AngularJS watch array of objects for data change

Question

I'm implementing a shopping cart and want to store the data in localStorage. I want to watch the variable $scope.cart for change so that I can update the localStorage

The cart variable looks like this:

[{'name':'foo', 'id':'bar', 'amount': 1 },...]

This is the code for the watch.

$scope.updateCart = function(){
    localStorageService.add('cart',JSON.stringify($scope.cart));
    alert('cart updated');
};

$scope.$watch($scope.cart, $scope.updateCart(), true);

This is the HTML where the user changes the model.

<li ng-repeat="item in cart">
  {{item.name}} <input type="text" ng-model="item.amount">
</li>

With the following code, the updateCart() method is never triggered. I'm not sure what I'm doing wrong. I check that the $scope.cart variable did change, but the update was not triggered.

Answer 1

$watch only evaluate string or function parameter in its first argument. Change your $watch like this :

$scope.$watch('cart.name + cart.id + cart.amount', $scope.updateCart());

OR

$scope.$watch('cart', $scope.updateCart, true);

See reference API

Answer 2

It's a common mistake that people use variable in the watch expression. for $scope.cart, you should use a string expression 'cart' instead. For example,

$scope.$watch('cart', function () {...}, true)

AngularJs has a new watch method, $watchCollection. It is basically the same as $watch with the object equality option set to true to watch the change in a property or an array item.

$scope.$watchCollection('cart', function () {...});

Here is the link to AngularJs doc for $watchCollection. http://docs.angularjs.org/api/ng/type/$rootScope.Scope. The syntax is basically the same as $watch except it does not have the 3rd param (object equality check)

Answer 3

try to change

$scope.$watch($scope.cart, $scope.updateCart(), true);

to

$scope.$watch('cart', $scope.updateCart(), true);

$watch only evaluate string or function parameter in its first argument