ng-disabled具有更改时触发的功能

I have a submit validation button that need to be enable when all fields is being entered correctly. As the validation is too complex and is not able to be solve by using formName.$invalid alone, i need to write a function to cater the validation. I expect that the shouldEnable function to be trigger for each of the model changes inside. But it does not. Os is there any alternative for this?I have a submit validation button that need to