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Understanding $emit, $broadcast and $on in AngularJS

05 Aug 2022
Advanced
195K Views
3 min read  

AngularJS provides $on, $emit, and $broadcast services for event-based communication between controllers.

$emit

It dispatches an event name upwards through the scope hierarchy and notify to the registered $rootScope.Scope listeners. The event life cycle starts at the scope on which $emit was called. The event traverses upwards toward the root scope and calls all registered listeners along the way. The event will stop propagating if one of the listeners cancels it.

<!DOCTYPE html>
<html>
<head>
 <title>Broadcasting</title>
 <script src="lib/angular.js"></script>
 <script>
 var app = angular.module('app', []);

 app.controller("firstCtrl", function ($scope) {
 $scope.$on('eventName', function (event, args) {
 $scope.message = args.message;
 console.log($scope.message);
 });
 });

 app.controller("secondCtrl", function ($scope) {
 $scope.handleClick = function (msg) {
 $scope.$emit('eventName', { message: msg });
 };
 });

 </script>
</head>
<body ng-app="app">
 <div ng-controller="firstCtrl" style="border:2px solid #E75D5C; padding:5px;">
 <h1>Parent Controller</h1>
 <p>Emit Message : {{message}}</p>
 <br />
 <div ng-controller="secondCtrl" style="border:2px solid #428bca;padding:5px;">
 <h1>Child Controller</h1>
 <input ng-model="msg">
 <button ng-click="handleClick(msg);">Emit</button>
 </div>
 </div>
</body>
</html>

How it works..

$broadcast

It dispatches an event name downwards to all child scopes (and their children) and notify to the registered $rootScope.Scope listeners. The event life cycle starts at the scope on which $broadcast was called. All listeners for the event on this scope get notified. Afterwards, the event traverses downwards toward the child scopes and calls all registered listeners along the way. The event cannot be canceled.

<!DOCTYPE html>
<html>
<head>
 <title>Broadcasting</title>
 <script src="lib/angular.js"></script>
 <script>
 var app = angular.module('app', []);

 app.controller("firstCtrl", function ($scope) {
 $scope.handleClick = function (msg) {
 $scope.$broadcast('eventName', { message: msg });
 };
 
 });

 app.controller("secondCtrl", function ($scope) {
 $scope.$on('eventName', function (event, args) {
 $scope.message = args.message;
 console.log($scope.message);
 });
 });

 </script>
</head>
<body ng-app="app">
 <div ng-controller="firstCtrl" style="border:2px solid #E75D5C; padding:5px;">
 <h1>Parent Controller</h1>
 <input ng-model="msg">
 <button ng-click="handleClick(msg);">Broadcast</button>
 <br /><br />
 <div ng-controller="secondCtrl" style="border:2px solid #428bca;padding:5px;">
 <h1>Child Controller</h1>
 <p>Broadcast Message : {{message}}</p>
 </div>
 </div>
</body>
</html>

How it works..

$on

It listen on events of a given type. It can catch the event dispatched by $broadcast and $emit.

Note

  1. If there is no parent-child relation between your scopes you can inject $rootScope into the controller and broadcast the event to all child scopes but you cannot emit your event.

  2. You can emit your event only when you have parent-child relation and event propagation is initiated by child. However, $emit can fire an event only for all $rootScope.$on listeners.

What do you think?

I hope you will enjoy the AngularJS event-based communication while developing your app with AngularJS. I would like to have feedback from my blog readers. Your valuable feedback, question, or comments about this article are always welcome.

About Author
Shailendra Chauhan (Microsoft MVP, Founder & CEO at Scholarhat by DotNetTricks)

Shailendra Chauhan is the Founder and CEO at ScholarHat by DotNetTricks which is a brand when it comes to e-Learning. He provides training and consultation over an array of technologies like Cloud, .NET, Angular, React, Node, Microservices, Containers and Mobile Apps development. He has been awarded Microsoft MVP 8th time in a row (2016-2023). He has changed many lives with his writings and unique training programs. He has a number of most sought-after books to his name which has helped job aspirants in cracking tough interviews with ease.
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