function setupModelWatcher(ctrl) {
  // model -> value
  // Note: we cannot use a normal scope.$watch as we want to detect the following:
  // 1. scope value is 'a'
  // 2. user enters 'b'
  // 3. ng-change kicks in and reverts scope value to 'a'
  //    -> scope value did not change since the last digest as
  //       ng-change executes in apply phase
  // 4. view should be changed back to 'a'
  ctrl.$$scope.$watch(function ngModelWatch(scope) {
    var modelValue = ctrl.$$ngModelGet(scope);

    // if scope model value and ngModel value are out of sync
    // This cannot be moved to the action function, because it would not catch the
    // case where the model is changed in the ngChange function or the model setter
    if (modelValue !== ctrl.$modelValue &&
      // checks for NaN is needed to allow setting the model to NaN when there's an asyncValidator
      // eslint-disable-next-line no-self-compare
      (ctrl.$modelValue === ctrl.$modelValue || modelValue === modelValue)
    ) {
      ctrl.$$setModelValue(modelValue);
    }

    return modelValue;
  });
}
This method is called internally when the bound scope value changes.