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.
-//
-// 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 {
- public:
-  virtual ~PostTaskAndReplyImpl() = default;
+using PostTaskCallback =
+    Callback<bool(const tracked_objects::Location&, const Closure& task)>;
 
-  // Implementation for TaskRunner::PostTaskAndReply and
-  // WorkerPool::PostTaskAndReply.
-  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;
-};
+// Posts |task| via |post_task_callback|. On completion, |reply| is posted to
+// the sequence or thread that called PostTaskAndReply(). Can only be called
+// when SequencedTaskRunnerHandle::IsSet(). Both |task| and |reply| are
+// guaranteed to be deleted on the sequence or thread from which
+// PostTaskAndReply() is invoked.
+bool BASE_EXPORT PostTaskAndReply(const tracked_objects::Location& from_here,
+                                  const Closure& task,
+                                  const Closure& reply,
+                                  const PostTaskCallback& post_task_callback);
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_THREADING_POST_TASK_AND_REPLY_IMPL_H_