Old values in $watchGroup's callback is not correct.

[tobyee]

I'm submitting a ...

• bug report
• feature request
• other (Please do not submit support requests here (see above))

Current behavior:

Old values in $watchGroup's parameters is not correct.

Expected / new behavior:

Correct values.

Minimal reproduction of the problem with instructions:

<input id="input1" ng-model="t1" ng-change="t2=t1">
<input id="input2" ng-model="t2">
<script>
var app = angular.module('plunker', []);
app.controller('MainCtrl', function($scope) {
  $scope.$watchGroup(['t1', 't2'], function(nvals, ovals) {
    console.log('nvals:' + angular.toJson(nvals) + ', ovals:' + angular.toJson(ovals))
  });
});
</script>

Steps:

1. initial state
   log: [null,null], ovals:[null,null]
2. type "a" in input1
   log: nvals:["a","a"], ovals:[null,null]
3. type "b" in input2
   log: nvals:["a","b"], ovals:[null,"a"] (??? ovals expect to be ["a","a"] but not.)

plunker:
https://plnkr.co/edit/slDfDzwvGT6jrZtn42C9?p=preview

Angular version: 1.6.4

Browser: all

Anything else:

[Narretz]

I think this works as egpected. Watch group old / new values are tracked individually, not as a collection. The first value didn't change when you changed the second value, it only changed from null to a, so it's correct that when the second value changed to b, the old value of the first is still null.

[tobyee]

I found some similar issues：
#8671
#12452

So it's by design behavior...
Even if the design is reasonable technically, it is weird result that confuse with the behavior of $watch.

I also found a PR #12643 that try to clarify the behavior, should it be merge?

[Narretz]

@tobyee thanks for unearthing the issues and PR. I will create a new PR, as I think the one you linked doesn't improve the explanation sufficiently.

[gkalpak]

Closing since this has been changed in master (1.7+) with #15854 and there is a PR for documenting it for 1.6.x (#16005).

[gkalpak]

See also #16024 for a discussion on the different behaviors.