Depcreate `PageTracker` in favor of `PluginAPI`

app/assets/javascripts/discourse/initializers/page-tracking.js.es6

@@ -1,5 +1,5 @@
 import { cleanDOM } from 'discourse/routes/discourse';
-import PageTracker from 'discourse/lib/page-tracker';
+import { startPageTracking, onPageChange } from 'discourse/lib/page-tracker';
 
 export default {
   name: "page-tracking",
@@ -34,13 +34,12 @@ export default {
       }
     };
 
-    const pageTracker = PageTracker.current();
-    pageTracker.start();
+    startPageTracking(router);
 
     // Out of the box, Discourse tries to track google analytics
     // if it is present
     if (typeof window._gaq !== 'undefined') {
-      pageTracker.on('change', function(url, title) {
+      onPageChange((url, title) => {
         window._gaq.push(["_set", "title", title]);
         window._gaq.push(['_trackPageview', url]);
       });
@@ -49,7 +48,7 @@ export default {
 
     // Also use Universal Analytics if it is present
     if (typeof window.ga !== 'undefined') {
-      pageTracker.on('change', function(url, title) {
+      onPageChange.on('change', (url, title) => {
         window.ga('send', 'pageview', {page: url, title: title});
       });
     }

app/assets/javascripts/discourse/lib/desktop-notifications.js.es6

@@ -1,6 +1,6 @@
 import DiscourseURL from 'discourse/lib/url';
-import PageTracker from 'discourse/lib/page-tracker';
 import KeyValueStore from 'discourse/lib/key-value-store';
+import { onPageChange } from 'discourse/lib/page-tracker';
 
 let primaryTab = false;
 let liveEnabled = false;
@@ -84,7 +84,8 @@ function setupNotifications() {
   if (document) {
     document.addEventListener("scroll", resetIdle);
   }
-  PageTracker.on("change", resetIdle);
+
+  onPageChange(resetIdle);
 }
 
 function resetIdle() {

app/assets/javascripts/discourse/lib/page-tracker.js.es6

@@ -1,37 +1,31 @@
-import Singleton from 'discourse/mixins/singleton';
+const PageTracker = Ember.Object.extend(Ember.Evented);
+let _pageTracker = PageTracker.create();
 
-/**
-  Called whenever the "page" changes. This allows us to set up analytics
-  and other tracking.
+let _started = false;
+export function startPageTracking(router) {
+  if (_started) { return; }
 
-  To get notified when the page changes, you can install a hook like so:
+  router.on('didTransition', function() {
+    this.send('refreshTitle');
+    const url = Discourse.getURL(this.get('url'));
 
-  ```javascript
-    PageTracker.current().on('change', function(url, title) {
-      console.log('the page changed to: ' + url + ' and title ' + title);
+    // Refreshing the title is debounced, so we need to trigger this in the
+    // next runloop to have the correct title.
+    Em.run.next(() => {
+      _pageTracker.trigger('change', url, Discourse.get('_docTitle'));
     });
-  ```
-**/
-const PageTracker = Ember.Object.extend(Ember.Evented, {
-  start: function() {
-    if (this.get('started')) { return; }
+  });
+  _started = true;
+}
 
-    var router = Discourse.__container__.lookup('router:main'),
-        self = this;
+export function onPageChange(fn) {
+  _pageTracker.on('change', fn);
+}
 
-    router.on('didTransition', function() {
-      this.send('refreshTitle');
-      var url = Discourse.getURL(this.get('url'));
-
-      // Refreshing the title is debounced, so we need to trigger this in the
-      // next runloop to have the correct title.
-      Em.run.next(function() {
-        self.trigger('change', url, Discourse.get('_docTitle'));
-      });
-    });
-    this.set('started', true);
+// backwards compatibility
+export default {
+  current() {
+    console.warn(`Using PageTracker.current() is deprecated. Your plugin should use the PluginAPI`);
+    return _pageTracker;
   }
-});
-PageTracker.reopenClass(Singleton);
-
-export default PageTracker;
+};

app/assets/javascripts/discourse/lib/plugin-api.js.es6

@@ -6,7 +6,7 @@ import { includeAttributes } from 'discourse/lib/transform-post';
 import { addToolbarCallback } from 'discourse/components/d-editor';
 import { addWidgetCleanCallback } from 'discourse/components/mount-widget';
 import { decorateWidget } from 'discourse/widgets/widget';
-import PageTracker from 'discourse/lib/page-tracker';
+import { onPageChange } from 'discourse/lib/page-tracker';
 
 let _decorateId = 0;
 function decorate(klass, evt, cb) {
@@ -139,8 +139,20 @@ class PluginApi {
     addWidgetCleanCallback('post-stream', fn);
   }
 
+  /**
+    Called whenever the "page" changes. This allows us to set up analytics
+    and other tracking.
+
+    To get notified when the page changes, you can install a hook like so:
+
+    ```javascript
+      api.onPageChange((url, title) => {
+        console.log('the page changed to: ' + url + ' and title ' + title);
+      });
+    ```
+  **/
   onPageChange(fn) {
-    PageTracker.current().on('change', fn);
+    onPageChange(fn);
   }
 
 }