Custom Events

custom-events.js

class CustomEvent {
  constructor() {
    var handlers = [];
    var isolatedHandlers = [];
    this.subscribe = (...subs) => {
      isolatedHandlers = handlers.slice(0);
      for (let sub of subs) {
        if (typeof sub === 'function') {
          isolatedHandlers.push(sub);
        }
      }
      handlers = isolatedHandlers.slice(0);
    };
    this.unsubscribe = (...subs) => {
      isolatedHandlers = handlers.slice(0);
      for (let sub of subs) {
        if (isolatedHandlers.includes(sub)) {
          const lastIndex = isolatedHandlers.lastIndexOf(sub);
          isolatedHandlers.splice(lastIndex, 1);
        }
        handlers = isolatedHandlers.slice(0);
      }
    };
    this.emit = function(...args) {
      for (let h of handlers) {
        h.call(this, ...args);
      }
    };
  }
}

// TEST
function l(arr) { arr.push('l'); }
function o(arr) { arr.push('o'); }
var e = new CustomEvent(), bucket = [];
e.subscribe(l, o, l);
e.emit(bucket); // bucket is ['l', 'o', 'l']
e.unsubscribe(o, l);
bucket = [];
e.emit(bucket); //bucket is ['l']

CustomEvents.markdown

Custom Events

Rules

• an event object has subscribe and unsubscribe methods to add and remove handlers
• subscribe and unsubscribe are able to take an arbitrary number of arguments and tolerate invalid arguments (not functions, or for unsubscribe, functions which are not subscribed) by skipping them
• multiple subscription of the same handler is allowed, and in this case unsubscription removes the last subscription of the same handler
• an event object has an emit method which must invoke all the handlers with the arguments provided
• emit uses its own invocation context as handers' invocation context
• the order of handlers invocation matches the order of subscription
• handler functions can subscribe and unsubscribe handlers, but the changes apply to the next emit call - the handlers for an ongoing emit call are not affected
• subscribe, unsubscribe and emit are the only public properties of event objects (apart from Object.prototype methods)

A script by V.

License.