wpf - Binding to time-dependent properties

Question

Some time ago i wrote a small widget-like application which was supposed to keep track of tasks, each task had a deadline specified as a DateTime, now if you want to display how much time is left until the deadline you might want to bind to a "virtual" (*curses the virtual keyword*) property like this:

public TimeSpan TimeLeft
{
    get { return Deadline - DateTime.Now; }
}

Obviously in theory this property changes every tick and you want to update your UI every now and then (e.g. by periodically pumping out a PropertyChanged event for that property).

Back when i wrote the widget i refreshed the whole task list every minute, but this is hardly ideal since if the user interacts with some item (e.g. by typing in a TextBox which binds to a Comments-property) that will be harshly interupted and updates to the source get lost.

So what might be the best approach to updating the UI if you have time-dependent properties like this?

(I don't use that application anymore by the way, just thought this was a very interesting question)

Answer 1

A timer is the only way I can think of. Since this is an interesting question, I'll put my .02 in. I would encapsulate it doing something like this:

public class CountdownViewModel : INotifyPropertyChanged
{
    Func<TimeSpan> calc;
    DispatcherTimer timer;

    public CountdownViewModel(DateTime deadline)
        : this(() => deadline - DateTime.Now)
    {
    }

    public CountdownViewModel(Func<TimeSpan> calculator)
    {
        calc = calculator;

        timer = new DispatcherTimer();
        timer.Interval = TimeSpan.FromSeconds(1);
        timer.Tick += timer_Tick;
        timer.Start();
    }

    void timer_Tick(object sender, EventArgs e)
    {
        var temp = PropertyChanged;
        if (temp != null)
        {
            temp(this, new PropertyChangedEventArgs("CurrentValue"));
        }
    }

    public TimeSpan CurrentValue
    {
        get
        {
            var result = calc();
            if (result < TimeSpan.Zero)
            {
                return TimeSpan.Zero;
            }
            return result;
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
}

public class MyViewModel
{
    public CountdownViewModel DeadlineCountdown { get; private set; }

    public DateTime Deadline { get; set; }

    public MyViewModel()
    {
        Deadline = DateTime.Now.AddSeconds(200);
        DeadlineCountdown = new CountdownViewModel(Deadline);
    }
}

Then you could bind to DeadlineCountdown.CurrentValue directly, or create a CountdownView. You could move the timer to the CountdownView, if you wanted. You could use a static timer so they all update at the same time.

Edit

If Deadline is going to change, you would have to construct the countdown like this:

DeadlineCountdown = new CountdownViewModel(() => this.Deadline - DateTime.Now);