Who uses NotifyCollectionChangedEventArgs' features?

Question

This is a follow-up to my question, How does WPF handle CollectionChanged events for custom collections?.

According to Alex.Wei's answer (and the source code of IndexedEnumerable) WPF ignores the specifics of the NotifyCollectionChangedEventArgs (e.g. Action) and always reacts the same, ultimately as if Action == Reset.

So my questions: Who uses NotifyCollectionChangedEventArgs' features and, if I raise the event manually (for a custom class) does it make sense to specifiy the details (if they are never evaluated)? Also, why does WPF behave like this - isn't this a potential performance killer?

Answer 1

I am sorry if I didn't make things clear in last anwser. Actually, WPF behaves according to specifics of the NotifyCollectionChangedEventArgs like it means to be, and IndexedEnumerable just a tool that let CollectionView or other componets of WPF access to the source collections that didn't implement IList throngh an index easily. For example, after you bind a collection to ItemsControl.ItemsSource, the things in below will happen.

1. ItemsControl will specify the collection as the source of its Items property.
2. Items property which is an ItemCollection will obtain a CollectionView by calling CollectionViewSource.GetDefaultCollectionView method.
3. The view will try to subscribe CollectionChanged event of the source collection and act accordingly.
4. ItemCollection will also subscribe CollectionChanged event of the view and act accordingly.
5. And also, ItemsControl subscribe CollectionChanged event of the Items and act accordingly form the beginning.

So, the answer to you questions is that a lot of WPF classes are using NotifyCollectionChangedEventArgs' features and you definitely need to riase CollectionChanged event correctly by providing all the details whatever you collection was implemented IList or not.

Answer 2

My experience is that WPF is going to raise an exception if you provide wrong indices or multiple elements in an Add or Remove action.

Apart from that, there is a range of frameworks (NMF Expressions, BindableLINQ, ContinuousLinq, Obtics) that make use of these CollectionChanged events in order to implement INotifyCollectionChanged on queries. These frameworks will heavily rely on the details that you put in the event, despite you can of course always resort to a reset event.

Answer 3

Take a look on my ObservableComputations library. That library uses NotifyCollectionChangedEventArgs very widely.