Add basic widgets framework (JS side).

static/js/widgetize.js

@@ -0,0 +1,77 @@
+var widgetize = (function () {
+
+var exports = {};
+
+var widgets = {};
+
+exports.activate = function (in_opts) {
+    var widget_type = in_opts.widget_type;
+    var extra_data = in_opts.extra_data;
+    var events = in_opts.events;
+    var row = in_opts.row;
+    var message = in_opts.message;
+    var post_to_server = in_opts.post_to_server;
+
+    events.shift();
+
+    if (!widgets[widget_type]) {
+        blueslip.warn('unknown widget_type', widget_type);
+        return;
+    }
+
+    var content_holder = row.find('.message_content');
+
+    var widget_elem;
+    if (message.widget) {
+        // Use local to work around linter.  We can trust this
+        // value because it comes from a template.
+        widget_elem = message.widget_elem;
+        content_holder.html(widget_elem);
+        return;
+    }
+
+    var callback = function (data) {
+        post_to_server({
+            msg_type: 'widget',
+            data: data,
+        });
+    };
+
+    // We depend on our widgets to use templates to build
+    // the HTML that will eventually go in this div.
+    widget_elem = $('<div>');
+    content_holder.html(widget_elem);
+
+    var widget = widgets[widget_type].activate({
+        elem: widget_elem,
+        callback: callback,
+        message: message,
+        extra_data: extra_data,
+    });
+
+    // This is hacky, we should just maintain our own list.
+    message.widget = widget;
+    message.widget_elem = widget_elem;
+
+    // Replay any events that already happened.  (This is common
+    // when you narrow to a message after other users have already
+    // interacted with it.)
+    if (events.length > 0) {
+        widget.handle_events(events);
+    }
+};
+
+exports.handle_event = function (widget_event) {
+    var message = message_store.get(widget_event.message_id);
+
+    var events = [widget_event];
+
+    message.widget.handle_events(events);
+};
+
+return exports;
+
+}());
+if (typeof module !== 'undefined') {
+    module.exports = widgetize;
+}