Angular's method of one-way binding to an object

Question

Angular's method of one-way binding to an object

Seeking a method for one-way (not one time) binding between an attribute on a directive without utilizing attrs.$observe. Currently considering binding via &attr and invoking the variables in the template like {{attr()}}.

app.controller('MainCtrl', function($scope) {
  $scope.names = ['Original'];
  setTimeout(function () {
    $scope.names.push('Asynchronously updated name');
    $scope.$apply();
  }, 1000);
});

app.directive('helloComponent', function () {
  return {
    scope: {
      'names': '&names'
    },
    template: '<li ng-repeat="name in names()">Hello {{name}}</li>'
  }
});

 <body ng-controller="MainCtrl">
    <ul>
      <hello-component names="names"/>
    </ul>
  </body>

Plunker

Exploring alternatives that maintain one-way binding without the necessity to invoke the bound properties.

Edit

Clarification added to emphasize the desire to bind to an object, not just a string. Consequently, @attr (designed for string attributes) is not considered a suitable solution.

javascript angularjs

Answer 1

Answer №1

The use of the symbol "&" is actually the correct approach in this scenario. While I have previously argued against it with @JoeEnzminger (view our discussion here and here) due to its semantic ambiguity, Joe's perspective on creating a one-way binding to an object using "&" instead of "@" has proven to be more suitable.

If you prefer not to utilize an isolate scope, a similar effect can be achieved by employing $parse:

var parsedName = $parse(attrs.name);
$scope.nameFn = function(){
  return parsedName($scope);
}

This can then be used in the template like so:

"<p>Hello {{nameFn()}}</p>"

Answer 2

The use of the symbol "&" is actually the correct approach in this scenario. While I have previously argued against it with @JoeEnzminger (view our discussion here and here) due to its semantic ambiguity, Joe's perspective on creating a one-way binding to an object using "&" instead of "@" has proven to be more suitable.

If you prefer not to utilize an isolate scope, a similar effect can be achieved by employing $parse:

var parsedName = $parse(attrs.name);
$scope.nameFn = function(){
  return parsedName($scope);
}

This can then be used in the template like so:

"<p>Hello {{nameFn()}}</p>"

Answer 3

Answer №2

The other responses did not cover this, but Angular 1.5 now supports one-way bindings for objects (check out the scope section in $compile docs for Angular 1.5.9):

< or <attr - establishes a one-way (one-directional) binding between a local scope property and an expression passed via the attribute attr. The expression is evaluated in the context of the parent scope. If no attr name is specified then the attribute name is assumed to be the same as the local name. You can also make the binding optional by adding ?: <? or <?attr.

For example, when using
<my-component my-attr="parentModel">
with directive definition
scope: { localModel:'<myAttr' }
, the isolated scope property localModel will mirror the value of parentModel on the parent scope. Changes made to parentModel will be reflected in localModel, but changes in localModel will not reflect in parentModel. There are, however, two important points to consider:

One-way binding does not duplicate the value from the parent to the isolate scope; it simply sets the same value. Therefore, if your bound value is an object, modifications to its properties in the isolated scope will be mirrored in the parent scope since they reference the same object.

One-way binding monitors changes to the identity of the parent value. This means that the $watch on the parent value only triggers if the reference to the value has changed. While this may not usually be an issue, it's important to note when you bind one-way to an object and then substitute that object in the isolated scope. If you change a property of the object in the parent scope, the change will not transfer to the isolated scope because the identity of the object in the parent scope remains the same. In this scenario, you must assign a new object instead.

One-way binding is beneficial if you do not intend to pass changes from your isolated scope bindings back to the parent. However, it doesn't rule out this possibility entirely.

In the following example, one-way binding is used to transmit changes in an object from the controller's scope to a directive.

Update

@Suamere pointed out that modifying properties of a bound object through one-way binding is possible; however, if the entire object is swapped out in the local model, the binding with the parent model will break because the parent and local scopes will refer to different objects. Two-way binding addresses this issue. The code snippet has been updated to emphasize these distinctions.

angular.module('App', [])

.directive('counter', function() {
  return {
    templateUrl: 'counter.html',
    restrict: 'E',
    scope: {
      obj1: '<objOneWayBinding',
      obj2: '=objTwoWayBinding'
    },
    link: function(scope) {
      scope.increment1 = function() {
        scope.obj1.counter++;
      };
      scope.increment2 = function() {
        scope.obj2.counter++;
      };

      scope.reset1 = function() {
        scope.obj1 = {
          counter: 0,
          id: Math.floor(Math.random()*10000),
          descr: "One-way binding",
          creator: "Directive"
        };
      };

      scope.reset2 = function() {
        scope.obj2 = {
          counter: 0,
          id: Math.floor(Math.random()*10000),
          descr: "Two-way binding",
          creator: "Directive"
        };
      };
    }
  };
})

.controller('MyCtrl', ['$scope', function($scope) {
  $scope.increment = function() {
    $scope.obj1FromController.counter++;
    $scope.obj2FromController.counter++;
  };

  $scope.reset = function() {
    $scope.obj1FromController = {
      counter: 0,
      id: Math.floor(Math.random()*10000),
      descr: "One-way binding",
      creator: "Parent"
    };

    $scope.obj2FromController = {
      counter: 0,
      id: Math.floor(Math.random()*10000),
      descr: "Two-way binding",
      creator: "Parent"
    };
  };

  $scope.reset();
}])

;

<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.5.6/angular.js"></script>

<div ng-app="App">

  <script type="text/ng-template" id="counter.html">
    <h3>In Directive</h3>
    <pre>{{obj1 | json:0}}</pre>

    <button ng-click="increment1()">
      Increment obj1 from directive
    </button>

    <button ng-click="reset1()">
      Replace obj1 from directive (breaks binding)
    </button>

    <pre>{{obj2 | json:0}}</pre>

    <button ng-click="increment2()">
      Increment obj2 from directive
    </button>

    <button ng-click="reset2()">
      Replace obj2 from directive (maintains binding)
    </button>

  </script>

  <div ng-controller="MyCtrl">
    
    <counter obj-one-way-binding="obj1FromController"
             obj-two-way-binding="obj2FromController">
    </counter>
    
    <h3>In Parent</h3>

    <pre>{{obj1FromController | json:0}}</pre>
    <pre>{{obj2FromController | json:0}}</pre>

    <button ng-click="increment()">
      Increment from parent
    </button>
    
    <button ng-click="reset()">
      Replace from parent (maintains binding)
    </button>

  </div>
</div>

Answer 4

The other responses did not cover this, but Angular 1.5 now supports one-way bindings for objects (check out the scope section in $compile docs for Angular 1.5.9):

< or <attr - establishes a one-way (one-directional) binding between a local scope property and an expression passed via the attribute attr. The expression is evaluated in the context of the parent scope. If no attr name is specified then the attribute name is assumed to be the same as the local name. You can also make the binding optional by adding ?: <? or <?attr.

For example, when using
<my-component my-attr="parentModel">
with directive definition
scope: { localModel:'<myAttr' }
, the isolated scope property localModel will mirror the value of parentModel on the parent scope. Changes made to parentModel will be reflected in localModel, but changes in localModel will not reflect in parentModel. There are, however, two important points to consider:

One-way binding does not duplicate the value from the parent to the isolate scope; it simply sets the same value. Therefore, if your bound value is an object, modifications to its properties in the isolated scope will be mirrored in the parent scope since they reference the same object.

One-way binding monitors changes to the identity of the parent value. This means that the $watch on the parent value only triggers if the reference to the value has changed. While this may not usually be an issue, it's important to note when you bind one-way to an object and then substitute that object in the isolated scope. If you change a property of the object in the parent scope, the change will not transfer to the isolated scope because the identity of the object in the parent scope remains the same. In this scenario, you must assign a new object instead.

One-way binding is beneficial if you do not intend to pass changes from your isolated scope bindings back to the parent. However, it doesn't rule out this possibility entirely.

In the following example, one-way binding is used to transmit changes in an object from the controller's scope to a directive.

Update

@Suamere pointed out that modifying properties of a bound object through one-way binding is possible; however, if the entire object is swapped out in the local model, the binding with the parent model will break because the parent and local scopes will refer to different objects. Two-way binding addresses this issue. The code snippet has been updated to emphasize these distinctions.

angular.module('App', [])

.directive('counter', function() {
  return {
    templateUrl: 'counter.html',
    restrict: 'E',
    scope: {
      obj1: '<objOneWayBinding',
      obj2: '=objTwoWayBinding'
    },
    link: function(scope) {
      scope.increment1 = function() {
        scope.obj1.counter++;
      };
      scope.increment2 = function() {
        scope.obj2.counter++;
      };

      scope.reset1 = function() {
        scope.obj1 = {
          counter: 0,
          id: Math.floor(Math.random()*10000),
          descr: "One-way binding",
          creator: "Directive"
        };
      };

      scope.reset2 = function() {
        scope.obj2 = {
          counter: 0,
          id: Math.floor(Math.random()*10000),
          descr: "Two-way binding",
          creator: "Directive"
        };
      };
    }
  };
})

.controller('MyCtrl', ['$scope', function($scope) {
  $scope.increment = function() {
    $scope.obj1FromController.counter++;
    $scope.obj2FromController.counter++;
  };

  $scope.reset = function() {
    $scope.obj1FromController = {
      counter: 0,
      id: Math.floor(Math.random()*10000),
      descr: "One-way binding",
      creator: "Parent"
    };

    $scope.obj2FromController = {
      counter: 0,
      id: Math.floor(Math.random()*10000),
      descr: "Two-way binding",
      creator: "Parent"
    };
  };

  $scope.reset();
}])

;

<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.5.6/angular.js"></script>

<div ng-app="App">

  <script type="text/ng-template" id="counter.html">
    <h3>In Directive</h3>
    <pre>{{obj1 | json:0}}</pre>

    <button ng-click="increment1()">
      Increment obj1 from directive
    </button>

    <button ng-click="reset1()">
      Replace obj1 from directive (breaks binding)
    </button>

    <pre>{{obj2 | json:0}}</pre>

    <button ng-click="increment2()">
      Increment obj2 from directive
    </button>

    <button ng-click="reset2()">
      Replace obj2 from directive (maintains binding)
    </button>

  </script>

  <div ng-controller="MyCtrl">
    
    <counter obj-one-way-binding="obj1FromController"
             obj-two-way-binding="obj2FromController">
    </counter>
    
    <h3>In Parent</h3>

    <pre>{{obj1FromController | json:0}}</pre>
    <pre>{{obj2FromController | json:0}}</pre>

    <button ng-click="increment()">
      Increment from parent
    </button>
    
    <button ng-click="reset()">
      Replace from parent (maintains binding)
    </button>

  </div>
</div>

Answer 5

Answer №3

Using an attribute essentially passes a string value. Rather than using the following syntax:

<hello-component name="name"/>

You have the option to use this instead:

<hello-component name="{{name}}"/>

Answer 6

Using an attribute essentially passes a string value. Rather than using the following syntax:

<hello-component name="name"/>

You have the option to use this instead:

<hello-component name="{{name}}"/>

Answer 7

Answer №4

Although the concept is similar to what New Dev suggested, I came up with a solution for a related issue by extracting an object from my isolate scope and creating a getter function for it that called scope.$parent.$eval(attrs.myObj).

In a revised version that closely resembles yours, I made the following adjustments:

app.directive('myDir', [function() {
    return {
        scope : {
            id : '@',
            otherScopeVar : '=',
            names : '='
        },
        template : '<li ng-repeat="name in names">{{name}}</li>'
    }
}]);

I changed it to:

app.directive('myDir', [function() {
    return {
        scope : {
            id : '@',
            otherScopeVar : '='
        },
        template : '<li ng-repeat="name in getNames()">{{name}}</li>',
        link : function(scope, elem, attrs) {
            scope.getNames() {
                return scope.$parent.$eval(attrs.myList);
            };
        }
    }
}]);

This way, during each digest cycle, your object is directly accessed from the parent scope. The benefit of this method for me was that I could switch the directive from two-way binding to one-way binding (which significantly improved performance) without needing to modify the views using the directive.

EDIT Upon further consideration, I realize that this may not be strictly one-way binding because while updating the variable and triggering a digest will always reflect the updated object, there isn't a built-in mechanism to execute additional logic when the object changes, as one would do with a $watch.

Answer 8

Although the concept is similar to what New Dev suggested, I came up with a solution for a related issue by extracting an object from my isolate scope and creating a getter function for it that called scope.$parent.$eval(attrs.myObj).

In a revised version that closely resembles yours, I made the following adjustments:

app.directive('myDir', [function() {
    return {
        scope : {
            id : '@',
            otherScopeVar : '=',
            names : '='
        },
        template : '<li ng-repeat="name in names">{{name}}</li>'
    }
}]);

I changed it to:

app.directive('myDir', [function() {
    return {
        scope : {
            id : '@',
            otherScopeVar : '='
        },
        template : '<li ng-repeat="name in getNames()">{{name}}</li>',
        link : function(scope, elem, attrs) {
            scope.getNames() {
                return scope.$parent.$eval(attrs.myList);
            };
        }
    }
}]);

This way, during each digest cycle, your object is directly accessed from the parent scope. The benefit of this method for me was that I could switch the directive from two-way binding to one-way binding (which significantly improved performance) without needing to modify the views using the directive.

EDIT Upon further consideration, I realize that this may not be strictly one-way binding because while updating the variable and triggering a digest will always reflect the updated object, there isn't a built-in mechanism to execute additional logic when the object changes, as one would do with a $watch.

Angular's method of one-way binding to an object

Plunker

Edit

Answer №1

Answer №2

Update

Answer №3

Answer №4

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