach(
 *       cssVar('--my-element-color'),
 *       cssVar('--my-element-color-complement', (val, host) => complementaryColor(value)),
 *     ),
 *   },
 * })
 * ```
 * @category Descriptor:Observe
 * @typeParam E - host element type
 * @typeParam V - property value type
 * @param observers a list of 'observe' Descriptor functions to execute
 * @returns An 'observe' Descriptor function
 */
export declare const forEach: <E, V>(...observers: Descriptor<E, V>["observe"][]) => NonNullable<Descriptor<E, V>["observe"]>;
/**
 * An observer which reflects the current property value to a custom CSS property on the host element.
 * ```
 * define<MyElement>({
 *   color: { value: 'red', observe: cssVar('--my-element-color') },
 * })
 * ```
 * @category Descriptor:Observe
 * @typeParam E - host element type
 * @typeParam V - property value type
 * @param customProperty CSS custom property name
 * @param transform am optional function converting the JS to CSS value
 * @returns An 'observe' Descriptor function
 */
export declare const cssVar: <E, V>(customProperty: string, transform?: (val: V, host: E) => any) => NonNullable<Descriptor<E, V>["observe"]>;
/**
 * Runs the observe function only when the current value is non-nullish.
 * ```
 * define({
 *   property: {
 *     ...getset(undefined),
 *     observe: nnull(cssVar(--custom-property)),
 *   },
 * })
 * ```
 * @category Descriptor:Observe
 * @typeParam E - host element type
 * @typeParam V - property value type
 * @param observe 'observe' function to execute when the value is not null or undefined
 * @param nullCallback optional 'observe' function when the value is nullish
 * @returns an 'observe' Descriptor
 */
export declare function nnull<E, V>(observe: Descriptor<E, V>['observe'], nullCallback?: Descriptor<E, V>['observe']): NonNullable<Descriptor<E, V>['observe']>;
/**
 * Runs the observe function only when the current value is truthy.
 * ```
 * define({
 *   property: {
 *     ...getset(''), // will not run on initial set
 *     observe: truthy(cssVar(--custom-property)),
 *   },
 * })
 * ```
 * @category Descriptor:Observe
 * @typeParam E - host element type
 * @typeParam V - property value type
 * @param observe 'observe' function to execute when the value is truthy
 * @param nullCallback optional 'observe' function when the value is falsy
 * @returns an 'observe' Descriptor
 */
export declare function truthy<E, V>(observe: Descriptor<E, V>['observe'], nullCallback?: Descriptor<E, V>['observe']): NonNullable<Descriptor<E, V>['observe']>;