ng-required changing checkbox from false to undefined - angularjs

Question

I've been banging my head for a while on this one.
I have a checkbox typed input using ng-model, ng-required, and ng-change
When I uncheck a checkbox I expect the ng-model to be set to false, but it is actually being set to undefined
Also when the ng-required expression toggles; the ng-model also bounces between undefined and false which in turn causes ng-change to trigger.
My actual ng-change code executes some code I do not want to run multiple times.
The simple snippet below shows what I'm running into. I threw a text input into the example to see if it did the same thing. It appears to do the same thing when a user enters a string then deletes it...
This question is not about text inputs...I'm mostly surprised that a false value is not a valid checkbox value when ng-required is true.
var app = angular.module('example', []);
app.controller('controller',['$scope', function($scope){
$scope.inputModel = {
isSelected: undefined,
textInput: undefined
};
$scope.onChangeCounter = 0;
$scope.requiredModel = true;
$scope.resolveCheckboxInput = function() {
if (typeof $scope.inputModel.isSelected === "undefined") {
return "I am undefined"
}
return "I am " + $scope.inputModel.isSelected
};
$scope.resolveTextInput = function() {
if (typeof $scope.inputModel.textInput === "undefined") {
return "I am undefined"
}
return "I am " + $scope.inputModel.textInput
};
$scope.onChangeCallback = function() {
$scope.onChangeCounter++;
};
$scope.isRequired = function() {
return $scope.requiredModel;
};
}]);
<script src="https://cdnjs.cloudflare.com/ajax/libs/angular.js/1.5.8/angular.min.js"></script>
<div ng-app="example">
<ul>
<li> Set Checkbox Input to TRUE</li>
<li> Set Checkbox Input to FALSE</li>
<li> Toggle ng-required</li>
<li> Checkbox Input is cleary affected</li>
</ul>
<div ng-controller="controller">
<label>
<input type="checkbox"
ng-required="isRequired()"
ng-change="onChangeCallback()"
ng-model='inputModel.isSelected' />
Checkbox Input
</label>
<br><br>
<div>
<b>Checkbox Input:</b> {{resolveCheckboxInput()}}
</div>
<br><br>
<label>
<input type="text"
ng-required="isRequired()"
ng-model='inputModel.textInput' />
Text Input
</label>
<br><br>
<div>
<b>Text Input:</b> {{resolveTextInput()}}
</div>
<br><br>
<label>
<input type="checkbox"
ng-model='requiredModel' />
ng-required
</label>
<br><br>
<div>
<b>Checkbox Input ng-change Counter:</b> {{onChangeCounter}}
</div>
</div>
</div>

Answer 1

Credit goes to this SO question/answer: Checkbox input undefined when unchecked - AngularJS
You need to add ng-model-options.
<input type="checkbox"
ng-required="isRequired()"
ng-change="onChangeCallback()"
ng-model='inputModel.isSelected'
ng-model-options="{ allowInvalid: true }" />
I'd usually mark this as duplicate but the answer in the link wasn't accepted, so it may not be apparent that this works.