Prevent redundant DoWorks due to canceled delayed tasks

third_party/WebKit/Source/platform/scheduler/base/time_domain.h

 // Copyright 2015 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 THIRD_PARTY_WEBKIT_SOURCE_PLATFORM_SCHEDULER_BASE_TIME_DOMAIN_H_
 #define THIRD_PARTY_WEBKIT_SOURCE_PLATFORM_SCHEDULER_BASE_TIME_DOMAIN_H_
 
 #include <map>
 
 #include "base/callback.h"
 #include "base/logging.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
 #include "base/time/time.h"
 #include "platform/scheduler/base/lazy_now.h"
 #include "platform/scheduler/base/task_queue_impl.h"
 
 namespace blink {
 namespace scheduler {
 namespace internal {
 class TaskQueueImpl;
 }  // internal
 class TaskQueueManager;
 class TaskQueueManagerDelegate;
 
-// The TimeDomain's job is to keep track of moments when delayed tasks have been
-// scheduled to fire and to notify their TaskQueues via UpdateDelayedWorkQueue.
+// The TimeDomain's job is to wake task queues up when their next delayed tasks
+// are due to fire. TaskQueues request a wake up via ScheduleDelayedWork, when
+// the WakeUp is due the TimeDomain calls TaskQueue::WakeUpForDelayedWork which
+// schedules the next non-canceled wakeup.
 //
-// The time domain keeps track of the next wakeup required to pump delayed tasks
-// and issues |RequestWakeup| calls to the subclass as needed.  Where possible
-// it tried to de-dupe these wakeups. Ideally it would be possible to cancel
-// them, but that's not currently supported by the base message loop.
+// To prevent spurious wake-ups for canceled tasks the TaskQueue should only
+// have a single wake up registered with its TimeDomain.  If should call
+// CancelDelayedWork as needed to ensure this. The TimeDomain communicates with
+// the TaskQueueManager to actually schedule the wake-ups on the underlying
+// base::MessageLoop. Various levels of de-duping are employed to prevent
+// unnecessary posting of TaskQueueManager::DoWork.
 //
 // The clock itself is provided by subclasses of the TimeDomain and it may be
 // the real wall clock or a synthetic (virtual) time base.
 class BLINK_PLATFORM_EXPORT TimeDomain {
  public:
   class BLINK_PLATFORM_EXPORT Observer {
    public:
     virtual ~Observer() {}
 
     // Called when an empty TaskQueue registered with this TimeDomain has a task
@@ skipping 38 matching lines @@
                     TimeDomain* destination_time_domain);
 
   // If there is a scheduled delayed task, |out_task_queue| is set to the queue
   // the next task was posted to and it returns true.  Returns false otherwise.
   bool NextScheduledTaskQueue(TaskQueue** out_task_queue) const;
 
   // Adds |queue| to the set of task queues that UpdateWorkQueues calls
   // UpdateWorkQueue on.
   void RegisterAsUpdatableTaskQueue(internal::TaskQueueImpl* queue);
 
-  // Schedules a call to TaskQueueImpl::MoveReadyDelayedTasksToDelayedWorkQueue
+  // Schedules a call to TaskQueueImpl::WakeUpForDelayedWork
   // when this TimeDomain reaches |delayed_run_time|.
   void ScheduleDelayedWork(internal::TaskQueueImpl* queue,
                            base::TimeTicks delayed_run_time,
                            base::TimeTicks now);
 
-  // Cancels a call to TaskQueueImpl::MoveReadyDelayedTasksToDelayedWorkQueue
+  // Cancels a call to TaskQueueImpl::WakeUpForDelayedWork
   // previously requested with ScheduleDelayedWork.  Note this only works if
   // delayed_run_time is _not_ the next scheduled run time.
   void CancelDelayedWork(internal::TaskQueueImpl* queue,
                          base::TimeTicks delayed_run_time);
 
   // Registers the |queue|.
   void RegisterQueue(internal::TaskQueueImpl* queue);
 
   // Removes |queue| from the set of task queues that UpdateWorkQueues calls
   // UpdateWorkQueue on. Returns true if |queue| was updatable.
@@ skipping 51 matching lines @@
 
   base::ThreadChecker main_thread_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(TimeDomain);
 };
 
 }  // namespace scheduler
 }  // namespace blink
 
 #endif  // THIRD_PARTY_WEBKIT_SOURCE_PLATFORM_SCHEDULER_BASE_TIME_DOMAIN_H_