import type { Unsubscribable } from './Unsubscribable.js';
/**
 * Object receiving data from an observable.
 *
 * @since 1.0.0
 * @see StrictObserver
 */
export interface Observer<in D = unknown> {
    /**
     * Called at the start of a subscription with the generated subscription object
     * @param subscription
     *
     * @since 1.0.0
     */
    start?(subscription: Unsubscribable): void;
    /**
     * Called with each emitted value
     * @param data
     *
     * @since 1.0.0
     */
    next(data: D): void;
    /**
     * Called when an error occurs in the observable
     * @param err
     *
     * @since 1.0.0
     */
    error(err: unknown): void;
    /**
     * Called when the observable completes. No other data or error will then be received.
     *
     * @since 1.0.0
     */
    complete(): void;
}
export type PartialObserver<in D = unknown> = {
    [K in keyof Observer<D>]?: Observer<D>[K] | undefined;
};
export type AnyObserver<D = any> = Observer<D>;