How to watch service variables ?

Last Updated : 14 Jul, 2022

$scope acts as a built-in object in AngularJs.

It consists of application data and methods. The $scope is acts as a link between a controller and view (HTML).

$scope is used to transfer the data.

$watch:

Scope in AngularJs object has $watch event which comes into picture when the model property gets changed or altered.

When a data binding is created from some point of program in the view to a variable on the $scope object, a watch is created internally in AngularJs.

The digest gets called multiple times, when we $watch() a function.

Every time when we bind the UI, We insert $watch in $watch list.

User: <input type="text" ng-model="user" />
Password: <input type="password" ng-model="pass" />

Example:

html

<!DOCTYPE html>
<html>
    <head>
        <title>
            AngularJs $watch() Function with GFG
        </title>
        <script src=
"https://ajax.googleapis.com/ajax/libs/angularjs/1.4.8/angular.min.js">
      </script>
        <script type="text/javascript">
            var app = angular.module("watchApp", []);
            app.controller("watchCtrl", function ($scope) {
                $scope.count = -1;
                $scope.$watch(
                  "txtval", function (newval, oldval) {
                    $scope.count = $scope.count + 1;
                });
            });
        </script>
    </head>
    <body>
        <div ng-app="watchApp" 
             ng-controller="watchCtrl">
            Enter Text:<input type="text" 
                              ng-model="txtval" />
          <br />
            <br />
            <span style="color: Green;">
              $watch() Function Fired: {{count}}</span>
        </div>
    </body>
</html>

$watch in AngularJs is a service.

It is used to keep the track of the changes on specific properties within a given scope. It is similar to an event listener. e changes are made in the value of specified variables.

When we register a watch we pass two functions as parameters to the $watch() function.These functions are:

A value function
A listener function

Example:

$scope.$watch(function() {}, //value function
             function() {}  //listener function
             );

A watcher can change in responses on:

Timeouts
UI
Complex asynchronous computations performed by web workers
Ajax calls

Methods Used: $scope.$watchGroup

$watchGroup() is a shortcut to set up watchers with the same callback, passing an array of watchExpressions.

$scope.$watchGroup(['obj.a', 'obj.b.bb[4]', 'letters[8]'], function(newValues, oldValues, scope) { //... });

$scope.$watchCollection is a shortcut to watch arrays or objects. In arrays, the listener is called when any of the elements are replaced, deleted, or added.

The $watch keeps track of the variable. This function takes two arguments:

new value

$scope.$watch('expression', function (newvalue, oldvalue) { //expression parameter //Code });

old value

$scope.$watch(function () {}, function (newvalue, oldvalue) { //Function Parameter // Code });

Example:

html

<!DOCTYPE html>
<html>
<head>
    <script src=
"https://ajax.googleapis.com/ajax/libs/angularjs/1.3.16/angular.min.js">
   </script>
</head>
<body ng-app="myNgApp">

    <div ng-controller="myController">
        Message: <input type="text"
                        ng-model="message" /> 
        <br />
        New Message: {{newMessage}} <br />
        Old Message: {{oldMessage}} 
    </div>
    <script>
        var ngApp = angular.module('myNgApp', []);
    
        ngApp.controller('myController', function ($scope) {
            $scope.message = "GeeksForGeeks!";

            $scope.$watch(
'message', function (newValue, oldValue) {
                $scope.newMessage = newValue;
                $scope.oldMessage = oldValue;
            });
        });
    </script>
</body>
</html>

Others:

$digest()

We call the $digest() function when AngularJS thinks it is necessary.

For example, after a button click handler is executed, or after an AJAX call returns.

$apply():

The $scope.$apply() function takes a function as parameter which it is executed, and after that $scope.$digest() is called internally. That makes it helpful to all the watches are checked

$scope.$apply(function() {
   $scope.data.myVar = "value";
});

Example of $Watch:

html

<div class="container" data-ng-app="app">
    <div class="well" data-ng-controller="FooCtrl">
        <p><strong>Controller1</strong></p>
        <div class="row">
            <div class="col-sm-6">
                <p><a href="" 
                      ng-click=
   "setItems([ { name: 'I am single item' } ])">
                  Send item one </a></p>
                <p><a href="" 
                      ng-click=
               "setItems([ { name: 'Item 1 of 2' },
                  { name: 'Item 2 of 2' } ])">
                  Send item two </a></p>
              
                <p><a href=""
                      ng-click=
                      "setItems([ { name: 'Item 1 of 3' },
                  { name: 'Item 2 of 3' },
                  { name: 'Item 3 of 3' } ])">
                  Send item three </a></p>
            </div>
            <div class="col-sm-6">
                <p><a href="" 
                      ng-click="setName('Sheldon')">
                  Send name: Geeks</a></p>
                <p><a href="" 
                      ng-click="setName('Leonard')">
                  Send name: For</a></p>
                <p><a href="" 
                      ng-click="setName('Penny')">
                  Send name: Geeks</a></p>
            </div>
        </div>
    </div>

    <div class="well" 
         data-ng-controller="BarCtrl">
        <p><strong>Controller2</strong></p>
        <p ng-if="name">Name is: {{ name }}</p>
        <div ng-repeat="item in items">
          {{ item.name }}</div>
    </div>
</div>

javascript

var app = angular.module('app', []);

app.factory('PostmanService', function() {
  var Postman = {};
  Postman.set = function(key, val) {
    Postman[key] = val;
  };
  Postman.get = function(key) {
    return Postman[key];
  };
  Postman.watch = function($scope, key, onChange) {
    return $scope.$watch(
      
      function() {
        return Postman.get(key);
      },
      
      function(newValue, oldValue) {
        if (newValue !== oldValue) {
          // Only update if the value changed
          $scope[key] = newValue;
          // Run onChange if it is function
          if (angular.isFunction(onChange)) {
            onChange(newValue, oldValue);
          }
        }
      }
    );
  };
  return Postman;
});

app.controller('FooCtrl', ['$scope',
                           'PostmanService', 
             function($scope, PostmanService) {
  $scope.setItems = function(items) {
    PostmanService.set('items', items);
  };
  $scope.setName = function(name) {
    PostmanService.set('name', name);
  };
}]);

app.controller('BarCtrl', ['$scope',
                           'PostmanService',
           function($scope, PostmanService) {
  $scope.items = [];
  $scope.name = '';
  PostmanService.watch($scope, 'items');
  PostmanService.watch(
       $scope,'name', function(newVal, oldVal) {
    alert('Hi, ' + newVal + '!');
  });
}]);