Create LibDispatchTaskRunner, a SingleThreadTaskRunner that is backed by a libdispatch queue.

base/mac/libdispatch_task_runner.h

Index: base/mac/libdispatch_task_runner.h
diff --git a/base/mac/libdispatch_task_runner.h b/base/mac/libdispatch_task_runner.h
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+// 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_MAC_LIBDISPATCH_SEQUENCED_TASK_RUNNER_H_
+#define BASE_MAC_LIBDISPATCH_SEQUENCED_TASK_RUNNER_H_
+
+#include <dispatch/dispatch.h>
+
+#include "base/single_thread_task_runner.h"
+
+namespace base {
+namespace mac {
+
+// This is an implementation of the TaskRunner interface that runs closures on
+// a thread managed by Apple's libdispatch. This has the benefit of being able
+// to PostTask() and friends to a dispatch queue, while being reusable as a
+// dispatch_queue_t.
+//
+// One would use this class if an object lives exclusively on one thread but
+// needs a dispatch_queue_t for use in a system API. This ensures all dispatch
+// callbacks happen on the same thread as Closure tasks.
+//
+// A LibDispatchTaskRunner will continue to run until all references to the
+// underlying dispatch queue are released.
+//
+// Important Notes:
+//   - There is no MessageLoop running on this thread, and ::current() returns
+//     NULL.
+//   - No nested loops can be run, and all tasks are run non-nested.
+//   - Work scheduled via libdispatch runs at the same priority as and is
+//     interleaved with posted tasks, though FIFO order is guaranteed.
+//
+class LibDispatchTaskRunner : public base::SingleThreadTaskRunner {
+ public:
+  // Starts a new serial dispatch queue with a given name.
+  explicit LibDispatchTaskRunner(const char* name);

[Mark Mentovai, 2012/12/06 22:11:45]

Dunno if it’s totally useful to have callers supply their own name here. It’s
just used as a label. Is there a useful label that we could derive at runtime or
from some other object somehow?

[Robert Sesek, 2012/12/06 22:18:05]

No, callers should supply it like they do for well-established thread names.
E.g., this will be "Audio Thread" where I first use it.

+
+  // base::TaskRunner:
+  virtual bool PostDelayedTask(const tracked_objects::Location& from_here,
+                               const Closure& task,
+                               base::TimeDelta delay) OVERRIDE;
+  virtual bool RunsTasksOnCurrentThread() const OVERRIDE;
+
+  // base::SequencedTaskRunner:
+  virtual bool PostNonNestableDelayedTask(
+      const tracked_objects::Location& from_here,
+      const Closure& task,
+      base::TimeDelta delay) OVERRIDE;
+
+  // Returns the dispatch queue associated with this task runner, for use with
+  // system APIs that take dispatch queues. The caller is responsible for
+  // retaining the result.
+  dispatch_queue_t GetDispatchQueue() const;
+
+ protected:
+  virtual ~LibDispatchTaskRunner();
+
+ private:
+  dispatch_queue_t queue_;
+};
+
+}  // namespace mac
+}  // namespace base
+
+#endif  // BASE_MAC_LIBDISPATCH_SEQUENCED_TASK_RUNNER_H_