angularjs 提交时才验证表单

<form name="signup_form"
      novalidate
      ng-submit="signupForm(signup_form)"
      ng-controller="signupController">
    <fieldset>
        <legend>Signup</legend>
        <div class="row">
            <div class="large-12 columns">
                <label>Your name</label>
                <input type="text"
                       placeholder="Name"
                       name="name"
                       ng-model="signup.name"
                       ng-minlength="3"
                       ng-maxlength="20" required />
                <div class="error"
                     ng-show="signup_form.name.$dirty && signup_form.name.$invalid && submitted">//修改了
                    <small>error here</small>
                </div>
            </div>
        </div>
        <button type="submit" >Submit</button>
    </fieldset>
</form>

<script>
    app.controller('signupController', function($scope) { 
        $scope.submitted = false;
        $scope.signupForm = function(form) {
            if (form.$valid) { //
            
            } else {
                $scope.submitted = true;//修改了
            } 
        }
    });
</script>