angular validation - prevent button ng-click() when form is invalid - angularjs

Question

I have an angular form as part of a single page App. The 'Continue' button moves a user to the next section of the app, and has an ng-click. I would like to prevent this ng-click from happening when the button is clicked and the form is invalid, but I don't want to disable the Continue button.
Example: A user comes to the page and immediately clicks the Continue button without providing a first name. The error message "You should provide a first name" is shown and the user remains on the page.
How can I do this? Here is the code:
<form name="form" novalidate >
<div ng-show="form.FirstName.$touched && form.FirstName.$error.required" class="errorMessage">
You must enter a first name
</div>
<div class="form-group">
<label for="firstName" class="form-label">First Name</label>
<input id="firstName" class="form-control" name="FirstName" ng-model="vm.user.firstName" required />
</div>
<div class="btn-group">
<button ng-click="vm.next()">Continue</button>
</div>
</form>

Answer 1

Just add form.$valid && before the function call:
<form name="form" novalidate >
<div ng-show="form.FirstName.$touched && form.FirstName.$error.required" class="errorMessage">
You must enter a first name
</div>
<div class="form-group">
<label for="firstName" class="form-label">First Name</label>
<input id="firstName" class="form-control" name="FirstName" ng-model="vm.user.firstName" required />
</div>
<div class="btn-group">
<button ng-click="form.$valid && vm.next()">Continue</button>
</div>
</form>

Answer 2

Either a button is disabled or it's clickable, you can't have it both ways.
What you could do in your controller for vm.next() is say:
myApp.controller('Page1Controller'), ['$scope', function ($scope)
{
$scope.vm = {};
$scope.vm.next = function()
{
if (! $scope.vm.isFormValid())
return;
//move to the next page with your current code
...
}
$scope.vm.isFormValid()
{
$scope.vm.shouldShowError = $scope.form.valid; //add vm.shouldShowError to your error message ng-show in the view so it will display only after clicking continue
return $scope.form.valid;
}
}]);