2 ways data binding does not work

Question

2 ways data binding does not work

127 Views Asked by Tally Barak At 06 July 2015 at 08:36

I am writing a simple controller which does few calculations. This is a reflection of a real project with more advanced information. The problem is that the result, when placed on the html as an expression is being recalcualted with every change, however, when I am calculating in the $scope as a variable, it does not update. see comment in the the markup.

Any idea what I am missing here?

markup

<body ng-app="myApp"> 
<div ng-controller="mainController">
  <h3> Numbers </h3> 
  <label> a </label>
  <input type="number" ng-model="numbers.a"/>

  <label> b </label> 
  <input type="number" ng-model="numbers.b">
<br>
<br>
  <span> Result : {{result}} {{numbers.a*numbers.b}} </span> // the first one does not update, but the second does. 

  <h3> Nums </h3> 

    <label> a </label>
  <input type="number" ng-model="a1">

  <label> b</label> 
  <input type="number" ng-model="b1">

<br>  
  Result2:  {{result2}} {{a1+b1}}
<ul> 
  <li ng-repeat=" i in cool"> {{i}} </li>
  </ul>
  </div>
</body>

javascript:

angular.module('myApp',[])

.controller('mainController', ['$scope', function($scope) {
  $scope.numbers = {a:11, b:10}; 
  $scope.a1 = 5; 
  $scope.b1 = 7; 
  $scope.result = $scope.numbers.a*$scope.numbers.b; 
  $scope.result2 = $scope.a1 +$scope.b1 ;

  $scope.cool = [$scope.result + $scope.result2, 
               $scope.result - $scope.result2]
}]);

http://codepen.io/Tallyb/pen/rVdebm

Original Q&A

There are 3 best solutions below

**Remigius Stalder** · Answer 1 · 2015-07-06T08:47:41.293000

The calculations in the controller are only executed when the controller is instantiated, typically when the corresponding view is displayed. You'll have to encapsulate the calculation in functions (no $watch necessary):

$scope.result = function() {
  return $scope.numbers.a * $scope.numbers.b;
}
$scope.result2 = function() {
  return $scope.a1 + $scope.b1;
}
$scope.cool = function() {
  return [
    $scope.result() + $scope.result2(),
    $scope.result() - $scope.result2()
  ];
}

and reference it in your view:

<span> Result : {{result()}} {{numbers.a*numbers.b}} </span>

and:

Result2:  {{result2()}} {{a1+b1}}

and:

<li ng-repeat=" i in cool(result, result2)"> {{i}} </li>

see: http://codepen.io/anon/pen/vORXpg

**Arun P Johny** · Answer 2 · 2015-07-06T08:48:01.930000

You need a watch to recalculate the value of result when the numbers is changed. Otherwise the value of result is calculated only once

var app = angular.module('my-app', [], function() {})

app.controller('mainController', ['$scope',
  function($scope) {
    $scope.numbers = {
      a: 11,
      b: 10
    };
    $scope.a1 = 5;
    $scope.b1 = 7;

    //if a or b is changed recalculate result
    $scope.$watch('[numbers.a, numbers.b]', function() {
        $scope.result = $scope.numbers.a * $scope.numbers.b;
      })
      //if a1 or b1 is changed recalculate result2
    $scope.$watch('[a1, b1]', function() {
      $scope.result2 = $scope.a1 + $scope.b1;
    })

    //if result or result2 is changed recalculate cool
    $scope.$watch('[result, result2]', function() {
      $scope.cool = [$scope.result + $scope.result2,
        $scope.result - $scope.result2
      ]
    })
  }
]);

<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.2.23/angular.min.js"></script>
<div ng-app="my-app">
  <div ng-controller="mainController">
    <h3> Numbers </h3> 
    <label>a</label>
    <input type="number" ng-model="numbers.a" />

    <label>b</label>
    <input type="number" ng-model="numbers.b" />
    <br/>
    <br/>
    <span> Result : {{result}} {{numbers.a*numbers.b}} </span> // the first one does not update, but the second does.

    <h3> Nums </h3> 

    <label>a</label>
    <input type="number" ng-model="a1" />

    <label>b</label>
    <input type="number" ng-model="b1" />

    <br/>Result2: {{result2}} {{a1+b1}}
    <ul>
      <li ng-repeat=" i in cool">{{i}}</li>
    </ul>
  </div>
</div>

**riptidebyte** · Answer 3 · 2015-07-06T08:50:22.973000

The first variable result doesn't update, because your mainController function only gets evaluated once - which is the first time angular interprets the html and first discovers the expression ng-controller="mainController".

In order to get result to update automatically, you have to set up watch listeners in the controller like this:

angular.module('myApp',[])

.controller('mainController', ['$scope', function($scope) {
  // ...
  $scope.$watch('[numbers.a, numbers.b]', function () {
    $scope.result = $scope.numbers.a*$scope.numbers.b;
  });

}]);

An expression like {{numbers.a*numbers.b}} will update automatically, because angular will set up watch listeners for those. In fact, the expression syntax in the html is just syntactic sugar - under the hood, angular just invokes the same $watch functions for every expression it finds in the html.

See the $watch documentation for more detail.

I personally prefer not to use the $watch syntax mentioned above, because it bloats the controller. Alternatively, you can call a function from your html:

{{ calculateResult() }}

In your controller, you would then define the function like this:

angular.module('myApp',[])

.controller('mainController', ['$scope', function($scope) {
  // ...
  $scope.calculateResult = function () {
    return $scope.numbers.a*$scope.numbers.b;
  };

}]);

Side note: If you're concerned about performance and your calculateResult() function is really slow, you might want to stick to the first version though.

2 ways data binding does not work

There are 3 best solutions below

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