Support PostTaskAndReply from a sequenced task.

base/threading/post_task_and_reply_impl.h

 // Copyright (c) 2011 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 file contains the implementation shared by
 // TaskRunner::PostTaskAndReply and WorkerPool::PostTaskAndReply.
 
 #ifndef BASE_THREADING_POST_TASK_AND_REPLY_IMPL_H_
 #define BASE_THREADING_POST_TASK_AND_REPLY_IMPL_H_
 
+#include "base/base_export.h"
 #include "base/callback_forward.h"
 #include "base/location.h"
 
 namespace base {
 namespace internal {
 
-// Inherit from this in a class that implements PostTask appropriately
-// for sending to a destination thread.
+// Inherit from this in a class that implements PostTask to send a task to a
+// custom execution context.
 //
-// Note that 'reply' will always get posted back to your current
-// MessageLoop.
-//
-// If you're looking for a concrete implementation of
-// PostTaskAndReply, you probably want base::SingleThreadTaskRunner, or you
-// may want base::WorkerPool.
-class PostTaskAndReplyImpl {
+// If you're looking for a concrete implementation of PostTaskAndReply, you
+// probably want base::TaskRunner, or you may want base::WorkerPool.
+class BASE_EXPORT PostTaskAndReplyImpl {
  public:
   virtual ~PostTaskAndReplyImpl() = default;
 
-  // Implementation for TaskRunner::PostTaskAndReply and
-  // WorkerPool::PostTaskAndReply.
+  // Posts |task| by calling PostTask(). On completion, |reply| is posted to the
+  // sequence or thread that called this. Can only be called when
+  // SequencedTaskRunnerHandle::IsSet(). Both |task| and |reply| are guaranteed
+  // to be deleted on the sequence or thread that called this.
   bool PostTaskAndReply(const tracked_objects::Location& from_here,
                         const Closure& task,
                         const Closure& reply);
 
  private:
   virtual bool PostTask(const tracked_objects::Location& from_here,
                         const Closure& task) = 0;
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_THREADING_POST_TASK_AND_REPLY_IMPL_H_