feat(InstantSearch): support onStateChange (#1149)

fixes #1148

Author: Haroenv

src/components/InstantSearch.js

@@ -46,6 +46,10 @@ export default createInstantSearchComponent({
       type: Function,
       default: undefined,
     },
+    onStateChange: {
+      type: Function,
+      default: undefined,
+    },
     initialUiState: {
       type: Object,
       default: undefined,
@@ -84,6 +88,7 @@ export default createInstantSearchComponent({
         routing: this.routing,
         stalledSearchDelay: this.stalledSearchDelay,
         searchFunction: this.searchFunction,
+        onStateChange: this.onStateChange,
         initialUiState: this.initialUiState,
       }),
     };

src/util/createInstantSearchComponent.js

@@ -30,6 +30,13 @@ export const createInstantSearchComponent = component =>
               'Please open a new issue: https://github.com/algolia/vue-instantsearch/issues/new?template=feature.md'
           );
         },
+        onStateChange() {
+          throw new Error(
+            'onStateChange configuration can not be changed dynamically at this point.' +
+              '\n\n' +
+              'Please open a new issue: https://github.com/algolia/vue-instantsearch/issues/new?template=feature.md'
+          );
+        },
         searchFunction(searchFunction) {
           // private InstantSearch.js API:
           this.instantSearchInstance._searchFunction = searchFunction;