为什么我的转换器（基于ListView.ItemsSource）没有被调用？

Question

I have a small WPF application in which I have a list of files. Below the list of files I have an "Upload" button. I would like the text in the upload button to display "Upload x files" where x is the number of items in the list.

I am using a converter which takes the ItemsSource property as input and returns the string, however if I add or remove items from the listview (ie. it's underlying collection) the converter on the button binding is not called.

What am I doing incorrectly?

I have the base class:

public class FileItem : INotifyPropertyChanged { ... }

And an ObservableCollection:

public class Files : ObservableCollection<FileItem> {}

An I assign the collection to the ListView.

lvw_FileList.ItemsSource = new Files();

The ListView has it's ItemSource property bound.

<ListView x:Name="lvw_FileList" ItemsSource="{Binding Mode=OneWay}">
...
</ListView>

The button which text content needs a converter based on the ListView.ItemsSource.

<Button Content="{Binding ElementName=lvw_FileList, Path=ItemsSource, Converter={StaticResource UpdateButtonConverter}}" x:Name="btn_Upload" /> 

Answer 1

Bindings only listen to PropertyChanged and not CollectionChanged notifications. If you need to run the conversion whenever the collection changes consider a MultiBinding to both the collection and its Count property. The count change will trigger a reevaluation and the collection value can be used in the converter.