How to Notify That View Should Get New Value of Calculated Field - silverlight

Question

I am working on a WP7 app that displays some times on one page. I have a code behind that has an ObservableCollection of objects. Each object has a calculated property that uses DateTime.Now to determine the time that's displayed on the page. I can't figure out how to "notify" that the property has changed since the property doesn't change, the current time is changing (just once per second). Any ideas? Here's the jist of what I've got:
//my business object
public class Widget
{
private string _name;
public string Name
{
get { return _name; }
set { _name = value; }
}
private DateTime? _start;
public DateTime? Start
{
get { return _start; }
set { _start = value; }
}
public TimeSpan? TimeSinceStart
{
get { return Start.HasValue ? DateTime.Now - Start.Value : default(TimeSpan); }
}
}
//my viewmodel
public class WidgetDisplayerViewModel : BaseViewModel
{
public WidgetDisplayerViewModel()
{
TimeUpdateTimer.Tick += new EventHandler(TimeUpdateTimer_Tick);
TimeUpdateTimer.Interval = new TimeSpan(0, 0, 1);
TimeUpdateTimer.Start();
}
public WidgetDisplayerViewModel(string selectedCategory) : this()
{
Category = MockDataService.GetCategory(selectedCategory);
Category.Widgets = MockDataService.GetWidgets(selectedCategory).ToObservableCollection();
}
public DispatcherTimer TimeUpdateTimer = new DispatcherTimer();
private DateTime _currentTime;
public DateTime CurrentTime
{
get { return _currentTime; }
set {
_currentTime = value;
NotifyPropertyChanged("CurrentTime");
}
}
public Category Category { get; set; }
void TimeUpdateTimer_Tick(object sender, EventArgs e)
{
CurrentTime = DateTime.Now;
}
}
And then the view is very simple and just needs to display the CurrentTime and then for each Widget in the collection it needs to show the TimeSinceStart. The CurrentTime is getting updated each second by the timer and that gets propogated to the view. That one is easy because the timer is setting it and so I have a chance to call NotifyPropertyChanged("CurrentTime"), but how would I "notify" that all of the TimeSinceStart getters should be called to update the calculated value for each Widget since I'm not setting them?
Thanks!

Answer 1

You'll have to manually refresh the property one way or another. I see you already have a timer ticking every second. So I can suggest you two solutions:
1/ Define a "UpdateTime" method in the Widget object. In this method, call NotifyPropertyChanged("TimeSinceStart"). When the timer is ticking, enumerate the list of widgets, and call the UpdateTime method on each.
2/ Create a global object implementing the INotifyPropertyChanged interface, and holding the value of CurrentTime. Make each of your Widget objects subscribe to the PropertyChanged event of this global class to know when the time is updated. Then, when the event is triggered, call NotifyPropertyChanged("TimeSinceStart").

Answer 2

This can be a tricky one to work out and it can get very messy very fast.
I would suggest you stick with your current approach of having only one timer which is initialised in the main viewmodel. You then have to ask yourself the question - does the age (TimeSinceStart) of the Widget belong on the Widget, or is it purely for display/informational purposes? Is it a core piece of information that each Widget must keep during its lifespan?
This looks to me like it is for display purposes only. So my suggestion is this: once you have called GetWidgets, you could enumerate through each Widget and wrap it in a thin viewmodel of its own. The constructor for that viewmodel takes two parameters - the timer from the main viewmodel, and the Widget. You then subscribe to the timer's Tick event, and from that you notify that the TimeSinceStart property has changed.
public class WidgetWrapper : INotifyPropertyChanged
{
public WidgetWrapper(DispatcherTimer timer, Widget widget)
{
_widget = widget;
timer.Tick += TimerTick;
}
private void TimerTick(object sender, EventArgs e)
{
OnPropertyChanged("TimeSinceStart");
}
public Widget Widget { get { return _widget; } }
public TimeSpan? TimeSinceStart
{
get { return _widget.Start.HasValue ? DateTime.Now - _widget.Start.Value : default(TimeSpan); }
}
private void OnPropertyChanged(string propertyName)
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
public event PropertyChangedEventHandler PropertyChanged;
private readonly Widget _widget;
}
public class WidgetDisplayerViewModel : BaseViewModel
{
public WidgetDisplayerViewModel(string selectedCategory) : this()
{
Category = MockDataService.GetCategory(selectedCategory);
var wrappedWidgets = new ObservableCollection<WidgetWrapper>();
MockDataService.GetWidgets(selectedCategory).ForEach(widget => wrappedWidgets.Add(new WidgetWrapper(TimeUpdateTimer, widget)));
Category.Widgets = wrappedWidgets;
}
}
Wrapping a DTO (entity, Data Transfer Object) with its own viewmodel is a quite common approach when adding functionality to an entity. If you use this appoach you will have to slightly modify any UI bindings that were targetting properties on the Widget, as those UI elements will now be dealing with a WidgetWrapper (or you can just surface the required properties in the WidgetWrapper itself, then no bindings have to change).

Answer 3

Invoke the NotifyPropertyChanged method for the specified property.
public DateTime CurrentTime
{
get { return _currentTime; }
set {
_currentTime = value;
NotifyPropertyChanged("CurrentTime");
NotifyPropertyChanged("TimeSinceStart");
}
}

Answer 4

Subscribe all widgets to CurrentTime PropertyChanged event in Widget constructor
private Widget()
{
App.ViewModel.PropertyChanged += (s, e) =>
{
if (e.PropertyName.Equals("CurrentTime")
{
NotifyPropertyChanged("TimeSinceStart");
}
};
}