Does the angular scope binding using the &(ampersand) operator only bind once or repeatedly?

Question

Does the angular scope binding using the &(ampersand) operator only bind once or repeatedly?

Is the angular scope binding &(ampersand) a one time binding? I see it referred to as a one-way binding, but is it also one-time?

Let's say I have:

<my-custom-directive data-item="item" />

And my directive is declared as follows:

.directive('myCustomDirective', [
'$log', function ($log) {
return {
    restrict: 'E',
    templateUrl: '/template.html',
    scope: {
        dataItem: '&'
    }
    controller: function ($scope) {
        // ....
    }
}])

The reason I'm asking if the binding is one-time is because that seems to be what I'm observing, that is. If item in the parent scope is updated, the one in the directive is not updated.

Am I correct in assuming that the binding is one-time?

To achieve what I want, where the directive keeps a copy without affecting the parent scope's item -- I did this:

.directive('myCustomDirective', [
'$log', function ($log) {
return {
    restrict: 'E',
    templateUrl: '/template.html',
    scope: {
        dataItemOriginal: '='
    },
    link: function ($scope) {
        $scope.$watch('dataItemOriginal', function () {
            $scope.dataItem = window.angular.copy($scope.dataItemOriginal);
        });
    },
    controller: function ($scope) {
   //....
   }
}])

Is this the correct approach or is there a more efficient way to achieve this?

javascript angularjs angularjs-scope angular-directive

Answer 1

Answer №1

Why settle for a solution that involves creating expensive watches? There is a more efficient way to achieve the same result.

Take a look at this alternative approach that eliminates the need for creating watches:

.directive('myCustomDirective', [
'$log', function ($log) {
return {
    restrict: 'E',
    templateUrl: '<div ng-click="clicked()">Click me for current value</div>',
    scope: {
        item: '&'
    },
    controller: function($scope) {
        $scope.clicked = function(){
            alert(item());  //item() returns current value of parent's $scope.item property
        }
        $scope.val = item();  //val is the initial value of $parent.item
        $scope.val = 42; //$parent.item is unaffected. 
    }

}])

The use of & in Angular directives can be confusing. While commonly used for passing functions into an isolated scope, its main purpose is to execute an expression in the context of the parent scope.

provides a way to execute an expression in the context of the parent scope

By using &, you create a function in the directive that executes the specified expression in the parent scope. This does not involve creating any watches and allows for flexibility with different expressions.

In your specific example:

<my-custom-directive data-item="item"></my-custom-directive>

scope: {
    item: '&'
}

The directive's $scope.item will act as a function that retrieves the referenced object from the parent scope without using any watches.

Contrary to common misconceptions, the one-way binding nature of & in directives prevents direct changes to $parent.item within the directive. This differs from two-way binding (=) where values can be updated through watches.

Furthermore, by utilizing $parse during function generation, & allows for passing local values that override those in the parent scope. For instance, calling item({item: 42}) within the directive will always return 42, regardless of the parent scope's item value.

This unique feature makes & a useful tool for executing function expressions with custom data from the directive in the parent scope.

Answer 2

Why settle for a solution that involves creating expensive watches? There is a more efficient way to achieve the same result.

Take a look at this alternative approach that eliminates the need for creating watches:

.directive('myCustomDirective', [
'$log', function ($log) {
return {
    restrict: 'E',
    templateUrl: '<div ng-click="clicked()">Click me for current value</div>',
    scope: {
        item: '&'
    },
    controller: function($scope) {
        $scope.clicked = function(){
            alert(item());  //item() returns current value of parent's $scope.item property
        }
        $scope.val = item();  //val is the initial value of $parent.item
        $scope.val = 42; //$parent.item is unaffected. 
    }

}])

The use of & in Angular directives can be confusing. While commonly used for passing functions into an isolated scope, its main purpose is to execute an expression in the context of the parent scope.

provides a way to execute an expression in the context of the parent scope

By using &, you create a function in the directive that executes the specified expression in the parent scope. This does not involve creating any watches and allows for flexibility with different expressions.

In your specific example:

<my-custom-directive data-item="item"></my-custom-directive>

scope: {
    item: '&'
}

The directive's $scope.item will act as a function that retrieves the referenced object from the parent scope without using any watches.

Contrary to common misconceptions, the one-way binding nature of & in directives prevents direct changes to $parent.item within the directive. This differs from two-way binding (=) where values can be updated through watches.

Furthermore, by utilizing $parse during function generation, & allows for passing local values that override those in the parent scope. For instance, calling item({item: 42}) within the directive will always return 42, regardless of the parent scope's item value.

This unique feature makes & a useful tool for executing function expressions with custom data from the directive in the parent scope.

Does the angular scope binding using the &(ampersand) operator only bind once or repeatedly?

Answer №1

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