Replace Closure in TaskRunner::PostTask with OneShotCallback

base/mac/libdispatch_task_runner.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_MAC_LIBDISPATCH_TASK_RUNNER_H_
+#define BASE_MAC_LIBDISPATCH_TASK_RUNNER_H_
+
+#include <dispatch/dispatch.h>
+
+#include "base/single_thread_task_runner.h"
+#include "base/synchronization/waitable_event.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 BASE_EXPORT LibDispatchTaskRunner : public base::SingleThreadTaskRunner {
+ public:
+  // Starts a new serial dispatch queue with a given name.
+  explicit LibDispatchTaskRunner(const char* name);
+
+  // base::TaskRunner:
+  bool PostDelayedTask(const tracked_objects::Location& from_here,
+                       OnceClosure task,
+                       base::TimeDelta delay) override;
+  bool RunsTasksOnCurrentThread() const override;
+
+  // base::SequencedTaskRunner:
+  bool PostNonNestableDelayedTask(const tracked_objects::Location& from_here,
+                                  OnceClosure task,
+                                  base::TimeDelta delay) override;
+
+  // This blocks the calling thread until all work on the dispatch queue has
+  // been run and the queue has been destroyed. Destroying a queue requires
+  // ALL retained references to it to be released. Any new tasks posted to
+  // this thread after shutdown are dropped.
+  void Shutdown();
+
+  // 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.
+  //
+  // All properties (context, finalizer, etc.) are managed by this class, and
+  // clients should only use the result of this for dispatch_async().
+  dispatch_queue_t GetDispatchQueue() const;
+
+ protected:
+  ~LibDispatchTaskRunner() override;
+
+ private:
+  static void Finalizer(void* context);
+
+  dispatch_queue_t queue_;
+
+  // The event on which Shutdown waits until Finalizer runs.
+  base::WaitableEvent queue_finalized_;
+};
+
+}  // namespace mac
+}  // namespace base
+
+#endif  // BASE_MAC_LIBDISPATCH_TASK_RUNNER_H_