Angular: why isn't $evalAsync called $applyAsync?

Question

Angular beginner question about scopes (docs here).

1. $eval executes an expression in the context of a scope.
2. $apply basically calls $eval and then $digest.
3. Why does $evalAsync call $digest too (or, more precisely, ensure $digest is called)?

It seems to be that $evalAsync should really be called $applyAsync, doesn't it?

I'm a beginner -- what am I missing?

Answer 1

$evalAsync and $applyAsync target to different situations.

$evalAsync: defers the expression to the next loop iteration of current digest cycle. One Angular digest cycle loops for a few times until no data is dirty. If no digest cycle is in progress, it will start a new digest cycle (call $apply method) and evaluate the expression (call $eval method) in it. This method is useful if you call a function out of Angular scope but still like to digest the dirty data when a digest cycle is already in progress, in which case you cannot call $apply.

$applyAsync: defers the expression to the next cycle of digest. It always starts a new cycle of digest to after the expression is evaluated (call $apply method). This method is useful if you frequently execute some service call back (like $http service) out Angular scope with dirty scope data. However, if it starts a digest for each callback, there may be bad performance. Therefore, this method optimize the process by share the digest cycles among several async callbacks, which outperforms the method $evalAsync.

Answer 2

$applyAsync already exists:

Schedule the invocation of $apply to occur at a later time. The actual time difference varies across browsers, but is typically around ~10 milliseconds.

This can be used to queue up multiple expressions which need to be evaluated in the same digest

evalAsync handles digests differently:

$evalAsync triggers a digest, and is unsuitable when it is expected that a digest should not occur.

Note: if this function is called outside of a $digest cycle, a new $digest cycle will be scheduled.

This behavior of this when called implicitly changed in AngularJS 1.3:

-Previously, even if invokeApply was set to false, a $rootScope digest would occur during promise resolution. This is no longer the case, as promises returned from $timeout and $interval will no longer trigger $evalAsync (which in turn causes a $digest) if invokeApply is false.

Workarounds include manually triggering $scope.$apply(), or returning $q.defer().promise from a promise callback, and resolving or rejecting it when appropriate.

References

• AngularJS API: $applyAsync

• applyAsync vs evalAsync