Required type validation only worked for the last checkbox.

Fixes formly-js#26

src/types/multiCheckbox.js

@@ -14,6 +14,14 @@ export default ngModule => {
           valueProp: c.string.optional
         })
       },
+      defaultOptions: {
+        ngModelAttrs:{
+          required: {
+            attribute: '',
+            bound: ''
+          }
+        }
+      },
       apiCheckInstance: c,
       controller: /* @ngInject */ function($scope) {
         const to = $scope.to;
@@ -23,15 +31,42 @@ export default ngModule => {
           change: setModel
         };
 
+        function checkValidity(expressionValue){
+          var valid = angular.isArray($scope.model[opts.key]) && 
+            $scope.model[opts.key].length > 0 && 
+            expressionValue;
+
+          $scope.fc.$setValidity('required', valid);
+        }
+
         function setModel() {
           $scope.model[opts.key] = [];
           angular.forEach($scope.multiCheckbox.checked, (checkbox, index) => {
             if (checkbox) {
               $scope.model[opts.key].push(to.options[index][to.valueProp || 'value']);
             }
           });
+
+          // Must make sure we mark as touched because only the last checkbox due to a bug in angular.
+          $scope.fc.$setTouched();
+          checkValidity(true);
+        }
+
+        if(opts.expressionProperties && opts.expressionProperties.required){
+          $scope.$watch($scope.options.expressionProperties.required, function(newValue){
+            checkValidity(newValue);
+          });
+        }
+
+        if($scope.to.required){
+          var unwatchFormControl = $scope.$watch('fc', function(newValue){
+            if(!newValue){ return; }
+            checkValidity(true);
+            unwatchFormControl;
+          });
         }
       }
     });
   }
+
 };