Silverlight UI not updating - ObservableCollection being reinstantiated

Question

I'm pretty new to ObservableCollections, but have built some code which I'm sure should work. Unfortunately it doesn't. The only thing that is not happening, is my GUI is not being updated. I know the values are being updated in the back (Checked using Debugger).

What am I doing wrong?

Here with a sample of my XAML for the Textblock:

<TextBlock Name="tbCallsOpen" Text="{Binding IndicatorValue}" />

Herewith sample of my code behind:

public partial class CurrentCalls : UserControl
{
    Microsoft.SharePoint.Client.ListItemCollection spListItems;
    ObservableCollection<CurrentCallIndicator> CallIndicators = new ObservableCollection<CurrentCallIndicator>();        

    public CurrentCalls()
    {            
        InitializeComponent();

        DispatcherTimer dispatchTimer = new DispatcherTimer();
        dispatchTimer.Interval = new TimeSpan(0, 0, 20);
        dispatchTimer.Tick += new EventHandler(BindData);
        dispatchTimer.Start();
    }

    private void BindData(object sender, EventArgs args)
    {
        //splistitems is a sharepoint list. Data is being retrieved succesfully, no issues here.
        foreach (var item in spListItems)
        {
            //My custom class which implements INotifyPropertyChanged
            CurrentCallIndicator indicator = new CurrentCallIndicator();
            indicator.IndicatorValue = item["MyValueColumn"];

            //Adding to ObservableCollection
            CallIndicators.Add(indicator);

        }
        //Setting Datacontext of a normal TextBlock
        tbCallsOpen.DataContext = CallIndicators.First(z => z.IndicatorName == "somevalue");
    }
}

score 2 · Answer 1 · answered Dec 08 '11 at 15:00

2

You are most likely assuming that changes to the underlying items in the collection will raise the CollectionChanged event; however that is not how the ObservableCollection<T> works.

If you wanted this behavior you would need to roll your own implmentation and when a PropertyChanged event is fired within an item within your collection, you would then need to fire the CollectionChanged event.

answered Dec 08 '11 at 15:00

Aaron McIver

24,527
5
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Ok, I've now also added bound to the CollectionChanged Event on the ObservableCollection and still no joy.. Values are updated, but Controls are not.. – Fox Dec 09 '11 at 07:36
Remove all unneeded code such as the DispatchTimer and see if it works. Need to narrow it down. You can also check the output window. – Aaron McIver Dec 09 '11 at 12:09

score 1 · Answer 2 · answered Dec 08 '11 at 19:45

Your code looks more-or-less correct to me, at first blush - though I wouldn't expect that you'd need to use an ObservableCollection<> to get the results you seem to be expecting: a simple List<> would work just fine.

If the debugger tells you that the DataContext is being updated correctly to the expected item, then the most likely issue is that there's a problem with how your binding is defined. If you're not seeing any binding errors reported in your debug window, then I'd look into Bea Stollnitz' article on debugging bindings. Most specifically, I often use the technique she suggests of a "DebugValueConverter", e.g.:

/// <summary>
/// Helps to debug bindings.  Use like this: Content="{Binding PropertyName, Converter={StaticResource debugConverter}}"
/// </summary>
public class DebugConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return value;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return value;
    }
}

And then set a breakpoint in your converter, and watch what's happening. It's a hack and a kludge, but until we're all on SL5 (which has binding debugging built in), it's your best bet.

score 0 · Answer 3 · answered Dec 12 '11 at 10:17

0

Ok, Sorted. I fixed the issue myself. Because I was updating the values in a loop, the ObservableCollection wasn't being updated properly. All I did in the beginning of the databinding method, was to Clear the collection : CallIndicators.Clear();

answered Dec 12 '11 at 10:17

Fox

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3
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Silverlight UI not updating - ObservableCollection being reinstantiated

3 Answers3