Introduce a new framework for back-and-forth tracked/protected preferences migration.

base/files/important_file_writer.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 BASE_FILES_IMPORTANT_FILE_WRITER_H_
 #define BASE_FILES_IMPORTANT_FILE_WRITER_H_
 
 #include <string>
 
 #include "base/base_export.h"
 #include "base/basictypes.h"
+#include "base/callback.h"
 #include "base/files/file_path.h"
 #include "base/memory/ref_counted.h"
 #include "base/threading/non_thread_safe.h"
 #include "base/time/time.h"
 #include "base/timer/timer.h"
 
 namespace base {
 
 class SequencedTaskRunner;
 class Thread;
@@ skipping 60 matching lines @@
   // to disk after the commit interval. If another ScheduleWrite is issued
   // before that, only one serialization and write to disk will happen, and
   // the most recent |serializer| will be used. This operation does not block.
   // |serializer| should remain valid through the lifetime of
   // ImportantFileWriter.
   void ScheduleWrite(DataSerializer* serializer);
 
   // Serialize data pending to be saved and execute write on backend thread.
   void DoScheduledWrite();
 
+  // Registers |on_next_successful_write| to be called once, on the next
+  // successful write event. Only one callback can be set at once.
+  void RegisterOnNextSuccessfulWriteCallback(
+      const base::Closure& on_next_successful_write);
+
   TimeDelta commit_interval() const {
     return commit_interval_;
   }
 
   void set_commit_interval(const TimeDelta& interval) {
     commit_interval_ = interval;
   }
 
  private:
+  // Helper method for WriteNow().
+  bool PostWriteTask(const std::string& data);
+
+  // If |result| is true and |on_next_successful_write_| is set, invokes
+  // |on_successful_write_| and then resets it; no-ops otherwise.
+  void ForwardSuccessfulWrite(bool result);
+
+  // Invoked once and then reset on the next successful write event.
+  base::Closure on_next_successful_write_;
+
   // Path being written to.
   const FilePath path_;
 
   // TaskRunner for the thread on which file I/O can be done.
   const scoped_refptr<base::SequencedTaskRunner> task_runner_;
 
   // Timer used to schedule commit after ScheduleWrite.
   OneShotTimer<ImportantFileWriter> timer_;
 
   // Serializer which will provide the data to be saved.
   DataSerializer* serializer_;
 
   // Time delta after which scheduled data will be written to disk.
   TimeDelta commit_interval_;
 
+  WeakPtrFactory<ImportantFileWriter> weak_factory_;
+
   DISALLOW_COPY_AND_ASSIGN(ImportantFileWriter);
 };
 
 }  // namespace base
 
 #endif  // BASE_FILES_IMPORTANT_FILE_WRITER_H_