Computing visible annotation tags and notifying host page

visible-anns.diff

diff --git a/src/sidebar/frame-sync.js b/src/sidebar/frame-sync.js
index 8a80563..6a505d7 100644
--- a/src/sidebar/frame-sync.js
+++ b/src/sidebar/frame-sync.js
@@ -2,6 +2,7 @@
 
 var events = require('./events');
 var bridgeEvents = require('../shared/bridge-events');
+var memoize = require('./util/memoize');
 var metadata = require('./annotation-metadata');
 var uiConstants = require('./ui-constants');
 
@@ -34,12 +35,27 @@ function formatAnnot(ann) {
 }
 
 /**
+ * Return the tags of annotations displayed in a list of threads.
+ *
+ * @param {Thread[]} threads - A list of visible threads
+ * @return {string[]} List of annotation tags
+ */
+var annotationTags = memoize(function (threads) {
+  return threads.reduce(function (tags, thread) {
+    if (thread.annotation) {
+      tags.push(thread.annotation.$tag);
+    }
+    return tags;
+  }, []);
+});
+
+/**
  * This service runs in the sidebar and is responsible for keeping the set of
  * annotations displayed in connected frames in sync with the set shown in the
  * sidebar.
  */
 // @ngInject
-function FrameSync($rootScope, $window, Discovery, annotationUI, bridge) {
+function FrameSync($rootScope, $window, Discovery, annotationUI, bridge, rootThread) {
 
   // Set of tags of annotations that are currently loaded into the frame
   var inFrame = new Set();
@@ -51,13 +67,18 @@ function FrameSync($rootScope, $window, Discovery, annotationUI, bridge) {
   function setupSyncToFrame() {
     // List of loaded annotations in previous state
     var prevAnnotations = [];
+    var prevFilterQuery = null;
     var prevFrames = [];
     var prevPublicAnns = 0;
+    var prevSelectedTab = null;
+    var prevVisibleAnns = [];
 
     annotationUI.subscribe(function () {
       var state = annotationUI.getState();
       if (state.annotations === prevAnnotations &&
-          state.frames === prevFrames) {
+          state.frames === prevFrames &&
+          state.filterQuery === prevFilterQuery &&
+          state.selectedTab === prevSelectedTab) {
         return;
       }
 
@@ -109,6 +130,21 @@ function FrameSync($rootScope, $window, Discovery, annotationUI, bridge) {
           }
         }
       }
+
+      // Notify the page which annotations are currently visible in the sidebar
+      prevFilterQuery = state.filterQuery;
+      prevSelectedTab = state.selectedTab;
+      var visibleAnns = annotationTags(rootThread.thread(state).children);
+      if (prevVisibleAnns !== visibleAnns) {
+
+        // TODO - Trigger hiding/showing of anns in the page depending on
+        // their visibility.
+
+        prevVisibleAnns = visibleAnns;
+      }
     });
   }