[NTP Snippets] Persist remote categories in prefs

components/ntp_snippets/category.h

 // Copyright 2016 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.
 
 #ifndef COMPONENTS_NTP_SNIPPETS_CATEGORY_H_
 #define COMPONENTS_NTP_SNIPPETS_CATEGORY_H_
 
 #include <ostream>
 
 namespace ntp_snippets {
 
 class CategoryFactory;
 
 // These are the categories that the client knows about.
 // The values before LOCAL_CATEGORIES_COUNT are the categories that are provided
 // locally on the device. Categories provided by the server (IDs strictly larger
 // than REMOTE_CATEGORIES_OFFSET) only need to be hard-coded here if they need
 // to be recognized by the client implementation.
+// NOTE: These are persisted, so don't reorder or remove values, and insert new
+// values only in the appropriate places marked below.
 // On Android builds, a Java counterpart will be generated for this enum.
 // GENERATED_JAVA_ENUM_PACKAGE: org.chromium.chrome.browser.ntp.snippets
 enum class KnownCategories {
   // Pages recently downloaded during normal navigation.
   RECENT_TABS,
 
   // Pages downloaded by the user for offline consumption.
   DOWNLOADS,
 
   // Recently used bookmarks.
   BOOKMARKS,
 
   // Physical Web page available in the vicinity.
   PHYSICAL_WEB_PAGES,
 
   // Pages recently browsed to on other devices.
   FOREIGN_TABS,
+  // INSERT NEW LOCAL CATEGORIES HERE!
 
   // Follows the last local category.
   LOCAL_CATEGORIES_COUNT,
 
   // Remote categories come after this.
   REMOTE_CATEGORIES_OFFSET = 10000,
 
   // Articles for you.
   ARTICLES,
+  // INSERT NEW REMOTE CATEGORIES HERE!
 };
 
 // A category groups ContentSuggestions which belong together. Use the
 // CategoryFactory to obtain instances.
 class Category {
  public:
   // An arbitrary but consistent ordering. Can be used to look up categories in
   // a std::map, but should not be used to order categories for other purposes.
   struct CompareByID;
 
   // Returns a non-negative identifier that is unique for the category and can
   // be converted back to a Category instance using
   // |CategoryFactory::FromIDValue(id)|.
-  // Note that these IDs are not necessarily stable across multiple runs of
-  // the application, so they should not be persisted.
   int id() const { return id_; }
 
   // Returns whether this category matches the given |known_category|.
   bool IsKnownCategory(KnownCategories known_category) const;
 
  private:
   friend class CategoryFactory;
 
   explicit Category(int id);
 
   int id_;
 
   // Allow copy and assignment.
 };
 
 bool operator==(const Category& left, const Category& right);
 
 bool operator!=(const Category& left, const Category& right);
 
 struct Category::CompareByID {
   bool operator()(const Category& left, const Category& right) const;
 };
 
 std::ostream& operator<<(std::ostream& os, const Category& obj);
 
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_CATEGORY_H_