AngularJS - Always keep an eye on a group of items

Question

AngularJS - Always keep an eye on a group of items

Looking to create a custom watcher for a collection within my application, I initially believed that Angular would have all the necessary tools at my disposal.

I had access to $watch, both shallow and deep, as well as $watchCollection. A $digest cycle was in place to monitor changes in variables exposed by $scope, using a dirty checking mechanism to trigger watchers accordingly...

All seemed well! What more could I ask for?

Wrong!

As it turned out, $watchCollection only responded to the initial change in the watched variable...and that was about it for the powerful watchers. Why did it have to be this way?

Following a reality check, I came to the realization that I needed a somewhat unpleasant loop to track this collection, or perhaps implement a callback system to handle modifications of the variable whenever they occurred.

Does anyone have insights on how this can be achieved in the most elegant manner?

Important note:
For some reason unknown to me, there appeared to be a pesky bug lurking in my code...

Upon resolving it, both $watchCollection(expr, foo) and $watch(expr, foo, true) functioned as intended...

I had been misled by a response on this SO post, where a user had commented:

[...] I don't see anything in your code that makes the subsequent requests (to check for new messages). Where does that happen?

I unfortunately took his comments as confirmation of my assumption...my mistake!

Leaving this question as a memento

javascript angularjs watch

Answer 1

Answer №1

Using the 3rd parameter (objectEquality) in a regular $watch function can help you check if objects are equal rather than just comparing references.

Here's an example of how you can use it:

$scope.$watch('prop', function(value) {
   // do something
   }, true);

Setting the value to true instructs Angular to compare the actual objects themselves, not just their memory addresses.

You can find more information about this feature in the documentation related to scope.

Answer 2

Using the 3rd parameter (objectEquality) in a regular $watch function can help you check if objects are equal rather than just comparing references.

Here's an example of how you can use it:

$scope.$watch('prop', function(value) {
   // do something
   }, true);

Setting the value to true instructs Angular to compare the actual objects themselves, not just their memory addresses.

You can find more information about this feature in the documentation related to scope.

Answer 3

Answer №2

Here is an alternative solution that can be used if $watchCollection is not effective. Instead of watching the array directly, watch on a JSON object.

$scope.$watch(function() { 
  return angular.toJson($scope.array); 
},
function() {
  // watch logic
}

I have implemented this solution to monitor multiple arrays as shown below:

$scope.$watch(function() { 
  return JSON.stringify([$scope.array1, $scope.array2]); 
},
function() {
  // watch logic
}

You can choose to use either JSON.stringify or angular.toJson for this purpose.

Answer 4

Here is an alternative solution that can be used if $watchCollection is not effective. Instead of watching the array directly, watch on a JSON object.

$scope.$watch(function() { 
  return angular.toJson($scope.array); 
},
function() {
  // watch logic
}

I have implemented this solution to monitor multiple arrays as shown below:

$scope.$watch(function() { 
  return JSON.stringify([$scope.array1, $scope.array2]); 
},
function() {
  // watch logic
}

You can choose to use either JSON.stringify or angular.toJson for this purpose.

AngularJS - Always keep an eye on a group of items

Answer №1

Answer №2

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