How To Watch A Variable In A Service

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

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

$scope is used to transfer the data.

$watch:

Scope in AngularJs object has $watch event which comes into moving-picture show when the model belongings gets inverse or altered.

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

The digest gets called multiple times, when nosotros $sentinel() a function.

Every time when nosotros bind the UI, We insert $watch in $watch listing.

User: <input blazon="text" ng-model="user" /> Countersign: <input type="password" ng-model="laissez passer" />

Example:

            <!DOCTYPE html>          
            <            html            >          
                        <            head            >          
                        <            championship            >          
                        AngularJs $spotter() Role with GFG          
                        </            championship            >          
                        <            script            src            =          
                        </            script            >          
                        <            script            type            =            "text/javascript"            >          
                        var app = athwart.module("watchApp", []);          
                        app.controller("watchCtrl", function ($scope) {          
                        $scope.count = -i;          
                        $scope.$lookout man(          
                        "txtval", function (newval, oldval) {          
                        $scope.count = $telescopic.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: Light-green;"            >          
                        $watch() Function Fired: {{count}}</            span            >          
                        </            div            >          
                        </            trunk            >          
            </            html            >          

$watch in AngularJs is a service.

Information technology is used to keep the track of the changes on specific properties inside a given scope. It is similar to an upshot listener. eastward changes are made in the value of specified variables.

When we register a watch we pass 2 functions equally parameters to the $watch() office.These functions are:

A value part
A listener function

Example:

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

A watcher can change in responses on:

Timeouts
UI
Circuitous asynchronous computations performed by web workers
Ajax calls

Methods Used: $telescopic.$watchGroup

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

$telescopic.$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 rails of the variable. This role takes two arguments:

new value

$scope.$picket('expression', role (newvalue, oldvalue) { //expression parameter
//Code
});
old value

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

Example:

            <!DOCTYPE html>          
            <            html            >          
            <            head            >          
                        <            script            src            =          
                        </            script            >          
            </            caput            >          
            <            torso            ng-app            =            "myNgApp"            >          
                        <            div            ng-controller            =            "myController"            >          
                        Message: <            input            type            =            "text"          
                        ng-model            =            "message"            />          
                        <            br            />          
                        New Message: {{newMessage}} <            br            />          
                        One-time Message: {{oldMessage}}          
                        </            div            >          
                        <            script            >          
                        var ngApp = angular.module('myNgApp', []);          
                        ngApp.controller('myController', role ($telescopic) {          
                        $scope.message = "GeeksForGeeks!";          
                        $telescopic.$watch(          
            'message', office (newValue, oldValue) {          
                        $scope.newMessage = newValue;          
                        $scope.oldMessage = oldValue;          
                        });          
                        });          
                        </            script            >          
            </            body            >          
            </            html            >          

Others:

$digest()

We telephone call the the $digest() function when AngularJS thinks information technology is necessary.

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

$apply():

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

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

Example of $Watch:

            <            div            class            =            "container"            information-ng-app            =            "app"            >          
                        <            div            grade            =            "well"            data-ng-controller            =            "FooCtrl"            >          
                        <            p            ><            strong            >Controller1</            strong            ></            p            >          
                        <            div            class            =            "row"            >          
                        <            div            course            =            "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([ { proper name: 'Detail one of 2' },          
                        { name: 'Item 2 of 2' } ])">          
                        Ship particular two </            a            ></            p            >          
                        <            p            ><            a            href            =            ""          
                        ng-click=          
                        "setItems([ { proper name: 'Detail one of 3' },          
                        { name: 'Detail 2 of three' },          
                        { proper noun: 'Item 3 of three' } ])">          
                        Send detail three </            a            ></            p            >          
                        </            div            >          
                        <            div            class            =            "col-sm-vi"            >          
                        <            p            ><            a            href            =            ""
                        ng-click            =            "setName('Sheldon')"            >          
                        Send name: Geeks</            a            ></            p            >          
                        <            p            ><            a            href            =            ""
                        ng-click            =            "setName('Leonard')"            >          
                        Ship 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            >          

            var            app = angular.module(            'app'            , []);          
            app.factory(            'PostmanService'            ,                        part            () {          
                        var            Postman = {};          
                        Postman.fix =                        function            (key, val) {          
                        Postman[primal] = val;          
                        };          
                        Postman.become =                        function            (key) {          
                        return            Postman[primal];          
                        };          
                        Postman.watch =                        function            ($telescopic, cardinal, onChange) {          
                        render            $scope.$spotter(          
                        office            () {          
                        render            Postman.get(fundamental);          
                        },          
                        function            (newValue, oldValue) {          
                        if            (newValue !== oldValue) {          
                        $scope[cardinal] = newValue;          
                        if            (athwart.isFunction(onChange)) {          
                        onChange(newValue, oldValue);          
                        }          
                        }          
                        }          
                        );          
                        };          
                        return            Postman;          
            });          
            app.controller(            'FooCtrl'            , [            '$scope'            ,          
                        'PostmanService'            ,          
                        part            ($scope, PostmanService) {          
                        $scope.setItems =                        part            (items) {          
                        PostmanService.ready(            'items'            , items);          
                        };          
                        $scope.setName =                        role            (proper name) {          
                        PostmanService.set(            'proper name'            , name);          
                        };          
            }]);          
            app.controller(            'BarCtrl'            , [            '$scope'            ,          
                        'PostmanService'            ,          
                        function            ($scope, PostmanService) {          
                        $scope.items = [];          
                        $scope.proper noun =                        ''            ;          
                        PostmanService.watch($scope,                        'items'            );          
                        PostmanService.watch(          
                        $scope,            'name'            ,                        function            (newVal, oldVal) {          
                        alarm(            'Hullo, '            + newVal +                        '!'            );          
                        });          
            }]);          

CSS:

            .well {          
                        margin-top            :                        10px            ;          
                        margin-bottom            :                        10px            ;          
            }          
            p:terminal-child {          
                        margin-bottom            :                        0            ;          
            }          