@johngw/stream/dist/subjects/StatefulSubject.js

Reactive programming tools using the WHATWG Streams API.

import { ForkableRecallStream } from '../sinks/ForkableRecallStream.js';
import { stateReducer, } from '../transformers/stateReducer.js';
import { Subject } from './Subject.js';
import { ControllableStream } from '../sources/ControllableStream.js';
/**
 * Another form of the {@link Subject:class} that reduces a piece
 * of state with actions and queues changes to the stream.
 *
 * It's important to note that if the result of an action does not change
 * the state then nothing is queued to the stream.
 *
 * @group Subjects
 * @see {@link stateReducer:function}
 * @example
 * ```
 * // The State will be the type of data you want to manipulate.
 * interface State {
 *   authors: string[]
 * }
 *
 * // The Actions are a record of "action name" to a parameter.
 * // If the action doesn't require a parameter, use `void`.
 * type Actions = {
 *   'add author': string
 *   'action that doesnt need a parameter': void
 * }
 *
 * // To get the best out of this API, pass the Actions type and State
 * // during construction.
 * const subject = new StatefulSubject<Actions, State>(
 *   {
 *     __INIT__: () => ({ authors: [] }),
 *
 *     'add author': (state, author) => state.authors.includes(author) ? state : {
 *        ...state,
 *        authors: [...state.authors, author],
 *     },
 *
 *     'action that doesnt need a parameter': (state) => state
 *   }
 * )
 *
 * subject.fork().pipeTo(write(console.info))
 * // { action: '__INIT__', state: { authors: [] } }
 *
 * subject.dispatch('add author', 'Jane Austin')
 * // { action: 'add author', param: 'Jane Austin', state: { authors: ['Jane Austin'] } }
 *
 * subject.dispatch('add author', 'Jane Austin')
 * // **Nothing will be queued as the state did not change.**
 * ```
 */
export class StatefulSubject extends Subject {
    constructor(reducers, options = {}) {
        super({
            ...options,
            forkable: new ForkableRecallStream(),
            transform: stateReducer(reducers),
        });
    }
    /**
     * Returns a new {@link StatefulControllableStream}. Once all controllers
     * have been closed, then the source is also closed.
     */
    control() {
        const subjectController = super.control();
        return Object.assign(subjectController, {
            /**
             * Parameterised version of enqueue for simpliclity.
             *
             * @example
             * ```
             * subject.dispatch('action', 'param')
             * // Instead of
             * subject.enqueue({ action: 'action', param: 'param' })
             * ```
             */
            dispatch(...args) {
                subjectController.enqueue({
                    action: args[0],
                    param: args[1],
                });
            },
        });
    }
}
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