$watch will not trigger data change - $watch not firing on data change

problem ：

I have a watch setup against the model of a ui-select2 dropdown (from ui-bootstrap). I have one for ui-select2 The watch settings of the drop-down list model （ come from ui-bootstrap）.The watch fires on load but not on data changes and I can't figure out why. The watch triggers when it is loaded , But it will not trigger when the data changes , I don't know why? .

It isn't the usual problem of not $apply'ing the model change or not using the third parameter for equality comparison (at least from my code). Do not apply model changes or use the third parameter for equality comparison （ At least from my code ） Not a common problem .

What do I need to do to get it to fire? What I need to do to make it burn ？

Here is a plunk demonstrating the issue. This is a demonstration of the problem plunk.

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Solution ：

Reference resources ： https://stackoom.com/en/question/1CfJN