Is there an easier way to combine multiple observable collections into a single observable collection?

Question

I'm using the CSLA Framework and MVVM Light for my application. For anyone unfamiliar with it, CSLA allows me to construct an object graph and provides observable binding lists, such as the following:

public class RootObject : BusinessBase<Customer>
{
    public FriendList Friends { get { ... } set { ... } }
}

public class FriendList : BusinessListBase<FriendsList, Friend> { }

public class Friend : BusinessBase<Friend>
{
    public AnnouncementList Announcements { get { ... } set { ... } }
}

The problem I have is I want to aggregate all of the Announcements together into a single ObservableCollection<Announcement> for my view model.

public class MainViewModel
{
    private RootObject _graph;

    public MainViewModel(int userId)
    {
        _graph = GetObjectGraph(userId);

        var announcements = _graph.Friends.SelectMany(x => x.Announcements);
        Announcements = new ObservableCollection<Announcement>(announcements);

        // Some way of synchronizing every new collection here!
    }

    public ObservableCollection<Announcement> Announcements { get; private set; }
}

I did have some code which attempted to subscribe to the collectionchanged event of the FriendList and then subsequently subscribe to the collectionchanged event of each new friend and unsubscribing from old friends and etc, but it got extremely complicated extremely fast.

I also tried looking at Reactive Extensions but I have no idea how to get it to remove items as friends are removed or announcements are removed.

I also came across some MVVM framework named ReactiveUI, but I don't want to redo everything out of MVVMLight and into ReactiveUI.

I'm really not sure if there is a better way, perhaps a way to just make Rx work without an entire framework, just for this scenario? I just don't want to have to go back to option one of managing a bunch of collection changed events with nested collection changed event subscriptions!

score 1 · Answer 1 · answered Jul 08 '14 at 03:37

1

I also came across some MVVM framework named ReactiveUI, but I don't want to redo everything out of MVVMLight and into ReactiveUI.

You don't have to rewrite anything to use ReactiveUI in your MVVM Light project, everything will work with your existing code.

That being said, while RxUI's Derived Collections support several operations to transform lists, SelectMany isn't one of them (i.e. to take a base list of lists and convert it into a flattened list). You'd have to write this bit yourself, though Rx and ReactiveList will make this task much easier to accomplish than doing it by-hand.

answered Jul 08 '14 at 03:37

Ana Betts

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I'd have to rewrite a lot of my view-model classes, which there are about 150 of them. Everything was working because I never needed this complicated situation, most of my view-models are simple and MVVMLight works just fine. :/ – michael Jul 08 '14 at 03:40
You're missing what I'm saying, you can totally use your MVVM Light ViewModels, you don't need to replace them with RxUI ViewModels, at all. You can use ReactiveList in a single class / place – Ana Betts Jul 08 '14 at 03:41
But using kind of observable-over-observable (that is, Reactive) there's no warn of leaks (i.e. event unsubscription)? – Mario Vernari Jul 08 '14 at 03:45
Rx makes sure to handle subscribing and unsubscribing correctly – Ana Betts Jul 08 '14 at 03:47
Don't want to get you losing time, but time ago I tried RX against WPF and it was a nightmare, just because the Disposable pattern. It comes useful only when you can trash everything: both the source and the target. – Mario Vernari Jul 08 '14 at 03:51
Really? That's pretty interesting because I had the exact opposite experience. I've been using WPF for about as long as you possibly can and when the reactive extensions got introduced, it felt to me like everything suddenly just got orders of magnitude simpler. – MojoFilter Jul 08 '14 at 12:34

score 1 · Answer 2 · edited May 23 '17 at 11:51

1

I know that this question is quite old, but it seems to me that Binding multiple ObservableCollections to One ObservableCollection is the answer you're looking for.

Basically, use a the CompositeCollection class that allows to treat multiple collections as a single collection.

edited May 23 '17 at 11:51

Community

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answered Jun 12 '15 at 14:52

GoJiTa972

121
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While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes. – Nikolay Kostov Jun 12 '15 at 15:42
Since I edited the answer, do I still deserve to be downvoted ? – GoJiTa972 Jun 22 '15 at 14:26
Sorry, I assumed wrong, then. Someone did, though :) – GoJiTa972 Jun 22 '15 at 17:47

Is there an easier way to combine multiple observable collections into a single observable collection?

2 Answers2