C++ readability review

components/timers/alarm_timer.h

Index: components/timers/alarm_timer.h
diff --git a/components/timers/alarm_timer.h b/components/timers/alarm_timer.h
index d2f93335d61c518c4516b523b94058df949a1577..a855a9df1a027212680e9fd57294f9a392ee3f8a 100644
--- a/components/timers/alarm_timer.h
+++ b/components/timers/alarm_timer.h
@@ -27,38 +27,6 @@ namespace timers {
 class AlarmTimer : public base::Timer,
                    public base::MessageLoop::DestructionObserver {
  public:
-  // The delegate is responsible for managing the system level details for
-  // actually setting up and monitoring a timer that is capable of waking the
-  // system from suspend.  This class is reference counted because it may need
-  // to outlive the timer in order to clean up after itself.
-  class Delegate : public base::RefCountedThreadSafe<Delegate> {
-   public:
-    // Initializes the timer.  Should return true iff the system has timers that
-    // can wake it up from suspend.  Will only be called once.
-    virtual bool Init(base::WeakPtr<AlarmTimer> timer) = 0;
-
-    // Stops the currently running timer.  It should be safe to call this more
-    // than once.
-    virtual void Stop() = 0;
-
-    // Resets the timer to fire after |delay| has passed.  Cancels any
-    // pre-existing delay.
-    virtual void Reset(base::TimeDelta delay) = 0;
-
-   protected:
-    virtual ~Delegate() {}
-
-   private:
-    friend class base::RefCountedThreadSafe<Delegate>;
-  };
-
-  AlarmTimer(bool retain_user_task, bool is_repeating);
-
-  AlarmTimer(const tracked_objects::Location& posted_from,
-             base::TimeDelta delay,
-             const base::Closure& user_task,
-             bool is_repeating);
-
   ~AlarmTimer() override;
 
   bool can_wake_from_suspend() const { return can_wake_from_suspend_; }
@@ -74,11 +42,21 @@ class AlarmTimer : public base::Timer,
   // MessageLoop::DestructionObserver overrides.
   void WillDestroyCurrentMessageLoop() override;
 
+ protected:
+  // The constructors for this class are protected because consumers should
+  // instantiate one of the specialized sub-classes defined below instead.
+  AlarmTimer(bool retain_user_task, bool is_repeating);
+  AlarmTimer(const tracked_objects::Location& posted_from,
+             base::TimeDelta delay,
+             const base::Closure& user_task,
+             bool is_repeating);
+
  private:
   // Initializes the timer with the appropriate delegate.
   void Init();
 
   // Delegate that will manage actually setting the timer.
+  class Delegate;
   scoped_refptr<Delegate> delegate_;
 
   // Keeps track of the user task we want to run.  A new one is constructed
@@ -100,6 +78,55 @@ class AlarmTimer : public base::Timer,
   DISALLOW_COPY_AND_ASSIGN(AlarmTimer);
 };
 
+// As its name suggests, a OneShotAlarmTimer runs a given task once.  It does
+// not remember the task that was given to it after it has fired and does not
+// repeat.  Useful for fire-and-forget tasks.
+class OneShotAlarmTimer : public AlarmTimer {
+ public:
+  // Constructs a basic OneShotAlarmTimer.  An AlarmTimer constructed this way
+  // requires that Start() is called before Reset() is called.
+  OneShotAlarmTimer() : AlarmTimer(false, false) {}
+};
+
+// A RepeatingAlarmTimer takes a task and delay and repeatedly runs the task
+// using the specified delay as an interval between the runs until it is
+// explicitly stopped.  It remembers both the task and the delay it was given
+// after it fires.
+class RepeatingAlarmTimer : public AlarmTimer {
+ public:
+  // Constructs a basic RepeatingAlarmTimer.  An AlarmTimer constructed this way
+  // requires that Start() is called before Reset() is called.
+  RepeatingAlarmTimer() : AlarmTimer(true, true) {}
+
+  // Constructs a RepeatingAlarmTimer with pre-populated parameters but does not
+  // start it.  Useful if |user_task| or |delay| are not going to change.
+  // Reset() can be called immediately after constructing an AlarmTimer in this
+  // way.
+  RepeatingAlarmTimer(const tracked_objects::Location& posted_from,
+                      base::TimeDelta delay,
+                      const base::Closure& user_task)
+      : AlarmTimer(posted_from, delay, user_task, true) {}
+};
+
+// A SimpleAlarmTimer only fires once but remembers the task that it was given
+// even after it has fired.  Useful if you want to run the same task multiple
+// times but not at a regular interval.
+class SimpleAlarmTimer : public AlarmTimer {
+ public:
+  // Constructs a basic SimpleAlarmTimer.  An AlarmTimer constructed this way
+  // requires that Start() is called before Reset() is called.
+  SimpleAlarmTimer() : AlarmTimer(true, false) {}
+
+  // Constructs a SimpleAlarmTimer with pre-populated parameters but does not
+  // start it.  Useful if |user_task| or |delay| are not going to change.
+  // Reset() can be called immediately after constructing an AlarmTimer in this
+  // way.
+  SimpleAlarmTimer(const tracked_objects::Location& posted_from,
+                   base::TimeDelta delay,
+                   const base::Closure& user_task)
+      : AlarmTimer(posted_from, delay, user_task, false) {}
+};
+
 }  // namespace timers
 
 #endif  // COMPONENTS_TIMER_ALARM_TIMER_H_