Keep emtpy List/Dictionary pref value with non-empty default.

chrome/common/json_pref_store.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.
 
 #ifndef CHROME_COMMON_JSON_PREF_STORE_H_
 #define CHROME_COMMON_JSON_PREF_STORE_H_
 #pragma once
 
+#include <set>
 #include <string>
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/file_path.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/observer_list.h"
 #include "chrome/common/important_file_writer.h"
 #include "chrome/common/persistent_pref_store.h"
 
@@ skipping 23 matching lines @@
   virtual size_t NumberOfObservers() const OVERRIDE;
   virtual bool IsInitializationComplete() const OVERRIDE;
 
   // PersistentPrefStore overrides:
   virtual ReadResult GetMutableValue(const std::string& key,
                                      base::Value** result) OVERRIDE;
   virtual void SetValue(const std::string& key, base::Value* value) OVERRIDE;
   virtual void SetValueSilently(const std::string& key,
                                 base::Value* value) OVERRIDE;
   virtual void RemoveValue(const std::string& key) OVERRIDE;
+  virtual void MarkNeedsEmptyValue(const std::string& key) OVERRIDE;
   virtual bool ReadOnly() const OVERRIDE;
   virtual PrefReadError GetReadError() const OVERRIDE;
   virtual PrefReadError ReadPrefs() OVERRIDE;
   virtual void ReadPrefsAsync(ReadErrorDelegate* error_delegate) OVERRIDE;
   virtual void CommitPendingWrite() OVERRIDE;
   virtual void ReportValueChanged(const std::string& key) OVERRIDE;
 
   // This method is called after JSON file has been read. Method takes
   // ownership of the |value| pointer. Note, this method is used with
   // asynchronous file reading, so class exposes it only for the internal needs.
@@ skipping 14 matching lines @@
   // Helper for safely writing pref data.
   ImportantFileWriter writer_;
 
   ObserverList<PrefStore::Observer, true> observers_;
 
   scoped_ptr<ReadErrorDelegate> error_delegate_;
 
   bool initialized_;
   PrefReadError read_error_;
 
+  std::set<std::string> keys_needs_empty_value_;

[battre, 2012/04/11 21:55:51]

grammar: keys (plural) vs. needs (singular)

[xiyuan, 2012/04/11 22:36:46]

Done.

+
   DISALLOW_COPY_AND_ASSIGN(JsonPrefStore);
 };
 
 #endif  // CHROME_COMMON_JSON_PREF_STORE_H_