base/timer/timer.h

// Copyright (c) 2012 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.

// OneShotTimer and RepeatingTimer provide a simple timer API.  As the names
// suggest, OneShotTimer calls you back once after a time delay expires.
// RepeatingTimer on the other hand calls you back periodically with the
// prescribed time interval.
//
// OneShotTimer and RepeatingTimer both cancel the timer when they go out of
// scope, which makes it easy to ensure that you do not get called when your
// object has gone out of scope.  Just instantiate a OneShotTimer or
// RepeatingTimer as a member variable of the class for which you wish to
// receive timer events.
//
// Sample RepeatingTimer usage:
//
//   class MyClass {
//    public:
//     void StartDoingStuff() {
//       timer_.Start(FROM_HERE, TimeDelta::FromSeconds(1),
//                    this, &MyClass::DoStuff);
//     }
//     void StopDoingStuff() {
//       timer_.Stop();
//     }
//    private:
//     void DoStuff() {
//       // This method is called every second to do stuff.
//       ...
//     }
//     base::RepeatingTimer timer_;
//   };
//
// Both OneShotTimer and RepeatingTimer also support a Reset method, which
// allows you to easily defer the timer event until the timer delay passes once
// again.  So, in the above example, if 0.5 seconds have already passed,
// calling Reset on |timer_| would postpone DoStuff by another 1 second.  In
// other words, Reset is shorthand for calling Stop and then Start again with
// the same arguments.
//
// These APIs are not thread safe. All methods must be called from the same
// sequence (not necessarily the construction sequence), except for the
// destructor and SetTaskRunner().
// - The destructor may be called from any sequence when the timer is not
// running and there is no scheduled task active, i.e. when Start() has never
// been called or after AbandonAndStop() has been called.
// - SetTaskRunner() may be called from any sequence when the timer is not
// running, i.e. when Start() has never been called or Stop() has been called
// since the last Start().
//
// By default, the scheduled tasks will be run on the same sequence that the
// Timer was *started on*, but this can be changed *prior* to Start() via
// SetTaskRunner().

#ifndef BASE_TIMER_TIMER_H_
#define BASE_TIMER_TIMER_H_

// IMPORTANT: If you change timer code, make sure that all tests (including
// disabled ones) from timer_unittests.cc pass locally. Some are disabled
// because they're flaky on the buildbot, but when you run them locally you
// should be able to tell the difference.

#include <memory>

#include "base/base_export.h"
#include "base/bind.h"
#include "base/bind_helpers.h"
#include "base/callback.h"
#include "base/location.h"
#include "base/macros.h"
#include "base/sequence_checker_impl.h"
#include "base/sequenced_task_runner.h"
#include "base/time/time.h"

namespace base {

class BaseTimerTaskInternal;
class TickClock;

//-----------------------------------------------------------------------------
// This class wraps TaskRunner::PostDelayedTask to manage delayed and repeating
// tasks. See meta comment above for thread-safety requirements.
//
class BASE_EXPORT Timer {
 public:
  // Construct a timer in repeating or one-shot mode. Start must be called later
  // to set task info. |retain_user_task| determines whether the user_task is
  // retained or reset when it runs or stops. If |tick_clock| is provided, it is
  // used instead of TimeTicks::Now() to get TimeTicks when scheduling tasks.
  Timer(bool retain_user_task, bool is_repeating);
  Timer(bool retain_user_task, bool is_repeating, const TickClock* tick_clock);

  // Construct a timer with retained task info. If |tick_clock| is provided, it
  // is used instead of TimeTicks::Now() to get TimeTicks when scheduling tasks.
  Timer(const Location& posted_from,
        TimeDelta delay,
        const base::Closure& user_task,
        bool is_repeating);
  Timer(const Location& posted_from,
        TimeDelta delay,
        const base::Closure& user_task,
        bool is_repeating,
        const TickClock* tick_clock);

  virtual ~Timer();

  // Returns true if the timer is running (i.e., not stopped).
  virtual bool IsRunning() const;

  // Returns the current delay for this timer.
  virtual TimeDelta GetCurrentDelay() const;

  // Set the task runner on which the task should be scheduled. This method can
  // only be called before any tasks have been scheduled. If |task_runner| runs
  // tasks on a different sequence than the sequence owning this Timer,
  // |user_task_| will be posted to it when the Timer fires (note that this
  // means |user_task_| can run after ~Timer() and should support that).
  void SetTaskRunner(scoped_refptr<SequencedTaskRunner> task_runner);

  // Start the timer to run at the given |delay| from now. If the timer is
  // already running, it will be replaced to call the given |user_task|.
  virtual void Start(const Location& posted_from,
                     TimeDelta delay,
                     const base::Closure& user_task);

  // Start the timer to run at the given |delay| from now. If the timer is
  // already running, it will be replaced to call a task formed from
  // |reviewer->*method|.
  template <class Receiver>
  void Start(const Location& posted_from,
             TimeDelta delay,
             Receiver* receiver,
             void (Receiver::*method)()) {
    Start(posted_from, delay,
          base::BindRepeating(method, base::Unretained(receiver)));
  }

  // Call this method to stop and cancel the timer.  It is a no-op if the timer
  // is not running.
  virtual void Stop();

  // Stop running task (if any) and abandon scheduled task (if any).
  void AbandonAndStop() {
    AbandonScheduledTask();

    Stop();
    // No more member accesses here: |this| could be deleted at this point.
  }

  // Call this method to reset the timer delay. The |user_task_| must be set. If
  // the timer is not running, this will start it by posting a task.
  virtual void Reset();

  const base::Closure& user_task() const { return user_task_; }
  const TimeTicks& desired_run_time() const { return desired_run_time_; }

 protected:
  // Returns the current tick count.
  TimeTicks Now() const;

  void set_user_task(const Closure& task) { user_task_ = task; }
  void set_desired_run_time(TimeTicks desired) { desired_run_time_ = desired; }
  void set_is_running(bool running) { is_running_ = running; }

  const Location& posted_from() const { return posted_from_; }
  bool retain_user_task() const { return retain_user_task_; }
  bool is_repeating() const { return is_repeating_; }
  bool is_running() const { return is_running_; }

 private:
  friend class BaseTimerTaskInternal;

  // Allocates a new |scheduled_task_| and posts it on the current sequence with
  // the given |delay|. |scheduled_task_| must be null. |scheduled_run_time_|
  // and |desired_run_time_| are reset to Now() + delay.
  void PostNewScheduledTask(TimeDelta delay);

  // Returns the task runner on which the task should be scheduled. If the
  // corresponding |task_runner_| field is null, the task runner for the current
  // sequence is returned.
  scoped_refptr<SequencedTaskRunner> GetTaskRunner();

  // Disable |scheduled_task_| and abandon it so that it no longer refers back
  // to this object.
  void AbandonScheduledTask();

  // Called by BaseTimerTaskInternal when the delayed task fires.
  void RunScheduledTask();

  // When non-null, the |scheduled_task_| was posted to call RunScheduledTask()
  // at |scheduled_run_time_|.
  BaseTimerTaskInternal* scheduled_task_;

  // The task runner on which the task should be scheduled. If it is null, the
  // task runner for the current sequence will be used.
  scoped_refptr<SequencedTaskRunner> task_runner_;

  // Location in user code.
  Location posted_from_;
  // Delay requested by user.
  TimeDelta delay_;
  // |user_task_| is what the user wants to be run at |desired_run_time_|.
  base::Closure user_task_;

  // The time at which |scheduled_task_| is expected to fire. This time can be a
  // "zero" TimeTicks if the task must be run immediately.
  TimeTicks scheduled_run_time_;

  // The desired run time of |user_task_|. The user may update this at any time,
  // even if their previous request has not run yet. If |desired_run_time_| is
  // greater than |scheduled_run_time_|, a continuation task will be posted to
  // wait for the remaining time. This allows us to reuse the pending task so as
  // not to flood the delayed queues with orphaned tasks when the user code
  // excessively Stops and Starts the timer. This time can be a "zero" TimeTicks
  // if the task must be run immediately.
  TimeTicks desired_run_time_;

  // Timer isn't thread-safe and must only be used on its origin sequence
  // (sequence on which it was started). Once fully Stop()'ed it may be
  // destroyed or restarted on another sequence.
  SequenceChecker origin_sequence_checker_;

  // Repeating timers automatically post the task again before calling the task
  // callback.
  const bool is_repeating_;

  // If true, hold on to the |user_task_| closure object for reuse.
  const bool retain_user_task_;

  // The tick clock used to calculate the run time for scheduled tasks.
  const TickClock* const tick_clock_;

  // If true, |user_task_| is scheduled to run sometime in the future.
  bool is_running_;

  DISALLOW_COPY_AND_ASSIGN(Timer);
};

//-----------------------------------------------------------------------------
// A simple, one-shot timer.  See usage notes at the top of the file.
class OneShotTimer : public Timer {
 public:
  OneShotTimer() : OneShotTimer(nullptr) {}
  explicit OneShotTimer(const TickClock* tick_clock)
      : Timer(false, false, tick_clock) {}
};

//-----------------------------------------------------------------------------
// A simple, repeating timer.  See usage notes at the top of the file.
class RepeatingTimer : public Timer {
 public:
  RepeatingTimer() : RepeatingTimer(nullptr) {}
  explicit RepeatingTimer(const TickClock* tick_clock)
      : Timer(true, true, tick_clock) {}
};

//-----------------------------------------------------------------------------
// A Delay timer is like The Button from Lost. Once started, you have to keep
// calling Reset otherwise it will call the given method on the sequence it was
// initially Reset() from.
//
// Once created, it is inactive until Reset is called. Once |delay| seconds have
// passed since the last call to Reset, the callback is made. Once the callback
// has been made, it's inactive until Reset is called again.
//
// If destroyed, the timeout is canceled and will not occur even if already
// inflight.
class DelayTimer : protected Timer {
 public:
  template <class Receiver>
  DelayTimer(const Location& posted_from,
             TimeDelta delay,
             Receiver* receiver,
             void (Receiver::*method)())
      : DelayTimer(posted_from, delay, receiver, method, nullptr) {}

  template <class Receiver>
  DelayTimer(const Location& posted_from,
             TimeDelta delay,
             Receiver* receiver,
             void (Receiver::*method)(),
             const TickClock* tick_clock)
      : Timer(posted_from,
              delay,
              base::Bind(method, base::Unretained(receiver)),
              false,
              tick_clock) {}

  using Timer::Reset;
};

}  // namespace base

#endif  // BASE_TIMER_TIMER_H_