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1-js/07-object-oriented-programming/13-mixins/head.html

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+<script>
+let eventMixin = {
+
+  /**
+   * Subscribe to event, usage:
+   *  menu.on('select', function(item) { ... }
+  */
+  on(eventName, handler) {
+    if (!this._eventHandlers) this._eventHandlers = {};
+    if (!this._eventHandlers[eventName]) {
+      this._eventHandlers[eventName] = [];
+    }
+    this._eventHandlers[eventName].push(handler);
+  },
+
+  /**
+   * Cancel the subscription, usage:
+   *  menu.off('select', handler)
+   */
+  off(eventName, handler) {
+    let handlers = this._eventHandlers && this._eventHandlers[eventName];
+    if (!handlers) return;
+    for(let i = 0; i < handlers.length; i++) {
+      if (handlers[i] == handler) {
+        handlers.splice(i--, 1);
+      }
+    }
+  },
+
+  /**
+   * Generate the event and attach the data to it
+   *  this.trigger('select', data1, data2);
+   */
+  trigger(eventName, ...args) {
+    if (!this._eventHandlers || !this._eventHandlers[eventName]) {
+      return; // no handlers for that event name
+    }
+
+    // call the handlers
+    this._eventHandlers[eventName].forEach(handler => handler.apply(this, args));
+  }
+};
+</script>