MD Settings: methods to get root pages and sections

chrome/browser/resources/settings/settings_main/settings_main.js

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * @fileoverview
  * 'settings-main' displays the selected settings page.
  */
 Polymer({
   is: 'settings-main',
@@ skipping 147 matching lines @@
    * content so that the last element may align with the top of the content
    * area.
    * @return {number}
    * @private
    */
   overscrollHeight_: function() {
     var route = settings.getCurrentRoute();
     if (route.subpage.length != 0 || this.showPages_.about)
       return 0;
 
-    var query = 'settings-section[section="' + route.section + '"]';
-    var topSection = this.$$('settings-basic-page').$$(query);
-    if (!topSection && this.showPages_.advanced)
-      topSection = this.$$('settings-advanced-page').$$(query);
-
+    var page = this.getPage_(route);
+    var topSection = page && page.getSection(route.section);
     if (!topSection || !topSection.offsetParent)
       return 0;
 
     // Offset to the selected section (relative to the scrolling window).
     let sectionTop = topSection.offsetParent.offsetTop + topSection.offsetTop;
     // The height of the selected section and remaining content (sections).
     let heightOfShownSections = this.$.overscroll.offsetTop - sectionTop;
     return Math.max(0, this.parentNode.scrollHeight - heightOfShownSections);
   },
 
   /** @private */
   toggleAdvancedPage_: function() {
     this.fire('toggle-advanced-page', !this.advancedToggleExpanded_);
   },
 
   /**
+   * Returns the root page (if it exists) for a route.
+   * @param {!settings.Route} route
+   * @return {(?SettingsAboutPageElement|?SettingsAdvancedPageElement|
+   *           ?SettingsBasicPageElement)}
+   */
+  getPage_: function(route) {

[dschuyler, 2016/08/10 17:16:17]

I'd like to hear more about why this function helps.

It seems to query the about page with each call
(which is unlikely to have the element in question).

Is it useful to separate the data types of the return
values?

It also seems like a lot of extra code to read.

[michaelpg, 2016/08/15 18:08:11]

No, it only queries the page for the current route -- so if the current route is
or descends from ABOUT, it will return the about page.
Separate, no; have, yes. The type is useful because MainPageBehavior has
functions like getSection which are useful to settings-main (rather than
re-implementing them here). We have to specify all the types because Closure
isn't smart enough to accept a behavior as a type.
We could try duplicating all of the public member docs from MainPageBehavior
onto an @interface, and claim to Closure that each of our main pages @implements
that interface, but that would be more duplication.

+    if (settings.Route.ABOUT.contains(route)) {
+      return /** @type {?SettingsAboutPageElement} */(
+          this.$$('settings-about-page'));
+    }
+    if (settings.Route.ADVANCED.contains(route)) {
+      return /** @type {?SettingsAdvancedPageElement} */(
+          this.$$('settings-advanced-page'));
+    }
+    if (settings.Route.BASIC.contains(route)) {
+      return /** @type {?SettingsBasicPageElement} */(
+          this.$$('settings-basic-page'));
+    }
+    assertNotReached();
+  },
+
+  /**
    * Navigates to the default search page (if necessary).
    * @private
    */
   ensureInDefaultSearchPage_: function() {
     settings.navigateTo(settings.Route.BASIC);
   },
 
   /**
    * @param {string} query
    * @return {!Promise} A promise indicating that searching finished.
    */
   searchContents: function(query) {
     this.ensureInDefaultSearchPage_();
     this.toolbarSpinnerActive = true;
 
     // Trigger rendering of the basic and advanced pages and search once ready.
     this.showPages_ = {about: false, basic: true, advanced: true};
 
     return new Promise(function(resolve, reject) {
       setTimeout(function() {
         var whenSearchDone = settings.getSearchManager().search(
-            query, assert(this.$$('settings-basic-page')));
+            query, assert(this.getPage_(settings.Route.BASIC)));
         assert(
             whenSearchDone ===
                 settings.getSearchManager().search(
-                    query, assert(this.$$('settings-advanced-page'))));
+                    query, assert(this.getPage_(settings.Route.ADVANCED))));
 
         whenSearchDone.then(function(request) {
           resolve();
           if (!request.finished) {
             // Nothing to do here. A previous search request was canceled
             // because a new search request was issued before the first one
             // completed.
             return;
           }
 
           this.toolbarSpinnerActive = false;
           this.showNoResultsFound_ =
               !request.isSame('') && !request.didFindMatches();
         }.bind(this));
       }.bind(this), 0);
     }.bind(this));
   },
 
   /**
    * @param {(boolean|undefined)} visibility
    * @return {boolean} True unless visibility is false.
    * @private
    */
   showAdvancedSettings_: function(visibility) {
     return visibility !== false;
   },
 });