cc/trees/blocking_task_runner.h

   1 // Copyright 2013 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 CC_TREES_BLOCKING_TASK_RUNNER_H_
   6 #define CC_TREES_BLOCKING_TASK_RUNNER_H_
   7
   8 #include <vector>
   9
  10 #include "base/location.h"
  11 #include "base/memory/ref_counted.h"
  12 #include "base/single_thread_task_runner.h"
  13 #include "base/synchronization/lock.h"
  14 #include "base/threading/platform_thread.h"
  15 #include "cc/base/cc_export.h"
  16
  17 namespace cc {
  18
  19 // This class wraps a SingleThreadTaskRunner but allows posted tasks to be
  20 // run without a round trip through the message loop. This shortcutting
  21 // removes guarantees about ordering. Tasks posted while the
  22 // BlockingTaskRunner is in a capturing state will run in order, and tasks
  23 // posted while the BlockingTaskRunner is /not/ in a capturing state will
  24 // run in order, but the two sets of tasks will *not* run in order relative
  25 // to when they were posted.
  26 //
  27 // To use this class, post tasks to the task runner returned by
  28 // BlockingTaskRunner::current() on the thread you want the tasks to run.
  29 // Hold a reference to the BlockingTaskRunner as long as you intend to
  30 // post tasks to it.
  31 //
  32 // Then, on the thread from which the BlockingTaskRunner was created, you
  33 // may instantiate a BlockingTaskRunner::CapturePostTasks. While this object
  34 // exists, the task runner will collect any PostTasks called on it, posting
  35 // tasks to that thread from anywhere. This CapturePostTasks object provides
  36 // a window in time where tasks can shortcut past the MessageLoop. As soon
  37 // as the CapturePostTasks object is destroyed (goes out of scope), all
  38 // tasks that had been posted to the thread during the window will be exectuted
  39 // immediately.
  40 //
  41 // Beware of re-entrancy, make sure the CapturePostTasks object is destroyed at
  42 // a time when it makes sense for the embedder to call arbitrary things.
  43 class CC_EXPORT BlockingTaskRunner
  44     : public base::RefCountedThreadSafe<BlockingTaskRunner> {
  45  public:
  46   // Returns the BlockingTaskRunner for the current thread, creating one if
  47   // necessary.
  48   static scoped_refptr<BlockingTaskRunner> current();
  49
  50   // While an object of this type is held alive on a thread, any tasks
  51   // posted to the thread will be captured and run as soon as the object
  52   // is destroyed, shortcutting past the MessageLoop.
  53   class CC_EXPORT CapturePostTasks {
  54    public:
  55     CapturePostTasks();
  56     ~CapturePostTasks();
  57
  58    private:
  59     scoped_refptr<BlockingTaskRunner> blocking_runner_;
  60
  61     DISALLOW_COPY_AND_ASSIGN(CapturePostTasks);
  62   };
  63
  64   // True if tasks posted to the BlockingTaskRunner will run on the current
  65   // thread.
  66   bool BelongsToCurrentThread();
  67
  68   // Posts a task using the contained SingleThreadTaskRunner unless |capture_|
  69   // is true. When |capture_| is true, tasks posted will be caught and stored
  70   // until the capturing stops. At that time the tasks will be run directly
  71   // instead of being posted to the SingleThreadTaskRunner.
  72   bool PostTask(const tracked_objects::Location& from_here,
  73                 const base::Closure& task);
  74
  75  private:
  76   friend class base::RefCountedThreadSafe<BlockingTaskRunner>;
  77
  78   explicit BlockingTaskRunner(
  79       scoped_refptr<base::SingleThreadTaskRunner> task_runner);
  80   virtual ~BlockingTaskRunner();
  81
  82   void SetCapture(bool capture);
  83
  84   base::PlatformThreadId thread_id_;
  85   scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
  86
  87   base::Lock lock_;
  88   int capture_;
  89   std::vector<base::Closure> captured_tasks_;
  90 };
  91
  92 }  // namespace cc
  93
  94 #endif  // CC_TREES_BLOCKING_TASK_RUNNER_H_