Build the recent subjects list on demand

We previously kept the lists in the DOM for all streams and updated
them all when new messages arrived.  This was very expensive for
large numbers of streams, so we now just build the subject lists on
demand.

(imported from commit 937ad4322ce2014200aeae8645f79875f6af576e)

zephyr/static/js/ui.js

@@ -533,6 +533,19 @@ exports.hide_loading_more_messages_indicator = function () {
     }
 };
 
+function rebuild_recent_subjects(stream, subject) {
+    $('.expanded_subjects').remove();
+    var stream_li = exports.get_filter_li('stream', stream);
+    var subjects = recent_subjects[stream] || [];
+
+    stream_li.append(templates.render('sidebar_subject_list',
+                                      {subjects: subjects,
+                                       stream: stream}));
+    if (subject !== undefined) {
+        exports.get_subject_filter_li(stream, subject).addClass('active-subject-filter');
+    }
+}
+
 $(function () {
     // NB: This just binds to current elements, and won't bind to elements
     // created after ready() is called.
@@ -1118,7 +1131,6 @@ $(function () {
     // side-bar-related handlers
     $(document).on('narrow_activated.zephyr', function (event) {
         $("ul.filters li").removeClass('active-filter active-subject-filter');
-        $("ul.expanded_subjects").addClass('hidden');
 
         // TODO: handle confused filters like "in:all stream:foo"
         var op_in = event.filter.operands('in');
@@ -1136,20 +1148,20 @@ $(function () {
         var op_stream = event.filter.operands('stream');
         if (op_stream.length !== 0 && subs.have(op_stream[0])) {
             var stream_li = exports.get_filter_li('stream', op_stream[0]);
-            $('ul.expanded_subjects', stream_li).removeClass('hidden');
             var op_subject = event.filter.operands('subject');
+            var subject;
             if (op_subject.length !== 0) {
-                exports.get_subject_filter_li(op_stream[0], op_subject[0])
-                       .addClass('active-subject-filter');
+                subject = op_subject[0];
             } else {
                 stream_li.addClass('active-filter');
             }
+            rebuild_recent_subjects(op_stream[0], subject);
         }
     });
 
     $(document).on('narrow_deactivated.zephyr', function (event) {
         $("ul.filters li").removeClass('active-filter active-subject-filter');
-        $("ul.expanded_subjects").addClass('hidden');
+        $("ul.expanded_subjects").remove();
         $("#global_filters li[data-name='home']").addClass('active-filter');
     });
 
@@ -1393,59 +1405,22 @@ exports.process_condensing = function (index, elem) {
     }
 };
 
-exports.update_recent_subjects = function () {
-    function same(arr1, arr2) {
-        var i = 0;
+exports.update_streams_sidebar = function () {
+    exports.sort_narrow_list();
 
-        if (arr1.length !== arr2.length) return false;
-        for (i = 0; i < arr1.length; i++) {
-            if (arr2[i] !== arr1[i]) {
-                return false;
-            }
-        }
-        return true;
+    if (! narrow.active()) {
+        return;
     }
 
-    $("#stream_filters > li").each(function (idx, elem) {
-        var stream = $(elem).attr('data-name');
-        var expander = $('.streamlist_expand', elem);
-        var subjects = recent_subjects[stream] || [];
-        var subject_names = $.map(subjects, function (elem, idx) {
-            return elem.subject;
-            });
-
-        expander.toggleClass('hidden', subjects.length === 0);
-
-        var currently_shown = $('ul.expanded_subjects li', elem).map(function(idx, elem) {
-            return $(elem).text().trim();
-        });
-
-        if (!same(currently_shown, subject_names)) {
-            var subject_list = $("ul.expanded_subjects", elem);
-
-            var was_hidden = subject_list.length === 0 || subject_list.hasClass('hidden');
-            // If this is the first subject in current narrow, show it regardless
-            var operators = narrow.operators();
-            if (subject_list.length === 0 && operators.length > 0 && operators[0][0] === 'stream') {
-                was_hidden = operators[0][1] !== stream;
-            }
-            var active_subject = $("ul.expanded_subjects li.active-subject-filter").text().trim();
-
-            subject_list.remove();
-
-            if (subjects.length > 0) {
-                $(elem).append(templates.render('sidebar_subject_list',
-                                                {subjects: subjects,
-                                                 stream: stream,
-                                                 hidden: was_hidden}));
-                if (active_subject !== '') {
-                    exports.get_subject_filter_li(stream, active_subject).addClass('active-subject-filter');
-                }
-            }
+    var op_stream = narrow.filter().operands('stream');
+    var op_subject = narrow.filter().operands('subject');
+    var subject;
+    if (op_stream.length !== 0) {
+        if (op_subject.length !== 0) {
+            subject = op_subject[0];
         }
-    });
-    // Resort the narrow list based on which streams have messages
-    exports.sort_narrow_list();
+        rebuild_recent_subjects(op_stream[0], subject);
+    }
 };
 
 return exports;

zephyr/static/js/zephyr.js

@@ -840,7 +840,7 @@ function get_updates(options) {
                 process_visible_unread_messages();
                 notifications.received_messages(messages);
                 compose.update_faded_messages();
-                ui.update_recent_subjects();
+                ui.update_streams_sidebar();
             }
 
             if (new_pointer !== undefined
@@ -920,7 +920,7 @@ function load_old_messages(opts) {
             add_messages(messages, opts.msg_list);
         }
 
-        ui.update_recent_subjects();
+        ui.update_streams_sidebar();
 
         if (opts.cont !== undefined) {
             opts.cont(messages);

zephyr/static/templates/sidebar_subject_list.handlebars

@@ -1,4 +1,4 @@
-<ul class='expanded_subjects {{#if hidden}}hidden{{/if}}' data-stream='{{stream}}'>
+<ul class='expanded_subjects' data-stream='{{stream}}'>
     {{#each subjects}}
     <li class='expanded_subject' data-name='{{subject}}'>
         <a href='#narrow/stream/{{../stream}}/subject/{{subject}}'>{{subject}}</a>