Set empty input field to ng-invalid on submit using AngularJS?

Question

I want to set input field to ng-invalid if a user clicks submit and the form fields are empty using AngularJS. I know a directive can be used to custom form validation and using $setValidity to manually set the input field but I'm not sure I need a directive to just highlight the empty/required fields on submit. Any help would be greatly appreciated. Thank you.

Here is the code I am working with: HTML

<form name="SignUpForm ng-submit="submitUser()" novalidate>
<input type="text" name="dob" ng-model="dob" placeholder="Date of Birth" required />
<span class="error" ng-show="signUpForm.dob.$dirty && signUpForm.dob.$invalid">Please provide a valid date of birth</span>
<input type="text" name="email" ng-model="email" ng-pattern="/^([0-9a-zA-Z]([-\.\w]*[0-9a-zA-Z])*@([0-9a-zA-Z][-\w]*[0-9a-zA-Z]\.)+[a-zA-Z]{2,9})$/" placeholder="Email" required />
<span class="error" ng-show="signUpForm.email.$dirty && signUpForm.email.$invalid">Please provide a valid email address</span>
<button type="submit">Submit</submit>
</form>

SASS/CSS

input {
&.ng-valid.ng-dirty {
color: #fff;
background-color: rgba(146, 253, 156, 0.5);
border-color: #92fd9c;
box-shadow: inset 2px 2px 2px #92fd9c;
}
&.ng-invalid.ng-dirty {
color: #850000;
background: rgba(254, 186, 186, 0.5);
border: 1px solid #850000;
}

AngularJS

$scope.submitUser = function() {
if ($scope.signUpForm.$valid) {
// Set form values & construct data to send
} else {
$scope.response = "Please fill in the required fields";
}

Solution

I'm not sure I completely follow, but does this do what you want?

Plunker:

http://plnkr.co/edit/L5qzEJRKkdGogsE7IzAr?p=preview

HTML:

<form name="signUpForm" ng-submit="submitUser()" novalidate>
<input type="text" ng-class="{ errorinput: submitted && signUpForm.dob.$invalid }" name="dob" ng-model="dob" placeholder="Date of Birth" required />
<span class="e" ng-show="submitted && signUpForm.dob.$invalid">Please provide a valid date of birth</span>
<br>
<input type="text" name="email" ng-class="{ errorinput: submitted && signUpForm.email.$invalid }" ng-model="email" ng-pattern="/^([0-9a-zA-Z]([-\.\w]*[0-9a-zA-Z])*@([0-9a-zA-Z][-\w]*[0-9a-zA-Z]\.)+[a-zA-Z]{2,9})$/" placeholder="Email" required />
<span class="e" ng-show="submitted && signUpForm.email.$invalid">Please provide a valid email address</span>
<br>
<button type="submit">Submit</button>
</form>

Javascript:

$scope.submitUser = function() {
  if ($scope.signUpForm.$valid) {

  } else {
    $scope.submitted = true;
  }
}