AngularJS - validation not based on form element - angularjs

Question

Is there a way to inject some validation - custom or otherwise - that isn't tied to a form element? Like - validate that some condition is met, but have it work with standard AngularJS validation?
Update:
Here's what I'm trying to accomplish:
I have a form contains a list of sections. Each section is controlled by a checkbox, and the section will display (via an ng-if) when the checkbox is checked.
Within each section, there's an opportunity for an item to be selected via a popup modal that is activated by a button click. Until an item is selected for that section, the form needs to be invalid. Once an item is selected for each selection that is checked, then the form needs to be valid.
I have a button at the bottom of the form with an ng-disabled="frm.$invalid". I want that to stay disabled until each section that has been checked contains an item that was selected via the modal.
Update 2:
Here's some example code:
<form class="form-horizontal" role="form" name="frm" novalidate>
<label>Name: </label>
<input type="text" ng-model="name" required/>
<div ng-repeat="orderItem in orderItems">
<input type="checkbox" name="items[]" ng-model="orderItem.selected"/>
<div ng-if="orderItem.selected">
... bunch of form fields
<button ng-click="openExchangeSelectionModal(orderItem)">Select Item</button>
<div ng-show="orderItem.exchange_item">
Selected Item: {{orderItem.exchange_item.name}} - ${{orderItem.exchange_item.price | number: 2}}
</div>
</div>
</div>
<button ng-disabled="frm.$invalid" ng-click="submitOrder">Submit</button>
</form>

Answer 1

Checking if the form is valid won't help you in this case since there is no required input that's not being filled.
What I would recommend is that the disable button would call a function that makes some logic about the number of check boxes expanded and the number of selected items and returns the buttons state (true for active otherwise false)
Let's take the example code you put up:
//some html tags...
<button ng-disabled="checkValid()" ng-click="submitOrder">Submit</button>
</form>
Notice that I changed the frm.$invalid to checkValid function, which is a function that is defined on your controller and can perform any logic you want to determine rather show your submit button or not.