Make AngularJS skip running a digest loop if $http.get() resulted in no new data

Question

I'm currently polling the server to check for new data, and then update the model in an AngularJS app accordingly. He're roughly what I'm doing:

setInterval(function () {
    $http.get('data.json').then(function (result) {
        if (result.data.length > 0) {
          // if data, update model here
        } else {
          // nothing has changed, but AngularJS will still start the digest cycle
        }
    });
}, 5000);

This works fine, but most of the requests will not result in any new data or data changes, but the $http service doesn't really know/care and will still trigger a digest cycle. I feel this is unnecessary (since the digest cycle is one of the heaviest operations in the app). Is there any way to still be able to use $http but somehow skip the digest if nothing has changed?

One solution would be to not use $http but jQuery instead, and then call $apply to let Angular know that the model has changed:

setInterval(function () {
    $.get('data.json', function (dataList) {
        if (dataList.length > 0) {
            // if data, update model
            $scope.value = dataList[0].value + ' ' + new Date();

            // notify angular manually that the model has changed.
            $rootScope.$apply();
        }
    });
}, 5000);

While this seems to work, I'm not sure it's a good idea. I would still like to use pure Angular if possible.

Anyone got any suggestions for improvements to the approach above or a more elegant solution entirely?

P.S. The reason I'm using setInterval instead of $timeout is because $timeout would also trigger a digest cycle which would be unnecessary in this case and only add to the "problem".

Answer 1

1. Solution provided by AngularJS @doc

AngularJS recommends to use a PERF trick that would bundle up a few $http responses in one $digest via $httpProvider. This again, is not fixing the problem, it's just a sedative :)

$httpProvider.useApplyAsync(true)

---

2. Saving the $$watchers solution - risky and not scalable

Firstly, the accepted solution is not scalable - there's no way you're going to do that $watchers trick on a 100K lines of JS code project - it's out of the question.

Secondly, even if the project is small, it's quite risky! What happens for instance if another ajax call arrives that actually needs those watchers?

---

3. Another (feasible) risky solution

The only alternative to achieve this without modifying AngularJS code would be to set the $rootScope.$$phase to true or '$digest', make the $http call, and set back the $rootScope.$$phase to null.

 $rootScope.$$phase = true;
 $http({...})
   .then(successcb, failurecb)
   .finally(function () {
       $rootScope.$$phase = null;            
   });

Risks:

1) other ajax calls might try to do the same thing --> they need to be synchronized via a wrapping ajax service (over $http)

2) user can trigger UI actions in between, actions that will change the $$phase to null and when the ajax call will come back, and still trigger the $digest

The solution popped after scanning AngularJS source code - here's the line that saves the situation: https://github.com/angular/angular.js/blob/e5e0884eaf2a37e4588d917e008e45f5b3ed4479/src/ng/http.js#L1272

---

4. The ideal solution

Because this is a problem that everyone is facing with AngularJS, I think it needs to be addressed systematically. The answers above are not fixing the problem, are only trying to avoid it. So we should create a AngularJS pull request that would allow us to specify via $httpProvider a config that would not trigger a digest for a specific $http request. Hopefully they agree that this needs to be addressed somehow.

Answer 2

You can do it by this trick :

var watchers;

scope.$on('suspend', function () {
  watchers = scope.$$watchers;
  scope.$$watchers = [];
});

scope.$on('resume', function () {
  scope.$$watchers = watchers;
  watchers = null;
});

With this you will remove your scope or reinsert it on the $digest cycle.

You have to manage events to do that of course.

Refer to this post :

Remove and restore Scope from digest cycles

Hope it helps !

Answer 3

Web sockets would seem to be the most elegant solution here. That way you don't need to poll the server. The server can tell your app when data or anything has changed.