Why Scope.$apply() calls $rootScope.$digest() rather than this.$digest()?

Question

In AngularJS scope.$apply() is called on every event handler (keydown/input events for input directive, change event for select directive, etc) and some other cases.

See small example. Seems that ngRepeat is recalculated and redrawn on every keydown despite the fact that changes occur in the other scope.

It would be interesting to know rationale for such decision.

Answer 1

Would be great to have AngularJS authors on this but I believe that the $digest() needs to be called on the $rootScope since changes triggered in a repeater's can have side-effects in other scopes (even the $rootScope).

The thing is that methods triggered in child scopes can influence objects in parent scopes (since child scopes inherit from parent ones). So, even if functions defined in child scopes can't modify object references from a parent scope they still can modify values in objects defined in a parent scope.

The above might sound a bit cryptic so let's consider an (a bit artificial) example with a list of items:

$scope.items = [{name: 'foo', value:0}, {name: 'bar', value:0}, {name: 'baz', value:0}];

Now, let's use ng-Repeat to display the above list and let's say that clicking on a item should increment a value of other items (once again, the example is a bit artificial but the point here is that an action triggered in one scope can have side effect in other scopes). It could go like this:

$scope.incOther = function(item) {
        for (var i=0; i<$scope.items.length; i++){
            if ($scope.items[i] !== item){
                $scope.items[i].value++;
            }
        }
    };

The example function would modify values in other scopes and AngularJS - to display correct results - need to evaluate watchers in parent scopes (up to the $rootScope since we don't know where an object was defined).

Here is the complete jsFiddle illustrating this: http://jsfiddle.net/pkozlowski_opensource/Z6e5g/3/

In fact the above jsFiddle also includes an object in a $rootScope to illustrate that the watchers evaluation really needs to start at the very top.