How does $scope.apply() work exactly in AngularJS? - angularjs

Question

I often updated model variables corresponding to DOM expression ({{}}) within the controllers. e.g.
$scope.myVar = new_value;
Some times the corresponding DOM expression {{myVar}} is updated automtically, others it's not.
I understand that sometimes we need to call $scope.$apply but...
I don't understand when I should call it
Some times I call it (let's say, just to be "sure") but I get this error (I guess since it's already being executed):
Error: [$rootScope:inprog]
http://errors.angularjs.org/1.3.6/$rootScope/inprog?p0=%24digest
Any clue?

Answer 1

Apply essentially "refreshes" your front end with the changes that had occurred to your scope.
Most of the time you dont need to do apply as it already is done for you.
Lets say that you do an ng-click(); Apply is done for you.
However, there are cases where apply is not triggered, and you must do it yourself.
Example with a directive:
.directive('someCheckbox', function(){
return {
restrict: 'E',
link: function($scope, $el, $attrs) {
$el.on('keypress', function(event){
event.preventDefault();
if(event.keyCode === 32 || event.keyCode === 13){
$scope.toggleCheckbox();
$scope.$apply();
}
});
}
}
})
I have made changes to my scope but no apply was done for me thus i need to do it myself.
Another example with a timeout:
$scope.getMessage = function() {
setTimeout(function() {
$scope.message = 'Fetched after two seconds';
console.log('message:' + $scope.message);
$scope.$apply(); //this triggers a $digest
}, 2000);
};
Understanding apply and digest

Answer 2

A good way to understand the purpose of $scope.apply() is to understand that basically it does an internal .digest() within angular to make sure the scope is in sync (double check if anything has changed, etc).
Most of the time, you never need it! Most typical angular things you'll do, ng-click for example, will automatically trigger it for you when you make any changes to the scope.
But take as an example a jQuery UI dialogBox.
Let's say you prompt the user something, and you need to update your scope when they push the OK button.
Angular isn't aware of that button, nor does it know when any event is fired on it.
Hence, this is a very common use-case for $scope.apply()
Inside of that OK buttons event, you'd simply do:
$scope.apply(function () {
// Angular is now aware that something might of changed
$scope.changeThisForMe = true;
});
In a nutshell $scope.$apply tells Angular and any watchers that values have been changed and to go back and check if there are any new values. This keeps things within the Angular context regardless of how you made a change, like in a DOM event, jQuery method, etc.