Alarm clock application in .Net

Question

I'm not really writing an alarm clock application, but it will help to illustrate my question.

Let's say that I have a method in my application, and I want this method to be called every hour on the hour (e.g. at 7:00 PM, 8:00 PM, 9:00 PM etc.). I could create a Timer and set its Interval to 3600000, but eventually this would drift out of sync with the system clock. Or I could use a while() loop with Thread.Sleep(n) to periodically check the system time and call the method when the desired time is reached, but I don't like this either (Thread.Sleep(n) is a big code smell for me).

What I'm looking for is some method in .Net that lets me pass in a future DateTime object and a method delegate or event handler, but I haven't been able to find any such thing. I suspect there's a method in the Win32 API that does this, but I haven't been able to find that, either.

Answer 1

Or, you could create a timer with an interval of 1 second and check the current time every second until the event time is reached, if so, you raise your event.

You can make a simple wrapper for that :

public class AlarmClock
{
    public AlarmClock(DateTime alarmTime)
    {
        this.alarmTime = alarmTime;

        timer = new Timer();
        timer.Elapsed += timer_Elapsed;
        timer.Interval = 1000;
        timer.Start();

        enabled = true;
    }

    void  timer_Elapsed(object sender, ElapsedEventArgs e)
    {
        if(enabled && DateTime.Now > alarmTime)
        {
            enabled = false;
            OnAlarm();
            timer.Stop();
        }
    }

    protected virtual void OnAlarm()
    {
        if(alarmEvent != null)
            alarmEvent(this, EventArgs.Empty);
    }


    public event EventHandler Alarm
    {
        add { alarmEvent += value; }
        remove { alarmEvent -= value; }
    }

    private EventHandler alarmEvent;
    private Timer timer;
    private DateTime alarmTime;
    private bool enabled;
}

Usage:

AlarmClock clock = new AlarmClock(someFutureTime);
clock.Alarm += (sender, e) => MessageBox.Show("Wake up!");

Please note the code above is very sketchy and not thread safe.

Answer 2

I know it's a bit of an old question, but I came across this when I was looking for an answer to something else. I thought I'd throw my two cents in here, since I recently had this particular issue.

Another thing you can do is schedule the method like so:

/// Schedule the given action for the given time.
public async void ScheduleAction ( Action action , DateTime ExecutionTime )
{
    try
    {
        await Task.Delay ( ( int ) ExecutionTime.Subtract ( DateTime.Now ).TotalMilliseconds );
        action ( );
    }
    catch ( Exception )
    {
        // Something went wrong
    }
}

Bearing in mind it can only wait up to the maximum value of int 32 (somewhere around a month), it should work for your purposes. Usage:

void MethodToRun ( )
{
    Console.WriteLine ("Hello, World!");
}

void CallingMethod ( )
{
    var NextRunTime = DateTime.Now.AddHours(1);
    ScheduleAction ( MethodToRun, NextRunTime );
}

And you should have a console message in an hour.

Answer 3

Interesting, I've actually come across a very similar issue and went looking for a method in the .Net framework that would handle this scenario. In the end, we ended up implementing our own solution that was a variation on a while loop w/ Thread.Sleep(n) where n gets smaller the closer you get to the desired target time (logarithmically actually, but with some reasonable thresholds so you're not maxing the cpu when you get close to the target time.) Here's a really simple implementation that just sleeps half the time between now and the target time.

class Program
{
    static void Main(string[] args)
    {
        SleepToTarget Temp = new SleepToTarget(DateTime.Now.AddSeconds(30),Done);
        Temp.Start();
        Console.ReadLine();
    }

    static void Done()
    {
        Console.WriteLine("Done");
    }
}

class SleepToTarget
{
    private DateTime TargetTime;
    private Action MyAction;
    private const int MinSleepMilliseconds = 250;

    public SleepToTarget(DateTime TargetTime,Action MyAction)
    {
        this.TargetTime = TargetTime;
        this.MyAction = MyAction;
    }

    public void Start()
    {
        new Thread(new ThreadStart(ProcessTimer)).Start();
    }

    private void ProcessTimer()
    {
        DateTime Now = DateTime.Now;

        while (Now < TargetTime)
        {
            int SleepMilliseconds = (int) Math.Round((TargetTime - Now).TotalMilliseconds / 2);
            Console.WriteLine(SleepMilliseconds);
            Thread.Sleep(SleepMilliseconds > MinSleepMilliseconds ? SleepMilliseconds : MinSleepMilliseconds);
            Now = DateTime.Now;
        }

        MyAction();
    }
}

Answer 4

You could simply reset the timer duration each time it fires, like this:

// using System.Timers;

private void myMethod()
{
    var timer = new Timer { 
        AutoReset = false, Interval = getMillisecondsToNextAlarm() };
    timer.Elapsed += (src, args) =>
    {
        // Do timer handling here.

        timer.Interval = getMillisecondsToNextAlarm();
        timer.Start();
    };
    timer.Start();
}

private double getMillisecondsToNextAlarm()
{
    // This is an example of making the alarm go off at every "o'clock"
    var now = DateTime.Now;
    var inOneHour = now.AddHours(1.0);
    var roundedNextHour = new DateTime(
        inOneHour.Year, inOneHour.Month, inOneHour.Day, inOneHour.Hour, 0, 0);
    return (roundedNextHour - now).TotalMilliseconds;
}