$apply vs $digest in directive testing

Question

If I have a directive that responds to the status of a particular attribute on the scope, and I want to change that attribute in my test and verify that it responds correctly, which is the best way of doing that change?

I've seen both these patterns:

scope.$apply(function() {
    scope.myAttribute = true;
});

and

scope.myAttribute = true;
scope.$digest();

What is the difference between them, and which is better and why?

score 207 · Accepted Answer · edited Feb 23 '16 at 02:03

207

scope.$digest() will fire watchers on the current scope, and on all of its children, too. scope.$apply will evaluate passed function and run $rootScope.$digest().

The first one is faster, as it needs to evaluate watchers for current scope and its children. The second one is slower, as it needs to evaluate watchers for$rootScope and all it's child scopes.

When an error occurs in one of the watchers and you use scope.$digest, it's not handled via $exceptionHandler service, so you need to handle exception yourself. scope.$apply uses a try-catch block internally and passes all exceptions to $exceptionHandler.

edited Feb 23 '16 at 02:03

activedecay

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answered Sep 09 '13 at 12:13

T W

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Will `$digest` also be called on the scope's children? – pilau May 17 '14 at 12:09
16

This can be misleading. `$digest` does not only fire watchers on current scope, but on all of its children too. – tasseKATT Nov 30 '14 at 20:15
@tasseKATT, I think I've captured what your comment is suggesting. – activedecay Feb 23 '16 at 00:48

score 12 · Answer 2 · edited Jan 22 '15 at 23:45

12

As the documentation itself mentions $digest cycle is peformed any time you do $scope.$apply. As per developer guide on scope

After evaluating the expression, the $apply method performs a $digest. In the $digest phase the scope examines all of the $watch expressions and compares them with the previous value.

And as per the Scope API documentation

Usually you don't call $digest() directly in controllers or in directives. Instead a call to $apply() (typically from within a directives) will force a $digest().

So you should not explicitly call $digest, you calling $apply method would trigger a digest cycle.

edited Jan 22 '15 at 23:45

Jackson

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answered Sep 09 '13 at 12:09

Chandermani

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this is not true as digest is sufficient for most of the cases where only the current scope and its children need to be evaluated. it is true that $scope.$apply calls $digest internally but it is very important to remember it will end up calling $rootScope.$digest() and hence very heavy – Srivathsa Harish Venkataramana Dec 29 '15 at 17:00

$apply vs $digest in directive testing

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