Why is using if(!$scope.$$phase) $scope.$apply() an anti-pattern?

Question

Sometimes I need to use $scope.$apply in my code and sometimes it throws a "digest already in progress" error. So I started to find a way around this and found this question: AngularJS : Prevent error $digest already in progress when calling $scope.$apply(). However in the comments (and on the angular wiki) you can read:

Don't do if (!$scope.$$phase) $scope.$apply(), it means your $scope.$apply() isn't high enough in the call stack.

So now i have two questions:

1. Why exactly is this an anti-pattern?
2. How can i safely use $scope.$apply?

Another "solution" to prevent "digest already in progress" error seems to be using $timeout:

$timeout(function() {
  //...
});

Is that the way to go? Is it safer? So here is the real question: How I can entirely eliminate the possibility of a "digest already in progress" error?

PS: I am only using $scope.$apply in non-angularjs callbacks that are not synchronous. (as far as I know those are situations where you must use $scope.$apply if you want your changes to be applied)

Answer 1

After some more digging i was able to solve the question whether it's always safe to use $scope.$apply. The short answer is yes.

Long answer:

Due to how your browser executes Javascript, it is not possible that two digest calls collide by chance.

The JavaScript code we write doesn’t all run in one go, instead it executes in turns. Each of these turns runs uninterupted from start to finish, and when a turn is running, nothing else happens in our browser. (from http://jimhoskins.com/2012/12/17/angularjs-and-apply.html)

Hence the error "digest already in progress" can only occur in one situation: When an $apply is issued inside another $apply, e.g.:

$scope.apply(function() {
  // some code...
  $scope.apply(function() { ... });
});

This situation can not arise if we use $scope.apply in a pure non-angularjs callback, like for example the callback of setTimeout. So the following code is 100% bulletproof and there is no need to do a if (!$scope.$$phase) $scope.$apply()

setTimeout(function () {
    $scope.$apply(function () {
        $scope.message = "Timeout called!";
    });
}, 2000);

even this one is safe:

$scope.$apply(function () {
    setTimeout(function () {
        $scope.$apply(function () {
            $scope.message = "Timeout called!";
        });
    }, 2000);
});

What is NOT safe (because $timeout - like all angularjs helpers - already calls $scope.$apply for you):

$timeout(function () {
    $scope.$apply(function () {
        $scope.message = "Timeout called!";
    });
}, 2000);

This also explains why the usage of if (!$scope.$$phase) $scope.$apply() is an anti-pattern. You simply don't need it if you use $scope.$apply in the correct way: In a pure js callback like setTimeout for example.

Read http://jimhoskins.com/2012/12/17/angularjs-and-apply.html for the more detailed explanation.