Memoize the `searchUris` selector

This is needed so that `searchUris` don’t change when non-relevant
frame state changes

src/sidebar/store/modules/frames.js

-import { createSelector } from 'reselect';
+import {
+  createSelector,
+  createSelectorCreator,
+  defaultMemoize,
+} from 'reselect';
+import shallowEqual from 'shallowequal';
 
 import * as util from '../util';
 
@@ -103,15 +108,23 @@ function searchUrisForFrame(frame) {
   return uris;
 }
 
-/**
- * Return the set of URIs that should be used to search for annotations on the
- * current page.
- */
-function searchUris(state) {
-  return state.frames.reduce(function (uris, frame) {
-    return uris.concat(searchUrisForFrame(frame));
-  }, []);
-}
+// "selector creator" that uses `shallowEqual` instead of `===` for memoization
+const createShallowEqualSelector = createSelectorCreator(
+  defaultMemoize,
+  shallowEqual
+);
+
+// Memoized selector will return the same array (of URIs) reference unless the
+// values of the array change (are not shallow-equal).
+const searchUris = createShallowEqualSelector(
+  state => {
+    return state.frames.reduce(
+      (uris, frame) => uris.concat(searchUrisForFrame(frame)),
+      []
+    );
+  },
+  uris => uris
+);
 
 export default {
   init: init,

src/sidebar/store/modules/test/frames-test.js

@@ -157,7 +157,12 @@ describe('sidebar/store/modules/frames', function () {
         testCase.frames.forEach(frame => {
           store.connectFrame(frame);
         });
-        assert.deepEqual(store.searchUris(), testCase.searchUris);
+        const firstResults = store.searchUris();
+        const secondResults = store.searchUris();
+        assert.deepEqual(firstResults, testCase.searchUris);
+        // The selector is memoized and should return the same Array reference
+        // assuming the list of search URIs hasn't changed
+        assert.equal(firstResults, secondResults);
       });
     });
   });