[#3639] Add new followers-counter module

This new module will update the UI for Followers counter immediately upon
clicking on Follow/Unfollow button, instead on page refresh.

ckan/public/base/javascript/modules/followers-counter.js

@@ -0,0 +1,59 @@
+/* Updates the Followers counter in the UI when the Follow/Unfollow button
+* is clicked.
+*
+* id - id of the object the user is trying to follow/unfollow.
+*
+* Example
+*
+*   <dd data-module="followers-counter" data-module-id="object-id">6</dd>
+*
+*/
+this.ckan.module('followers-counter', function($) {
+  'use strict';
+
+  return {
+    options: {id: null},
+
+    /* Subscribe to events when the Follow/Unfollow button is clicked.
+    *
+    * Returns nothing.
+    */
+    initialize: function() {
+      $.proxyAll(this, /_on/);
+
+      this.counterEl = this.$('span');
+      this.counterVal = this.counterEl.text();
+      this.counterVal = parseInt(this.counterVal, 10);
+      this.objId = this.options.id;
+
+      this.sandbox.subscribe('follow-follow-' + this.objId, this._onFollow);
+      this.sandbox.subscribe('follow-unfollow-' + this.objId, this._onUnfollow);
+    },
+
+    /* Handles updating the UI for Followers counter on Follow button click.
+    *
+    * Returns nothing.
+    */
+    _onFollow: function() {
+      this.counterEl.text(++this.counterVal);
+    },
+
+    /* Handles updating the UI for Followers counter on Unfollow button click.
+    *
+    * Returns nothing.
+    */
+    _onUnfollow: function() {
+      this.counterEl.text(--this.counterVal);
+    },
+
+    /* Remove any subscriptions to prevent memory leaks. This function is
+     * called when a module element is removed from the page.
+     *
+     * Returns nothing.
+     */
+    teardown: function() {
+      this.sandbox.unsubscribe('follow-follow-' + this.objId, this._onFollow);
+      this.sandbox.unsubscribe('follow-unfollow-' + this.objId, this._onUnfollow);
+    }
+  }
+});