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Sandra Emily

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Ravi Vishwakarma

Ravi Vishwakarma

25-Jun-2024

The digest cycle is a loop that processes all the watchers in the $scope and checks for changes in their values. If a change is detected, it updates the view to reflect the new value. The digest cycle continues until all watchers report no further changes.

How the Digest Cycle Works

  1. Watchers: AngularJS keeps track of all variables and expressions that need to be watched for changes using Watchers. Each watcher is a function that checks the current value of a variable or expression against its previous value.
  2. Dirty Checking: During the digest cycle, AngularJS performs dirty checking, which means it compares the current value of each watched variable or expression with its previous value. If a change is detected, AngularJS marks the variable as dirty.
  3. Updates: If any watchers are dirty, AngularJS will continue to loop through all the watchers until no changes are detected (typically, it runs up to 10 iterations to prevent infinite loops).

 

Describe the digest cycle in AngularJS and its importance.

Triggering the Digest Cycle

The digest cycle can be triggered in several ways:

  • Event Handlers: AngularJS automatically triggers the digest cycle when an event, such as a click or a keypress, is detected.
  • HTTP Responses: When data is returned from an HTTP request, AngularJS triggers the digest cycle to update the view.
  • Timeouts and Intervals: Using $timeout or $interval services automatically trigger the digest cycle.
  • Manual Triggering: You can manually trigger the digest cycle using $scope.$apply() or $scope.$digest().

Example 

<!DOCTYPE html>
<html ng-app="myApp">
<head>
    <title>Digest Cycle Example</title>
    <script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.8.2/angular.min.js"></script>
    <script>
        var app = angular.module('myApp', []);
        app.controller('MainController', ['$scope', function($scope) {
            $scope.counter = 0;

            $scope.increment = function() {
                $scope.counter++;
            };

            setInterval(function() {
                $scope.counter++;
                $scope.$apply();  // Manually triggering the digest cycle
            }, 1000);
        }]);
    </script>
</head>
<body ng-controller="MainController">
    <p>Counter: {{ counter }}</p>
    <button ng-click="increment()">Increment</button>
</body>
</html>

Explanation

  • Watchers: AngularJS sets up a watcher for counter and the expression {{ counter }} in the view.
  • Event Handling: Clicking the "Increment" button triggers the digest cycle through AngularJS’s event handling system.
  • Manual Triggering: The setInterval function manually triggers the digest cycle using $scope.$apply().

 

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