Add FetchStatus class.

We will use this in subsequent commits to replace the
`load_more_enabled` flag.

.eslintrc.json

@@ -91,6 +91,7 @@
         "gear_menu": false,
         "hashchange": false,
         "hash_util": false,
+        "FetchStatus": false,
         "message_list": false,
         "Filter": false,
         "flatpickr": false,

frontend_tests/node_tests/fetch_status.js

@@ -0,0 +1,82 @@
+var FetchStatus = zrequire('fetch_status');
+
+var fetch_status = FetchStatus();
+
+function reset() {
+    fetch_status = FetchStatus();
+}
+
+function can_load_newer() {
+    assert.equal(fetch_status.can_load_newer_messages(), true);
+}
+
+function blocked_newer() {
+    assert.equal(fetch_status.can_load_newer_messages(), false);
+}
+
+function can_load_older() {
+    assert.equal(fetch_status.can_load_older_messages(), true);
+}
+
+function blocked_older() {
+    assert.equal(fetch_status.can_load_older_messages(), false);
+}
+
+(function test_basics() {
+    reset();
+
+    can_load_older();
+
+    fetch_status.start_older_batch();
+
+    blocked_older();
+    can_load_newer();
+
+    fetch_status.finish_older_batch({
+        found_oldest: false,
+    });
+
+    can_load_older();
+    can_load_newer();
+
+    fetch_status.start_older_batch();
+
+    blocked_older();
+    can_load_newer();
+
+    fetch_status.finish_older_batch({
+        found_oldest: true,
+    });
+
+    blocked_older();
+    can_load_newer();
+
+    reset();
+
+    can_load_older();
+    can_load_newer();
+
+    fetch_status.start_newer_batch();
+
+    can_load_older();
+    blocked_newer();
+
+    fetch_status.finish_newer_batch({
+        found_newest: false,
+    });
+
+    can_load_older();
+    can_load_newer();
+
+    fetch_status.start_newer_batch();
+
+    can_load_older();
+    blocked_newer();
+
+    fetch_status.finish_newer_batch({
+        found_newest: true,
+    });
+
+    can_load_older();
+    blocked_newer();
+}());

static/js/fetch_status.js

@@ -0,0 +1,41 @@
+var FetchStatus = function () {
+
+    var self = {};
+
+    var loading_older = false;
+    var loading_newer = false;
+    var found_oldest = false;
+    var found_newest = false;
+
+    self.start_older_batch = function () {
+        loading_older = true;
+    };
+
+    self.finish_older_batch = function (opts) {
+        loading_older = false;
+        found_oldest = opts.found_oldest;
+    };
+
+    self.can_load_older_messages = function () {
+        return !loading_older && !found_oldest;
+    };
+
+    self.start_newer_batch = function () {
+        loading_newer = true;
+    };
+
+    self.finish_newer_batch = function (opts) {
+        loading_newer = false;
+        found_newest = opts.found_newest;
+    };
+
+    self.can_load_newer_messages = function () {
+        return !loading_newer && !found_newest;
+    };
+
+    return self;
+
+};
+if (typeof module !== 'undefined') {
+    module.exports = FetchStatus;
+}

tools/test-js-with-node

@@ -34,8 +34,9 @@ enforce_fully_covered = {
     'static/js/composebox_typeahead.js',
     'static/js/dict.js',
     'static/js/emoji.js',
-    'static/js/filter.js',
     'static/js/fenced_code.js',
+    'static/js/fetch_status.js',
+    'static/js/filter.js',
     'static/js/hash_util.js',
     'static/js/markdown.js',
     'static/js/message_store.js',

zproject/settings.py

@@ -1026,6 +1026,7 @@ JS_SPECS = {
             'js/top_left_corner.js',
             'js/stream_list.js',
             'js/filter.js',
+            'js/fetch_status.js',
             'js/message_list_view.js',
             'js/message_list.js',
             'js/message_live_update.js',