Portable Class Library and ObservableCollection, updating UI Thread - observablecollection

Question

I'm not very experienced with this topic so forgive me if this isn't very clear.
I've created a Portable Class Library that has an ObservableCollection of Sections, and each secion has an ObservableCollection of Items.
Both of these collections are bound to the UI of separate Win8 and WP8 apps.
I'm trying to figure out the correct way to populate these collections correctly so that the UI gets updated from the PCL class.
If the class was inside the win8 project I know I could do something like Dispatcher.BeginInvoke, but this doesn't translate to the PCL, nor would I be able to reuse that in the WP8 project.
In this thread (Portable class library equivalent of Dispatcher.Invoke or Dispatcher.RunAsync) I discovered the SynchroniationContext class.
I passed in a reference to the main app's SynchroniationContext, and when I populate the sections I can do so because it's only the one object being updated:
if (SynchronizationContext.Current == _synchronizationContext)
{
// Execute the CollectionChanged event on the current thread
UpdateSections(sections);
}
else
{
// Post the CollectionChanged event on the creator thread
_synchronizationContext.Post(UpdateSections, sections);
}
However, when I try to do the same thing with articles, I have to have a reference to both the section AND the article, but the Post method only allows me to pass in a single object.
I attempted to use a lambda expression:
if (SynchronizationContext.Current == _synchronizationContext)
{
// Execute the CollectionChanged event on the current thread
section.Items.Add(item);
}
else
{
// Post the CollectionChanged event on the creator thread
_synchronizationContext.Post((e) =>
{
section.Items.Add(item);
}, null);
}
but I'm guessing this is not correct as I'm getting an error about being "marshalled for a different thread".
So where am I going wrong here? how can I update both collections correctly from the PCL so that both apps can also update their UI?
many thanks!

Answer 1

Hard to say without seeing the rest of the code but I doubt is has anything to do with Portable Class Libraries. It would be good to see the details about the exception (type, message and stack trace).
The way you call Post() with more than argument looks correct. What happens if you remove the if check and simply always go through SynchronizationContext.Post()?
BTW: I don't explicitly pass in the SynchronizationContext. I assume that the ViewModel is created on the UI Thread. This allows me to capture it like this:
public class MyViewModel
{
private SynchronizationContext _context = SynchronizationContext.Current;
}

Answer 2

I would recommend that at least in your ViewModels, all publicly observable state changes (ie property change notifications and modifications to ObservableCollections) happen on the UI thread. I’d recommend doing the same thing with your model state changes, but it might make sense to let them make changes on different threads and marshal those changes to the UI thread in your ViewModels.
To do this, of course, you need to be able to switch to the UI thread in portable code. If SynchronizationContext isn’t working for you, then just create your own abstraction for the dispatcher (ie IRunOnUIThread).

Answer 3

The reason you were getting the "marshalled on a different thread" error is that you weren't passing the item to add to the list as the "state" object on the Post(action, state) method.
Your code should look like this:
if (SynchronizationContext.Current == _synchronizationContext)
{
// Execute the CollectionChanged event on the current thread
section.Items.Add(item);
}
else
{
// Post the CollectionChanged event on the creator thread
_synchronizationContext.Post((e) =>
{
var item = (YourItemnType) e;
section.Items.Add(item);
}, item);
}
If you make that change, your code will work fine from a PCL.