Improve profile stats performace.

components/prefs/pref_service.h

 // Copyright (c) 2012 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.
 
 // This provides a way to access the application's current preferences.
 
 // Chromium settings and storage represent user-selected preferences and
 // information and MUST not be extracted, overwritten or modified except
 // through Chromium defined APIs.
 
@@ skipping 232 matching lines @@
   // Returns the default value of the given preference. |path| must point to a
   // registered preference. In that case, will never return NULL.
   const base::Value* GetDefaultPrefValue(const std::string& path) const;
 
   // Returns true if a value has been set for the specified path.
   // NOTE: this is NOT the same as FindPreference. In particular
   // FindPreference returns whether RegisterXXX has been invoked, where as
   // this checks if a value exists for the path.
   bool HasPrefPath(const std::string& path) const;
 
-  // Returns a dictionary with effective preference values.
-  std::unique_ptr<base::DictionaryValue> GetPreferenceValues() const;
+  // Issues a callback for every preference value. The preferences must not be
+  // mutated during iteration.
+  void IteratePreferenceValues(
+      base::RepeatingCallback<void(const std::string& key,
+                                   const base::Value& value)> callback) const;
 
-  // Returns a dictionary with effective preference values, omitting prefs that
-  // are at their default values.
-  std::unique_ptr<base::DictionaryValue> GetPreferenceValuesOmitDefaults()
-      const;
-
-  // Returns a dictionary with effective preference values. Contrary to
-  // GetPreferenceValues(), the paths of registered preferences are not split on
-  // '.' characters. If a registered preference stores a dictionary, however,
-  // the hierarchical structure inside the preference will be preserved.
-  // For example, if "foo.bar" is a registered preference, the result could look
-  // like this:
-  //   {"foo.bar": {"a": {"b": true}}}.
-  std::unique_ptr<base::DictionaryValue>
-  GetPreferenceValuesWithoutPathExpansion() const;
+  // Returns a dictionary with effective preference values. This is an expensive
+  // operation which does a deep copy. Use only if you really need the results
+  // in a base::Value (for example, for JSON serialization). Otherwise use
+  // IteratePreferenceValues above to avoid the copies.
+  //
+  // If include_defaults is set, preferences set to their default values will
+  // be included. Otherwise, these will be omitted from the returned
+  // dictionary.
+  std::unique_ptr<base::DictionaryValue> GetPreferenceValues(
+      bool include_defaults) const;

[Roger Tawa OOO till Jul 10th, 2017/04/11 21:04:29]

Would be better if this were an enum instead of a bool to make it more explicit.
 As I started this code review, I could not tell what true or false meant until
I got to the end.

[brettw, 2017/04/18 19:28:38]

Done

 
   bool ReadOnly() const;
 
   PrefInitializationStatus GetInitializationStatus() const;
 
   // Tell our PrefValueStore to update itself to |command_line_store|.
   // Takes ownership of the store.
   virtual void UpdateCommandLinePrefStore(PrefStore* command_line_store);
 
   // We run the callback once, when initialization completes. The bool
@@ skipping 101 matching lines @@
 
   // Local cache of registered Preference objects. The pref_registry_
   // is authoritative with respect to what the types and default values
   // of registered preferences are.
   mutable PreferenceMap prefs_map_;
 
   DISALLOW_COPY_AND_ASSIGN(PrefService);
 };
 
 #endif  // COMPONENTS_PREFS_PREF_SERVICE_H_