Extract AD policy scheduler into separate class

components/policy/core/common/policy_scheduler.h

+// Copyright 2017 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_POLICY_CORE_COMMON_POLICY_SCHEDULER_H_
+#define COMPONENTS_POLICY_CORE_COMMON_POLICY_SCHEDULER_H_
+
+#include <memory>
+
+#include "base/callback.h"
+#include "base/cancelable_callback.h"
+#include "base/macros.h"
+#include "base/memory/weak_ptr.h"
+#include "base/sequence_checker.h"
+#include "base/time/time.h"
+#include "components/policy/policy_export.h"
+
+namespace policy {
+
+// Scheduler for driving repeated asynchronous tasks such as e.g. policy
+// fetches. Subsequent tasks are guaranteed not to overlap. Tasks are posted to
+// the current thread and therefore must not block (suitable e.g. for
+// asynchronous D-Bus calls).
+// Tasks scheduling begins immediately after instantiation of the class. Upon
+// destruction, scheduled but not yet started tasks are cancelled. The result of
+// started but not finished tasks is NOT reported.
+class POLICY_EXPORT PolicyScheduler {
+ public:
+  // Callback for the task to report success or failure.
+  using TaskCallback = base::OnceCallback<void(bool success)>;
+
+  // Task to be performed at regular intervals. The task takes a |callback| to
+  // return success or failure.
+  using Task = base::RepeatingCallback<void(TaskCallback callback)>;
+
+  // Callback for PolicyScheduler to report success or failure of the tasks.
+  using SchedulerCallback = base::RepeatingCallback<void(bool success)>;
+
+  // Defines the |task| to be run every |interval| and the |callback| for the
+  // scheduler to report the result. (Intervals are computed as the time
+  // difference between the end of the previous and the start of the subsequent
+  // task.) Calling the constructor starts the loop and schedules the first task
+  // to be run without delay.
+  PolicyScheduler(Task task,
+                  SchedulerCallback callback,
+                  base::TimeDelta interval);
+  ~PolicyScheduler();
+
+  // Schedules a task to run immediately. Deletes any previously scheduled but
+  // not yet started tasks. In case a task is running currently, the new task is
+  // scheduled to run immediately after the end of the currently running task.
+  void ScheduleTaskNow();
+
+ private:
+  // Schedules next task to run in |delay|. Deletes any previously scheduled
+  // tasks.
+  void ScheduleDelayedTask(base::TimeDelta delay);
+
+  // Schedules next task to run in |interval_| or immediately in case of
+  // overlap. Deletes any previously scheduled tasks.
+  void ScheduleNextTask();
+
+  // Actually executes the scheduled task.
+  void RunScheduledTask();
+
+  // Reports back the |result| of the previous task and schedules the next one.
+  void OnTaskDone(bool result);
+
+  Task task_;
+  SchedulerCallback callback_;
+  const base::TimeDelta interval_;
+
+  // Whether a task is in progress.
+  bool task_in_progress_ = false;
+
+  // Whether there had been an overlap of tasks and thus the next task needs to
+  // be scheduled without delay.
+  bool overlap_ = false;
+
+  // End time of the previous task. Zero in case no task has ended yet.
+  base::TimeTicks last_task_;
+
+  std::unique_ptr<base::CancelableClosure> job_;
+
+  SEQUENCE_CHECKER(sequence_checker_);
+
+  // Must be last member.
+  base::WeakPtrFactory<PolicyScheduler> weak_ptr_factory_{this};
+
+  DISALLOW_COPY_AND_ASSIGN(PolicyScheduler);
+};
+
+}  // namespace policy
+
+#endif  // COMPONENTS_POLICY_CORE_COMMON_POLICY_SCHEDULER_H_