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_TASK_RUNNER_H_
6 #define BASE_MAC_LIBDISPATCH_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_TASK_RUNNER_H_
81