When should I set the ItemsSource property of a ListBox? - silverlight

Question

I am writing a WP7 app which restores its state on the main UI thread in its page's OnNavigatedTo() override handler. In this handler, it sets the page's listbox ItemsSource property to the deserialized ObservableCollection of data items. The deserialization is from isolated storage and is quick enough that it doesn't hang the thread.
However, when the page comes up, the listbox is empty. Upon setting a breakpoint and inspecting the state of the page, the Items property is correctly populated and non-empty.
If I delay the settings of the ItemsSource property like so:
protected override void OnNavigatedTo(NavigationEventArgs e)
{
base.OnNavigatedTo(e);
int delayMs = 100; // Why 100 ?
runDelayedDispatch(Deployment.Current.Dispatcher,
delayMs, delegate()
{
deserializeFromStorageAndSetItemsSource();
});
}
...
// Does a BeginInvoke() after the specified delay.
public static void runDelayedDispatch(Dispatcher dispatcher,
int delayInMilliseconds, Action action)
{
Action dispatcherAction = delegate()
{
dispatcher.BeginInvoke(action);
};
BackgroundWorker worker = new BackgroundWorker();
worker.DoWork += (s, e) => Thread.Sleep(delayInMilliseconds);
worker.RunWorkerCompleted += (s, e) => dispatcherAction.Invoke();
worker.RunWorkerAsync();
}
Then everything works fine.
What am I doing wrong? Should I be reading from isolated storage on a different handler or later in the application lifecycle?
Articles about app lifecycle have not shed any light on this :(
http://msdn.microsoft.com/en-us/library/system.windows.controls.page.onnavigatedto(v=vs.95).aspx
http://msdn.microsoft.com/en-us/library/cc838245(v=vs.95).aspx
http://windowsphonegeek.com/articles/WP7-Application-Lifecycle-and-Tombstoning
http://visualstudiomagazine.com/articles/2011/06/01/pcmob_app-lifecycle.aspx
Thanks!

Answer 1

It sounds like maybe your data context isn't set correctly, your binding isn't set correctly, or your INotifyPropertyChanged isn't firing.
PS: I think you should reword your question to get rid of your attempt to bypass the problem with a delay - that is giving you answers along lines you don't want and I'm pretty sure it isn't needed at all. Instead, put the ALL the RELEVANT code for the list and page in your question so we can see what you're doing.

Answer 2

All you have to do is to refresh the UI using INotifyPropertyChanged
protected override void OnNavigatedTo(NavigationEventArgs e)
{
base.OnNavigatedTo(e);
int delayMs = 100; // Why 100 ?
runDelayedDispatch(Deployment.Current.Dispatcher,
delayMs, delegate()
{
deserializeFromStorageAndSetItemsSource();
});
NotifyPropertyChange("UI");
}
public static void runDelayedDispatch(Dispatcher dispatcher,
int delayInMilliseconds, Action action)
{
Action dispatcherAction = delegate()
{
dispatcher.BeginInvoke(action);
};
BackgroundWorker worker = new BackgroundWorker();
worker.DoWork += (s, e) => Thread.Sleep(delayInMilliseconds);
worker.RunWorkerCompleted += (s, e) => dispatcherAction.Invoke();
worker.RunWorkerAsync();
NotifyPropertyChanged("UI");
}
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged(String info)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(info));
}
}

Answer 3

I found it, thanks all! I went back to the basics and tested this scenario from a bare bones list box and it worked in the OnNavigatedTo() without any problem.
The culprit was that I was hooking into receiving notification when the DataContext had changed in order to bind the width of the listbox items to the listbox width, but I could easily accomplish this by hooking into the LayoutUpdated event.
Here was the culprit code that I removed:
http://www.codeproject.com/Articles/38559/Silverlight-DataContext-Changed-Event
Thanks guys and sorry for wasting your time :|