how How often does the AngularJS digest loop run?

 

Questions


When discussing the merits of AngularJS, two-way data binding is often touted as a major benefit of Angular over other JS frameworks. Digging deeper, the documentation suggests this process is done through dirty-checking rather than through event-driven measures. At first, it seems that the digest-loop works by having a method fire off in the background at periodic intervals, checking all the $watches during each cycle. However, reading further, it seems that the digest-loop is actually triggered by rootScope.digest(), which in turn is triggered by $.apply, which is in turn triggered by an event(!), such as an onClick event called through ng-click.

But, how can this be? I thought Angular doesn’t use change listeners. So how does the digest-loop really operate? Does Angular automatically kick-off the digest-loop internally, or is the digest-loop triggered by events? If the digest-loop is run automatically, how often does it run?


Some clarification points:

  • I’m not asking about how the digest-loop runs when manually binding to changes. In this case, if you want to force a digest loop, you can do so by calling $.apply()
  • I’m also not asking about how often digest loop runs in response to user events. For example, if ng-model is on an input box, Angular will kick-off a digest loop when a user starts typing. The confusing part is that in order to know a user was typing, doesn’t Angular use an event-based onKeyUp somewhere?
  • I already know that there is a limit of 10 cycles max per digest-loop. My question is less about the number of cycles per digest-loop, but rather the number of digest-loops that run, say, per second.
  • Bonus questions: How does the digest-loop relate to the JavaScript event-loop? Does the JS event loop run periodically in the background? Is the digest-loop the same thing as the event loop, but only in the “Angular Context”? Are these totally different ideas?

 

 

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Answer

Angular digests are triggered – they don’t happen through polling.

Code executes, after code is done, angular triggers a digest.

Example:

 element.on('click', function() {
     $scope.$apply(function() { 
         // do some code here, after this, $digest cycle will be triggered
     });
 });

Angular will also trigger a $digest after the compile/link phase:

Compile > Link > Digest

And as to how many digest cycles are triggered? It depends on how soon the scope variables stabalise. Usually it takes at least 2 cycles to determine that.

angularjs,event-handling,javascript,javascript-events