fix(index): only set listeners on init once (#4634)

* fix(index): only set listeners on init once

Without this, if init is called multiple times, multiple event listeners are attached, as well as the helper beign recreated.

This fixes algolia/vue-instantsearch#911 by avoiding to have a different helper instance when init gets called multiple times

* test: add tests for this behaviour

src/widgets/index/__tests__/index-test.ts

@@ -2341,6 +2341,58 @@ See documentation: https://www.algolia.com/doc/api-reference/widgets/index-widge
         expect.anything()
       );
     });
+
+    describe('multiple init calls', () => {
+      it('does not recreate helper', () => {
+        const instance = index({ indexName: 'test' });
+
+        expect(instance.getHelper()).toBe(null);
+
+        instance.init(createInitOptions());
+
+        const helper1 = instance.getHelper()!;
+
+        instance.init(createInitOptions());
+
+        const helper2 = instance.getHelper()!;
+
+        expect(helper1).toBe(helper2);
+      });
+
+      it('does not listen on change again multiple times', () => {
+        const instance = index({ indexName: 'test' });
+
+        expect(instance.getHelper()).toBe(null);
+
+        instance.init(createInitOptions());
+
+        const helper = instance.getHelper()!;
+
+        expect(helper.listenerCount('change')).toBe(2);
+
+        instance.init(createInitOptions());
+
+        expect(helper.listenerCount('change')).toBe(2);
+      });
+
+      it('derives only once', () => {
+        const instance = index({ indexName: 'test' });
+
+        const mainHelper = algoliasearchHelper(createSearchClient(), '');
+
+        const instantSearchInstance = createInstantSearch({ mainHelper });
+
+        expect(instance.getHelper()).toBe(null);
+
+        instance.init(createInitOptions({ instantSearchInstance }));
+
+        expect(mainHelper.derivedHelpers.length).toBe(1);
+
+        instance.init(createInitOptions({ instantSearchInstance }));
+
+        expect(mainHelper.derivedHelpers.length).toBe(1);
+      });
+    });
   });
 
   describe('render', () => {

src/widgets/index/index.ts

@@ -372,6 +372,12 @@ const index = (props: IndexProps): Index => {
     },
 
     init({ instantSearchInstance, parent, uiState }: IndexInitOptions) {
+      if (helper !== null) {
+        // helper is already initialized, therefore we do not need to set up
+        // any listeners
+        return;
+      }
+
       localInstantSearchInstance = instantSearchInstance;
       localParent = parent;
       localUiState = uiState[indexId] || {};
@@ -416,7 +422,7 @@ const index = (props: IndexProps): Index => {
         return mainHelper.search();
       };
 
-      (helper as any).searchWithoutTriggeringOnStateChange = () => {
+      helper.searchWithoutTriggeringOnStateChange = () => {
         return mainHelper.search();
       };