frontend: Implement list_render class.

This implements a list_render closure class that allows for
progressive, responsive rendering of long, scrollable lists, with
filtering support.

It isn't used, at present.

.eslintrc.json

@@ -109,6 +109,7 @@
         "message_fetch": false,
         "favicon": false,
         "condense": false,
+        "list_render": false,
         "floating_recipient_bar": false,
         "tab_bar": false,
         "emoji": false,

static/js/list_rendering.js

@@ -0,0 +1,186 @@
+var list_render = (function () {
+    var DEFAULTS = {
+        INITIAL_RENDER_COUNT: 80,
+        LOAD_COUNT: 20,
+        instances: {},
+    };
+
+    // @params
+    // container: jQuery object to append to.
+    // list: The list of items to progressively append.
+    // opts: An object of random preferences.
+    var func = function ($container, list, opts) {
+        // this memoizes the results and will return a previously invoked
+        // instance's prototype.
+        if (opts.name && DEFAULTS.instances[opts.name]) {
+            return DEFAULTS.instances[opts.name].data(list);
+        }
+
+        var meta = {
+            offset: 0,
+            listRenders: {},
+            filtered_list: list,
+        };
+
+        // this is a list that could be filtered by the value of the
+        // `opts.filter.element` this value should never change.
+        Object.defineProperty(meta, "list", {
+            configurable: false,
+            writeable: false,
+            value: list,
+        });
+
+        if (!opts) {
+            return;
+        }
+
+        // we want to assume below that `opts.filter` exists, but may not necessarily
+        // have any defined specs.
+        if (!opts.filter) {
+            opts.filter = {};
+        }
+
+        var $nearestScrollingContainer = $container;
+        while ($nearestScrollingContainer.length) {
+            if ($nearestScrollingContainer.is("body, html")) {
+                blueslip.warn("Please wrap progressive scrolling lists in an element with 'max-height' attribute. Error found in:\n" + util.preview_node($container));
+                break;
+            }
+
+            if ($nearestScrollingContainer.css("max-height") !== "none") {
+                break;
+            }
+
+            $nearestScrollingContainer = $nearestScrollingContainer.parent();
+        }
+
+        var prototype = {
+            // Reads the provided list (in the scope directly above)
+            // and renders the next block of messages automatically
+            // into the specified contianer.
+            render: function (load_count) {
+                load_count = load_count || opts.load_count || DEFAULTS.LOAD_COUNT;
+
+                // Stop once the offset reaches the length of the original list.
+                if (meta.offset >= meta.filtered_list.length) {
+                    return;
+                }
+
+                var slice = meta.filtered_list.slice(meta.offset, meta.offset + load_count);
+
+                var html = _.reduce(slice, function (acc, item) {
+                    var _item = opts.modifier(item);
+
+                    // if valid jQuery selection, attempt to grab the first elem.
+                    if (_item.constructor === jQuery) {
+                        _item = _item[0];
+                    }
+
+                    // if is a valid element, get the outerHTML.
+                    if (_item instanceof Element) {
+                        _item = _item.outerHTML;
+                    }
+
+                    // return the modified HTML or nothing if corrupt (null, undef, etc.).
+                    return acc + (_item || "");
+                }, "");
+
+                $container.append($(html));
+                meta.offset += load_count;
+
+                return this;
+            },
+
+            // Fills the container with an initial batch of items.
+            // Needs to be enough to exceed the max height, so that a
+            // scrollable area is created.
+            init: function () {
+                this.render(DEFAULTS.INITIAL_RENDER_COUNT);
+                return this;
+            },
+
+            filter: function (map_function) {
+                meta.filtered_list = meta.list(map_function);
+            },
+
+            // reset the data associated with a list. This is so that instead of
+            // initializing a new progressive list render instance, you can just
+            // update the data of an existing one.
+            data: function (data) {
+                if (Array.isArray(data)) {
+                    meta.list = data;
+                    meta.filtered_list = data;
+
+                    prototype.clear().init();
+                    return this;
+                }
+
+                blueslip.warn("The data object provided to the progressive" +
+                              " list render is invalid");
+                return this;
+            },
+
+            clear: function () {
+                $container.html("");
+                meta.offset = 0;
+                return this;
+            },
+        };
+
+        // on scroll of the nearest scrolling container, if it hits the bottom
+        // of the container then fetch a new block of items and render them.
+        $nearestScrollingContainer.scroll(function () {
+            if (this.scrollHeight - (this.scrollTop + this.clientHeight) < 10) {
+                prototype.render();
+            }
+        });
+
+        if (opts.filter.element) {
+            opts.filter.element.on(opts.filter.event || "input", function () {
+                var self = this;
+                var value = self.value.toLowerCase();
+
+                meta.filtered_list = meta.list.filter(function (item) {
+                    if (opts.filter.callback) {
+                        return opts.filter.callback(item, value);
+                    }
+
+                    return !!item.toLowerCase().match(value);
+                });
+
+                // clear and re-initialize the list with the newly filtered subset
+                // of items.
+                prototype.clear().init();
+            });
+        }
+
+        // Save the instance for potential future retrieval if a name is provided.
+        if (opts.name) {
+            DEFAULTS.instances[opts.name] = prototype;
+        }
+
+        return prototype;
+    };
+
+    func.get = function (name) {
+        return DEFAULTS.instances[name] || false;
+    };
+
+    // this can delete list render issues and free up memory if needed.
+    func.delete = function (name) {
+        if (DEFAULTS.instances[name]) {
+            delete DEFAULTS.instances[name];
+            return true;
+        }
+
+        blueslip.warn("The progressive list render instance with the name '" +
+                      name + "' does not exist.");
+        return false;
+    };
+
+    return func;
+}());
+
+if (typeof module !== 'undefined') {
+    module.exports = list_render;
+}

zproject/settings.py

@@ -862,6 +862,7 @@ JS_SPECS = {
             'js/message_edit.js',
             'js/condense.js',
             'js/resize.js',
+            'js/list_rendering.js',
             'js/floating_recipient_bar.js',
             'js/lightbox.js',
             'js/ui_state.js',