js-draw/dist/mjs/EventDispatcher.mjs

Draw pictures using a pen, touchscreen, or mouse! JS-draw is a drawing library for JavaScript and TypeScript.

// Code shared with Joplin (from when it was MIT licensed)
// (js-draw was originally intended to be part of Joplin).
/**
 * Handles notifying listeners of events.
 *
 * `EventKeyType` is used to distinguish events (e.g. a `ClickEvent` vs a `TouchEvent`)
 * while `EventMessageType` is the type of the data sent with an event (can be `void`).
 *
 * @example
 * ```
 * const dispatcher = new EventDispatcher<'event1'|'event2'|'event3', void>();
 * dispatcher.on('event1', () => {
 *   console.log('Event 1 triggered.');
 * });
 * dispatcher.dispatch('event1');
 * ```
 *
 */
export default class EventDispatcher {
    constructor() {
        this.listeners = {};
    }
    dispatch(eventName, event) {
        const listenerList = this.listeners[eventName];
        if (listenerList) {
            for (let i = 0; i < listenerList.length; i++) {
                listenerList[i](event);
            }
        }
    }
    on(eventName, callback) {
        if (!this.listeners[eventName])
            this.listeners[eventName] = [];
        this.listeners[eventName].push(callback);
        return {
            // Retuns false if the listener has already been removed, true otherwise.
            remove: () => {
                const originalListeners = this.listeners[eventName];
                this.off(eventName, callback);
                return originalListeners.length !== this.listeners[eventName].length;
            },
        };
    }
    /** Removes an event listener. This is equivalent to calling `.remove()` on the object returned by `.on`. */
    off(eventName, callback) {
        const listeners = this.listeners[eventName];
        if (!listeners)
            return;
        // Replace the current list of listeners with a new, shortened list.
        // This allows any iterators over this.listeners to continue iterating
        // without skipping elements.
        this.listeners[eventName] = listeners.filter((otherCallback) => otherCallback !== callback);
    }
}