1. Common obstacles in the functionality of data binding2. Constraints

Question

1. Common obstacles in the functionality of data binding2. Constraints

Working on a basic controller to perform some calculations, which is a simplified version of a more complex project. The issue I'm facing is that the result displayed in the HTML gets recalculated every time there's a change, but when calculating within the $scope as a variable, it doesn't update. Refer to the markup comments for details.

Any insights on what could be causing this behavior?

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

javascript angularjs angular-data

Answer 1

Answer №1

The variable result does not update because the mainController function is only evaluated once, which happens when Angular first interprets the HTML and discovers the ng-controller="mainController" expression.

To automatically update the result, you need to set up watch listeners in the controller as shown below:

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 as Angular sets up watch listeners for it. The expression syntax in HTML is just syntactic sugar - Angular internally uses $watch functions for every expression found in the HTML.

Refer to the $watch documentation for more details.

I personally avoid using the $watch syntax mentioned above as it can make the controller bulky. Instead, I prefer calling a function from the HTML like this:

{{ calculateResult() }}

In the controller, define the function as follows:

angular.module('myApp',[])

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

}]);

Side note: If performance is a concern and the calculateResult() function is slow, sticking to the initial version might be preferable.

Answer 2

The variable result does not update because the mainController function is only evaluated once, which happens when Angular first interprets the HTML and discovers the ng-controller="mainController" expression.

To automatically update the result, you need to set up watch listeners in the controller as shown below:

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 as Angular sets up watch listeners for it. The expression syntax in HTML is just syntactic sugar - Angular internally uses $watch functions for every expression found in the HTML.

Refer to the $watch documentation for more details.

I personally avoid using the $watch syntax mentioned above as it can make the controller bulky. Instead, I prefer calling a function from the HTML like this:

{{ calculateResult() }}

In the controller, define the function as follows:

angular.module('myApp',[])

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

}]);

Side note: If performance is a concern and the calculateResult() function is slow, sticking to the initial version might be preferable.

Answer 3

Answer №2

When the controller is initialized, the calculations are performed only at that time when the corresponding view is shown. To incorporate these calculations, you need to place them within functions (no $watch needed):

$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()
  ];
}

Then, make sure to refer to them in your view:

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

Also:

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

Lastly:

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

For more information, visit: http://codepen.io/anon/pen/vORXpg

Answer 4

When the controller is initialized, the calculations are performed only at that time when the corresponding view is shown. To incorporate these calculations, you need to place them within functions (no $watch needed):

$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()
  ];
}

Then, make sure to refer to them in your view:

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

Also:

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

Lastly:

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

For more information, visit: http://codepen.io/anon/pen/vORXpg

Answer 5

Answer №3

To ensure the correct calculation of the result when numbers are changed, it is essential to have a watch in place. Otherwise, the value of result will be computed 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;

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

    // Recalculate 'cool' if result or result2 change
    $scope.$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>

Answer 6

To ensure the correct calculation of the result when numbers are changed, it is essential to have a watch in place. Otherwise, the value of result will be computed 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;

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

    // Recalculate 'cool' if result or result2 change
    $scope.$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>

1. Common obstacles in the functionality of data binding2. Constraints

Answer №1

Answer №2

Answer №3

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