ckeditor/ckeditor5-utils

/**
 * @license Copyright (c) 2003-2020, CKSource - Frederico Knabben. All rights reserved.
 * For licensing, see LICENSE.md or https://ckeditor.com/legal/ckeditor-oss-license
 */

/**
 * @module utils/keystrokehandler
 */

import DomEmitterMixin from './dom/emittermixin';
import { getCode, parseKeystroke } from './keyboard';

/**
 * Keystroke handler allows registering callbacks for given keystrokes.
 *
 * The most frequent use of this class is through the {@link module:core/editor/editor~Editor#keystrokes `editor.keystrokes`}
 * property. It allows listening to keystrokes executed in the editing view:
 *
 *        editor.keystrokes.set( 'Ctrl+A', ( keyEvtData, cancel ) => {
 *            console.log( 'Ctrl+A has been pressed' );
 *            cancel();
 *        } );
 *
 * However, this utility class can be used in various part of the UI. For instance, a certain {@link module:ui/view~View}
 * can use it like this:
 *
 *        class MyView extends View {
 *            constructor() {
 *                this.keystrokes = new KeystrokeHandler();
 *
 *                 this.keystrokes.set( 'tab', handleTabKey );
 *            }
 *
 *            render() {
 *                super.render();
 *
 *                this.keystrokes.listenTo( this.element );
 *            }
 *        }
 *
 * That keystroke handler will listen to `keydown` events fired in this view's main element.
 *
 */
export default class KeystrokeHandler {
    /**
     * Creates an instance of the keystroke handler.
     */
    constructor() {
        /**
         * Listener used to listen to events for easier keystroke handler destruction.
         *
         * @protected
         * @member {module:utils/dom/emittermixin~Emitter}
         */
        this._listener = Object.create( DomEmitterMixin );
    }

    /**
     * Starts listening for `keydown` events from a given emitter.
     *
     * @param {module:utils/emittermixin~Emitter} emitter
     */
    listenTo( emitter ) {
        // The #_listener works here as a kind of dispatcher. It groups the events coming from the same
        // keystroke so the listeners can be attached to them with different priorities.
        //
        // E.g. all the keystrokes with the `keyCode` of 42 coming from the `emitter` are propagated
        // as a `_keydown:42` event by the `_listener`. If there's a callback created by the `set`
        // method for this 42 keystroke, it listens to the `_listener#_keydown:42` event only and interacts
        // only with other listeners of this particular event, thus making it possible to prioritize
        // the listeners and safely cancel execution, when needed. Instead of duplicating the Emitter logic,
        // the KeystrokeHandler re–uses it to do its job.
        this._listener.listenTo( emitter, 'keydown', ( evt, keyEvtData ) => {
            this._listener.fire( '_keydown:' + getCode( keyEvtData ), keyEvtData );
        } );
    }

    /**
     * Registers a handler for the specified keystroke.
     *
     * @param {String|Array.<String|Number>} keystroke Keystroke defined in a format accepted by
     * the {@link module:utils/keyboard~parseKeystroke} function.
     * @param {Function} callback A function called with the
     * {@link module:engine/view/observer/keyobserver~KeyEventData key event data} object and
     * a helper funcion to call both `preventDefault()` and `stopPropagation()` on the underlying event.
     * @param {Object} [options={}] Additional options.
     * @param {module:utils/priorities~PriorityString|Number} [options.priority='normal'] The priority of the keystroke
     * callback. The higher the priority value the sooner the callback will be executed. Keystrokes having the same priority
     * are called in the order they were added.
     */
    set( keystroke, callback, options = {} ) {
        const keyCode = parseKeystroke( keystroke );
        const priority = options.priority;

        // Execute the passed callback on KeystrokeHandler#_keydown.
        // TODO: https://github.com/ckeditor/ckeditor5-utils/issues/144
        this._listener.listenTo( this._listener, '_keydown:' + keyCode, ( evt, keyEvtData ) => {
            callback( keyEvtData, () => {
                // Stop the event in the DOM: no listener in the web page
                // will be triggered by this event.
                keyEvtData.preventDefault();
                keyEvtData.stopPropagation();

                // Stop the event in the KeystrokeHandler: no more callbacks
                // will be executed for this keystroke.
                evt.stop();
            } );

            // Mark this keystroke as handled by the callback. See: #press.
            evt.return = true;
        }, { priority } );
    }

    /**
     * Triggers a keystroke handler for a specified key combination, if such a keystroke was {@link #set defined}.
     *
     * @param {module:engine/view/observer/keyobserver~KeyEventData} keyEvtData Key event data.
     * @returns {Boolean} Whether the keystroke was handled.
     */
    press( keyEvtData ) {
        return !!this._listener.fire( '_keydown:' + getCode( keyEvtData ), keyEvtData );
    }

    /**
     * Destroys the keystroke handler.
     */
    destroy() {
        this._listener.stopListening();
    }
}