What could be the reason why modifications made to $scope in a directive's link function do not appear on the user interface?

Question

What could be the reason why modifications made to $scope in a directive's link function do not appear on the user interface?

The AngularJS directives' link function causes changes to isolate scope data that are not reflected in the UI.

Take a look at this example:

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

myApp.directive('myDirective', function () {
    return {
        restrict: 'A',
        template: '<span>Title: {{myParams.title}}</span>',
        scope: {
            myParams: '='
        },
        link: function ($scope) {
            // the updated value does not reflect on the UI
            $scope.myParams.title = 'this is myDirective';
        }
    };
});

HTML:

 <div my-directive my-params="{title: 'this is title'}"></div>

I expect the HTML page to show this is myDirective, but instead, it displays this is title.

Furthermore, can anyone explain why this is happening? Thank you.

javascript angularjs angularjs-directive

Answer 1

Answer №1

The issue arises from the fact that the

my-params="{title: 'this is title'}"

is considered a constant expression and cannot be changed. Therefore, any attempt to modify the value of the 'title' property will be overridden by the constant itself. You can see an example of this behavior in this code snippet, which demonstrates how Angular 1.4 handles directives with constant expressions compared to dynamic values from a controller:

<div ng-app="app" ng-controller="Ctrl as ctrl">
  <div my-directive my-params="{title: 'this is title'}"></div>
  <div my-directive my-params="data"></div>
</div>

In the second instance where a non-constant expression is used, it works as expected.

If you switch the Angular version to 1.2, you will notice that Angular throws an error related to infinite digest cycle:

VM1033 angular.js:9101 Error: [$rootScope:infdig] 10 $digest() iterations reached. Aborting!
Watchers fired in the last 5 iterations: [["fn: parentValueWatch; newVal: {\"title\":\"this is title\"}; oldVal: {\"title\":\"this is title\"}"],["fn: parentValueWatch; newVal: {\"title\":\"this is title\"}; oldVal: {\"title\":\"this is title\"}"],["fn: parentValueWatch; newVal: {\"title\":\"this is title\"}; oldVal: {\"title\":\"this is title\"}"],["fn: parentValueWatch; newVal: {\"title\":\"this is title\"}; oldVal: {\"title\":\"this is title\"}"],["fn: parentValueWatch; newVal: {\"title\":\"this is title\"}; oldVal: {\"title\":\"this is title\"}"]]

This problem occurs because the expression within the directive (

my-params="{title: 'this is title'}"

) attempts to overwrite the scope attribute of the directive continuously (creating new objects in Angular 1.2, resulting in an infinite digest loop).

Answer 2

The issue arises from the fact that the

my-params="{title: 'this is title'}"

is considered a constant expression and cannot be changed. Therefore, any attempt to modify the value of the 'title' property will be overridden by the constant itself. You can see an example of this behavior in this code snippet, which demonstrates how Angular 1.4 handles directives with constant expressions compared to dynamic values from a controller:

<div ng-app="app" ng-controller="Ctrl as ctrl">
  <div my-directive my-params="{title: 'this is title'}"></div>
  <div my-directive my-params="data"></div>
</div>

In the second instance where a non-constant expression is used, it works as expected.

If you switch the Angular version to 1.2, you will notice that Angular throws an error related to infinite digest cycle:

VM1033 angular.js:9101 Error: [$rootScope:infdig] 10 $digest() iterations reached. Aborting!
Watchers fired in the last 5 iterations: [["fn: parentValueWatch; newVal: {\"title\":\"this is title\"}; oldVal: {\"title\":\"this is title\"}"],["fn: parentValueWatch; newVal: {\"title\":\"this is title\"}; oldVal: {\"title\":\"this is title\"}"],["fn: parentValueWatch; newVal: {\"title\":\"this is title\"}; oldVal: {\"title\":\"this is title\"}"],["fn: parentValueWatch; newVal: {\"title\":\"this is title\"}; oldVal: {\"title\":\"this is title\"}"],["fn: parentValueWatch; newVal: {\"title\":\"this is title\"}; oldVal: {\"title\":\"this is title\"}"]]

This problem occurs because the expression within the directive (

my-params="{title: 'this is title'}"

) attempts to overwrite the scope attribute of the directive continuously (creating new objects in Angular 1.2, resulting in an infinite digest loop).

Answer 3

Answer №2

The function for link is executed only once during the initialization of the directive. The value assigned to myParams.title within this link function is set at this point (though it may be overridden later)

Afterwards, when you update the value of my-params in the attribute (which is watched separately and updated by Angular), the latest value for myParams.title from the attribute is reflected in the UI.

If you need to change the value, you can have a click function (or other event handler) inside the link like this:

In controller:

$scope.myparams = {title: 'this is title'};

In UI:

<div my-directive my-params="myparams"></div>

In directive:

.directive('myDirective', function () {
    return {
        restrict: 'A',
        template: '<span> Title: {{myParams.title}} </span>',
        scope: {
            myParams: '='
        },
        link: function ($scope,element) {
            // the new value is not reflected in ui

            element.click(function(){
                $scope.myParams.title = 'this is myDirective';
                $scope.$apply();
            })
        }
    };
});

Answer 4

The function for link is executed only once during the initialization of the directive. The value assigned to myParams.title within this link function is set at this point (though it may be overridden later)

Afterwards, when you update the value of my-params in the attribute (which is watched separately and updated by Angular), the latest value for myParams.title from the attribute is reflected in the UI.

If you need to change the value, you can have a click function (or other event handler) inside the link like this:

In controller:

$scope.myparams = {title: 'this is title'};

In UI:

<div my-directive my-params="myparams"></div>

In directive:

.directive('myDirective', function () {
    return {
        restrict: 'A',
        template: '<span> Title: {{myParams.title}} </span>',
        scope: {
            myParams: '='
        },
        link: function ($scope,element) {
            // the new value is not reflected in ui

            element.click(function(){
                $scope.myParams.title = 'this is myDirective';
                $scope.$apply();
            })
        }
    };
});

What could be the reason why modifications made to $scope in a directive's link function do not appear on the user interface?

Answer №1

Answer №2

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