[scheduler]: Avoid waking up the scheduler to end long idle periods.

components/scheduler/child/single_thread_idle_task_runner.h

 // Copyright 2014 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 COMPONENTS_SCHEDULER_CHILD_SINGLE_THREAD_IDLE_TASK_RUNNER_H_
 #define COMPONENTS_SCHEDULER_CHILD_SINGLE_THREAD_IDLE_TASK_RUNNER_H_
 
 #include "base/bind.h"
 #include "base/callback.h"
 #include "base/memory/ref_counted.h"
 #include "base/single_thread_task_runner.h"
 #include "base/time/time.h"
+#include "components/scheduler/scheduler_export.h"
 
 namespace scheduler {
 
 // A SingleThreadIdleTaskRunner is a task runner for running idle tasks. Idle
 // tasks have an unbound argument which is bound to a deadline
 // (in base::TimeTicks) when they are run. The idle task is expected to
 // complete by this deadline.
 class SingleThreadIdleTaskRunner
     : public base::RefCountedThreadSafe<SingleThreadIdleTaskRunner> {
  public:
   typedef base::Callback<void(base::TimeTicks)> IdleTask;
 
+  // Used request idle task deadlines and signal posting of idle tasks.

[Sami, 2015/05/26 13:35:41]

"Used to"?

[rmcilroy, 2015/05/27 11:58:37]

Done.

+  class SCHEDULER_EXPORT Observer {

[Sami, 2015/05/26 13:35:41]

"Delegate" or "Controller" perhaps since I think computing the deadline goes a
little beyond observing. WDYT?

[rmcilroy, 2015/05/27 11:58:37]

Yeah (I originally had Delegate, then moved to Observer when things were simple,
but I agree they are more complex now and Delegate is more appropriate). Done.

+   public:
+    Observer();
+    virtual ~Observer();
+
+    // Signals that an idle task has been posted.
+    virtual void OnIdleTaskPosted() = 0;

[Sami, 2015/05/26 13:35:41]

Maybe mention that this will be called on the posting thread?

[rmcilroy, 2015/05/27 11:58:37]

Done.

+
+    // Signals that a new idle task is about to be run and returns the deadline
+    // for this idle task.
+    virtual void OnWillRunIdleTask(base::TimeTicks* deadline_out) const = 0;

[Sami, 2015/05/26 13:35:41]

Could we name these WillProcessIdleTask/DidProcessIdleTask for consistency with
the messageloop observer? In any case, use only "On" or "Will/Did" but not both.

[rmcilroy, 2015/05/27 11:58:37]

Done.

+
+    // Signals that an idle task has finished being run.
+    virtual void OnDidRunIdleTask() = 0;
+
+   private:
+    DISALLOW_COPY_AND_ASSIGN(Observer);
+  };
+
+
   // NOTE Category strings must have application lifetime (statics or
   // literals). They may not include " chars.
   SingleThreadIdleTaskRunner(
       scoped_refptr<base::SingleThreadTaskRunner> idle_priority_task_runner,
       scoped_refptr<base::SingleThreadTaskRunner> after_wakeup_task_runner,
-      base::Callback<void(base::TimeTicks*)> deadline_supplier,
+      Observer* observer,
       const char* tracing_category);
 
   virtual void PostIdleTask(const tracked_objects::Location& from_here,
                             const IdleTask& idle_task);
 
   virtual void PostNonNestableIdleTask(
       const tracked_objects::Location& from_here,
       const IdleTask& idle_task);
 
   virtual void PostIdleTaskAfterWakeup(
       const tracked_objects::Location& from_here,
       const IdleTask& idle_task);
 
   bool RunsTasksOnCurrentThread() const;
 
  protected:
   virtual ~SingleThreadIdleTaskRunner();
 
  private:
   friend class base::RefCountedThreadSafe<SingleThreadIdleTaskRunner>;
 
   void RunTask(IdleTask idle_task);
 
   scoped_refptr<base::SingleThreadTaskRunner> idle_priority_task_runner_;
   scoped_refptr<base::SingleThreadTaskRunner> after_wakeup_task_runner_;
-  base::Callback<void(base::TimeTicks*)> deadline_supplier_;
+  Observer* observer_;  // NOT OWNED
   const char* tracing_category_;
   base::WeakPtr<SingleThreadIdleTaskRunner> weak_scheduler_ptr_;
   base::WeakPtrFactory<SingleThreadIdleTaskRunner> weak_factory_;
   DISALLOW_COPY_AND_ASSIGN(SingleThreadIdleTaskRunner);
 };
 
 }  // namespace scheduler
 
 #endif  // COMPONENTS_SCHEDULER_CHILD_SINGLE_THREAD_IDLE_TASK_RUNNER_H_