What is the best approach to schedule events?

Question

I have an application in which the user is able to create different notifications, like sticky notes and set their starting times. When he presses the start button a timer starts and these reminders should pop up at the time they were set for. I've searched for other answers, like this one, but the problem here is the notifications' times are different.

So what is the best way to schedule the events that activate the notifications? I can think of two possible ways with their Pros and Cons:

1. Run a DispatcherTimer, that ticks every second and checks whether the time for a notification has come and pop it up. Pros: single DispatcherTimer instance. Cons: ticking every second and checking all notifications is an overhead.

2. Create a DispatcherTimer for each notification and let them handle time themselves. Pros: every timer ticks just once to pop the notification. Cons: too many timers is an overhead and may be hard to control.

Am I on the right track? Which of the two approaches is better, resource wise? Is there a third better way I am overlooking?

EDIT: If it makes any difference, the notifications should also auto close after some user-defined time and repeat at regular user-defined intervals.

Answer 1

I've used many methods to schedule events in C# Applications (Threads, Timers, Quartz API...), and I think that the Quertz.NET API -link- is the best tool you'll find (For me at least). It's easy and simple to use.

Example of your job class:

public class HelloJob : IJob
{
    public void Execute(IJobExecutionContext context)
    {
        Console.WriteLine("Greetings from HelloJob!");
    }
}

Example from the internet:

// Instantiate the Quartz.NET scheduler
var schedulerFactory = new StdSchedulerFactory();
var scheduler = schedulerFactory.GetScheduler();

// Instantiate the JobDetail object passing in the type of your
// class. Your class needs to implement a IJob interface
var job = new JobDetail("job1", "group1", typeof(HelloJob));

// Instantiate a trigger using the basic cron syntax.
// Example : run at 1AM every Monday - Friday.
var trigger = new CronTrigger(
    "trigger1", "group1", "job1", "group1", "0 0 1 ? * MON-FRI");

// Add the job to the scheduler
scheduler.AddJob(job, true);
scheduler.ScheduleJob(trigger);

You'll find a helpful code example in the QuickSart guide here.

Regards.

Answer 2

If the notification system is going to be used inside single process, continue with single dispatcher timer. Make sure the dispatcher timer is set to the near notification. and each time a new notification is created or timer hit ,change the time to next nearest notification.

That way you can avoid processing every time. eg: First time when somebody creates notification point timer to that time. If someone else create another notification before first hits change the timer to second. If the second time is after the first time, change the timer after dispatching the first notification call back. If its threaded you may need to work hard to get thread safety.

If notification is needed across process use windows task scheduler which already knows how to run timer and call our code on time. You may need to use some sort of IPC (WCF net.pipe, msmq etc...) to achieve notification.