What is $$phase in AngularJS?

Question

I found this code snippet which is part of a angular directive someone wrote for bootstrap modal.

//Update the visible value when the dialog is closed                                                                                                                                                                                                                             //through UI actions (Ok, cancel, etc.)                                                                                                                                                                                                                                          element.bind("hide.bs.modal", function () {                                                                                                                                                                                                                                          scope.modalVisible = false;                                                                                                                                                                                                                                                      if (!scope.$$phase && !scope.$root.$$phase)                                                                                                                                                                                                                                          scope.$apply();                                                                                                                                                                                                                                                          });   

I understood that this part is for the latter half of two way binding we bind to hide.bs.modal event and update modal when UI changes.

I just wanted to know why is the person checking $$phase for scope and rootScope before calling apply ?

Can't we straightaway call apply ?

What is $$phase here?

I tried searching a lot, couldn't find any good explanation.

EDIT:

I found where I saw the example: Simple Angular Directive for Bootstrap Modal

Answer 1

$$phase is a flag set while angular is in a $digest cycle.

Sometimes (in rare cases), you want to check $$phase on the scope before doing an $apply. An error occurs if you try to $apply during a $digest:

Error: $apply already in progress

Answer 2

Davin is totally correct that it's a flag that angular sets during the digest cycle.

But don't use it in your code.

I recently got a chance to ask Misko (angular author) about $$phase, and he said to never use it; it's an internal implementation of the digest cycle, and it's not future safe.

To make sure your code continues to work in the future, he suggested wrapping whatever you want to "safe apply" inside of a $timeout

$timeout(function() {   // anything you want can go here and will safely be run on the next digest. }) 

This comes up a lot when you have callbacks or other things that might resolve during the digest cycle (but don't always)

Here's an example snippet from when I was dealing with one of google's libraries: (The rest of the service this was from has been left off.)

window.gapi.client.load('oauth2', 'v2', function() {     var request = window.gapi.client.oauth2.userinfo.get();     request.execute(function(response) {         // This happens outside of angular land, so wrap it in a timeout          // with an implied apply and blammo, we're in action.         $timeout(function() {             if(typeof(response['error']) !== 'undefined'){                 // If the google api sent us an error, reject the promise.                 deferred.reject(response);             }else{                 // Resolve the promise with the whole response if ok.                 deferred.resolve(response);             }         });     }); }); 

Note that the delay argument for $timeout is optional and will default to 0 if left unset ($timeout calls $browser.defer which defaults to 0 if delay isn't set)

A little non-intuitive, but that's the answer from the guys writing Angular, so it's good enough for me!