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.