如何将WPF UserControl挂钩到绑定的ViewModel对象的事件？

Question

Say I have a UserControl with a DependencyProperty to bind to an ObservableCollection<Foo> object. The collection of Foo objects is actually sent to some Flash ActiveX object. Because of this, I can't "WPF bind" to properties of Foo objects.

When properties of a Foo object are updated by something else than the UserControl, I want the UserControl to be notified so it refreshes (ie resends all the Foo objects to the Flash object).

I'm not quite sure how to handle this. I can think of two not so distinct possibilities:

1. The UserControl subscribes to some event of the object that contains the ObservableCollection<Foo> object. Whenever a property of a Foo object is changed, the event would fire, and so the UserControl would know to resend the Foo objects the Flash object.

2. Having the UserControl subscribed to the CollectionChanged event of the ObservableCOllection object, somehow cause the ObservableCOllection<Foo> object to raise the event with the Action NotifyCollectionChangedAction.Reset .

    void OnFooCollectionChanged(object sender, NotifyCollectionChangedEventArgs e) { if (!_flashIsLoaded) return; if (e.Action == NotifyCollectionChangedAction.Reset) { ClearAndSendFoos(this, Foos); return; } .... 

Any idea how I can get any of the two above done, or how else I can get the UserControl to know it's time to refresh the collection?

Answer 1

Something seems amiss here. You tagged the post with MVVM, so I suppose you are using it.

See, with Model, View, ViewModel, you should be doing this work between the ViewModel and the Model. So the ViewModel would subscribe to the model's event and update. If the Model doesn't have such an event, then inherit it and add it. Then because the ViewModel implements INotifyPropertyChanged and because it uses an ObservableCollection, the View will already update.

Get the ViewModel's ObservableCollection to change and you don't have to have the UserControl subscribe to any event.

The view should just display the ViewModel updates.

Answer 2

You should consider having multiple ViewModels.

If you are interested in changes that happen in an item that you have in your ViewModel, create an ViewModel for those Items and change the ViewModel collection to a ObservableCollection<ItemViewModel> .

That way, you can bind to the properties you are actually interested in.

Let's have an example:

// the overall viewModel:
public class OverallViewModel : ViewModelBase
{
    public ObservableCollection<ProductViewModel> Products { get; set; };
}

// the item viewmodel:
public class ProductViewModel : ViewModelBase
{
    public string ProductName { get; set; }
    public int CountOfSold { get; set; }
}

So, you sell 5 Bananas. The count of Banana's sold changes. Since you bind directly to the item, there is no longer any need to have an extra event, the view (in your case the usercontrol) can update without any need to listen to an artificial collection event.

I do not know much about Flash, but perhaps having a converter in your usercontrol that hosts the flash can help. At least you do not have to crawl all the list, but just a single item.

Hope this helps you