Client: Implement event system (#4)

public_html/js/events.js

@@ -0,0 +1,42 @@
+"use strict";
+
+/**
+ * Base class to implement an event system in any class. Events can be dispatched and
+ * subscribed/unsubscribed. Events can have multiple registered callbacks.
+ */
+class EventDispatcher {
+    constructor() {
+        this._events = {};
+    }
+
+    /**
+     * Dispatch a named event. Calls all callbacks that are subscribed to this event.
+     */
+    dispatch(eventName, data = null) {
+        if (this._events[eventName]) {
+            this._events[eventName].forEach((callback) => {
+                callback(data);
+            });
+        }
+    }
+
+    /**
+     * Subscribe to an event.
+     */
+    on(eventName, callback) {
+        if (!this._events[eventName]) {
+            this._events[eventName] = [];
+        }
+        this._events[eventName].push(callback);
+    }
+
+    /**
+     * Unsubscribe from an event. Not sure how to unsubscribe anonymous functions though.
+     * If callback is not found, nothing happens.
+     */
+    off(eventName, callback) {
+        if (this._events[eventName]) {
+            this._events[eventName] = this._events[eventName].filter(item => item !== callback);
+        }
+    }
+}