base/mac/libdispatch_task_runner.h

   1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #ifndef BASE_MAC_LIBDISPATCH_SEQUENCED_TASK_RUNNER_H_
   6 #define BASE_MAC_LIBDISPATCH_SEQUENCED_TASK_RUNNER_H_
   7
   8 #include <dispatch/dispatch.h>
   9
  10 #include "base/single_thread_task_runner.h"
  11 #include "base/synchronization/waitable_event.h"
  12
  13 namespace base {
  14 namespace mac {
  15
  16 // This is an implementation of the TaskRunner interface that runs closures on
  17 // a thread managed by Apple's libdispatch. This has the benefit of being able
  18 // to PostTask() and friends to a dispatch queue, while being reusable as a
  19 // dispatch_queue_t.
  20 //
  21 // One would use this class if an object lives exclusively on one thread but
  22 // needs a dispatch_queue_t for use in a system API. This ensures all dispatch
  23 // callbacks happen on the same thread as Closure tasks.
  24 //
  25 // A LibDispatchTaskRunner will continue to run until all references to the
  26 // underlying dispatch queue are released.
  27 //
  28 // Important Notes:
  29 //   - There is no MessageLoop running on this thread, and ::current() returns
  30 //     NULL.
  31 //   - No nested loops can be run, and all tasks are run non-nested.
  32 //   - Work scheduled via libdispatch runs at the same priority as and is
  33 //     interleaved with posted tasks, though FIFO order is guaranteed.
  34 //
  35 class BASE_EXPORT LibDispatchTaskRunner : public base::SingleThreadTaskRunner {
  36  public:
  37   // Starts a new serial dispatch queue with a given name.
  38   explicit LibDispatchTaskRunner(const char* name);
  39
  40   // base::TaskRunner:
  41   bool PostDelayedTask(const tracked_objects::Location& from_here,
  42                        const Closure& task,
  43                        base::TimeDelta delay) override;
  44   bool RunsTasksOnCurrentThread() const override;
  45
  46   // base::SequencedTaskRunner:
  47   bool PostNonNestableDelayedTask(const tracked_objects::Location& from_here,
  48                                   const Closure& task,
  49                                   base::TimeDelta delay) override;
  50
  51   // This blocks the calling thread until all work on the dispatch queue has
  52   // been run and the queue has been destroyed. Destroying a queue requires
  53   // ALL retained references to it to be released. Any new tasks posted to
  54   // this thread after shutdown are dropped.
  55   void Shutdown();
  56
  57   // Returns the dispatch queue associated with this task runner, for use with
  58   // system APIs that take dispatch queues. The caller is responsible for
  59   // retaining the result.
  60   //
  61   // All properties (context, finalizer, etc.) are managed by this class, and
  62   // clients should only use the result of this for dispatch_async().
  63   dispatch_queue_t GetDispatchQueue() const;
  64
  65  protected:
  66   ~LibDispatchTaskRunner() override;
  67
  68  private:
  69   static void Finalizer(void* context);
  70
  71   dispatch_queue_t queue_;
  72
  73   // The event on which Shutdown waits until Finalizer runs.
  74   base::WaitableEvent queue_finalized_;
  75 };
  76
  77 }  // namespace mac
  78 }  // namespace base
  79
  80 #endif  // BASE_MAC_LIBDISPATCH_SEQUENCED_TASK_RUNNER_H_