What differentiates $rootScope from $rootScope.$emit/$broadcast in AngularJS?

This represents the structure of my page.

// app.html
<wrapper ng-if="initialized && $root.user.type!='guest'">
  <header-component></header-component>
  <div class="app-body">
    <sidebar-component></sidebar-component>
    <main class="main-content" style="height:{{$root.pageHeight}}px; overflow-y: scroll">
      <ng-view></ng-view>
    </main>
    <aside-component></aside-component>
  </div>
</wrapper>

Within the ng-view directive, I have a controller that needs to send data to the header-component. It is not directly associated with the header-component.

Consider the current controller for ng-view:

// some-screen.js
$scope.foo = "bar";

I want to display bar in the header.

This can be achieved using either $rootScope (without an event) or by utilizing the $broadcast event.

First method - utilizing $rootScope directly:

// some-screen.js 
$rootScope.foo = "bar";

// header.js 
app.directive("headerComponent", ($rootScope) => {
  return {
    templateUrl: "/structure/header/header.html",
    scope: {},
    link: function($scope, element, attrs) {
      console.log($rootScope.foo) // "bar"
    }
  }
});

Second method - employing the $broadcast event:

// some-screen.js 
$rootScope.$emit("SomeNameOfTheEvent", $scope.foo);

// header.js 
app.directive("headerComponent", ($rootScope) => {
  return {
    templateUrl: "/structure/header/header.html",
    scope: {},
    link: function($scope, element, attrs) {
      $rootScope.$on("SomeNameOfTheEvent", function(event, info) {
        console.log(info.foo) // "bar"
      });
    }
  }
});

When using the $broadcast event, take note of these two factors:

  1. You must name each event - this can become cumbersome in larger applications as recalling specific event names may prove challenging. Generating documentation to reference these names is necessary for maintaining consistency.

  2. Both methods achieve the same result - $broadcast simply involves more code implementation.

AngularJS introduced the $broadcast event for a reason - what am I overlooking?

Answer №1

When using $emit, an event is dispatched upwards, while with $broadcast, an event is dispatched downwards

Explaining further

$rootScope.$emit ensures that only other listeners within the $rootScope can catch the event. This is useful when you want to limit who receives the event, similar to adults having a private conversation in a room away from children.

$rootScope.$broadcast allows the event to be heard by almost everything. It's like parents loudly announcing that dinner is ready for everyone in the house to hear.

$scope.$emit means the event will be picked up by the current $scope, its parent scopes, and the $rootScope. It's akin to a child complaining to their parents at home without other kids listening in (unlike in a grocery store).

$scope.$broadcast targets the current $scope and its children only. It's like a child whispering secrets to their stuffed animals without their parents overhearing.

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