Binding to INotifyCollecti onChanged event

[Some Bloke]

I am trying to bind a WPF dependency property to an aggregate of a
collection. I've implemented a wrapper around a Dictionary that
implements INotifyCollecti onChanged to fire the event when items are
added or removed.

Debugging through it I notice that there is nothing listening to the
event after I bind to it which I expected the SetBinding to do as I
don't know what delegate to attach to the event. It's the binding that
needs updating after all.

I've tried a test with just an ObservableColle ction to see if I was
missing something but that doesn't work either.

How do I wire up the event to the binding or do whatever it takes to
get my test to pass?

Here's my test code:

[TestClass()]
public class ObservableDicti onaryTest : DependencyObjec t
{
public class MaxDoubleConver ter : IValueConverter
{
public object Convert(object value, Type targetType, object
parameter, System.Globaliz ation.CultureIn fo culture)
{
if (value is IEnumerable<dou ble>)
{
return ((IEnumerable<d ouble>)value).M ax(); // using
Linq
}

throw new ArgumentExcepti on("Cannot covert " +
value.GetType() + " to IEnumerable<dou ble>", "value");
}
public object ConvertBack(obj ect value, Type targetType,
object parameter, System.Globaliz ation.CultureIn fo culture)
{
throw new NotImplementedE xception();
}
}

public double Max
{
get { return (double)GetValu e(MaxProperty); }
}

public static readonly DependencyPrope rty MaxProperty =
DependencyPrope rty.Register("M ax", typeof(double),
typeof(Observab leDictionaryTes t), new PropertyMetadat a(0.0));

[TestMethod()]
public void TestBindingToOb servableCollect ion()
{
ObservableColle ction<doubleval ues = new
ObservableColle ction<double>() ;
values.Add(1.0) ;
values.Add(2.0) ;
values.Add(3.0) ;

Binding b = new Binding();
b.Converter = new MaxDoubleConver ter();
b.Source = values;
b.Mode = BindingMode.One Way;
BindingOperatio ns.SetBinding(t his, MaxProperty, b);

Assert.AreEqual (3.0, Max); // this works

values.Add(4.0) ;
Assert.AreEqual (4.0, Max); // this doesn't

values.Remove(4 .0);
Assert.AreEqual (3.0, Max);
}
}

[Peter Morris]

What about using ObservableColle ction<T>?

--
Pete
=============== =============== ===========
I use Enterprise Core Objects (Domain driven design)
http://www.capableobjects.com/
=============== =============== ===========

[Some Bloke]

On 9 Jun, 16:22, "Peter Morris" <mrpmorris at gmail dot comwrote:

What about using ObservableColle ction<T>?

--
Pete
=============== =============== ===========
I use Enterprise Core Objects (Domain driven design)http://www.capableobjects.com/
=============== =============== ===========

That's what the test I've posted does or do you mean use it in another
way? It's only my test class that is called ObservableDicti onaryTest
(maybe I should have renamed it if that caused confusion).

I'd expect the Binding.Source setter to check if the source is
IEnumerable and if it implements INotifyCollecti onChanged then listen
to the event and update the bound value when it fires.

I'm obviously not understanding how binding to an ObservableColle ction
works.

[Chris Jobson]

>I am trying to bind a WPF dependency property to an aggregate of a

collection. I've implemented a wrapper around a Dictionary that
implements INotifyCollecti onChanged to fire the event when items are
added or removed.

Debugging through it I notice that there is nothing listening to the
event after I bind to it which I expected the SetBinding to do as I
don't know what delegate to attach to the event. It's the binding that
needs updating after all.

I've tried a test with just an ObservableColle ction to see if I was
missing something but that doesn't work either.

How do I wire up the event to the binding or do whatever it takes to
get my test to pass?

I think the problem is that only bindings which expect to bind to a
collection (like ListBox.ItemsSo urce) check for and subscribe to
INotifyCollecti onChanged (though I don't know exactly how they do it).

What you could do is make your collection a property of your class, bind to
the property, implement INotifyProperty Changed, then subscribe to the
collection's INotifyCollecti onChanged yourself and raise a PropertyChanged
event. This seems rather clumsy and I'm sure there ought to be a better
way - maybe someone more knowledgeable will point it out.

Chris Jobson

[Some Bloke]

On 9 Jun, 17:54, "Chris Jobson" <chris.job...@b tinternet.comwr ote:

I am trying to bind a WPF dependency property to an aggregate of a
collection. I've implemented a wrapper around a Dictionary that
implements INotifyCollecti onChanged to fire the event when items are
added or removed.

Debugging through it I notice that there is nothing listening to the
event after I bind to it which I expected the SetBinding to do as I
don't know what delegate to attach to the event. It's the binding that
needs updating after all.

I've tried a test with just an ObservableColle ction to see if I was
missing something but that doesn't work either.

How do I wire up the event to the binding or do whatever it takes to
get my test to pass?

I think the problem is that only bindings which expect to bind *to a
collection (like ListBox.ItemsSo urce) check for and subscribe to
INotifyCollecti onChanged (though I don't know exactly how they do it).

What you could do is make your collection a property of your class, bind to
the property, implement INotifyProperty Changed, then subscribe to the
collection's INotifyCollecti onChanged yourself and raise a PropertyChanged
event. This seems rather clumsy and I'm sure there ought to be a better
way - maybe someone more knowledgeable will point it out.

Chris Jobson

Thanks.

One other idea I had was that it was the DependencyPrope rty that
needed refreshing so I tried subscribing to the event and calling
InvalidatePrope rty(MaxProperty ), but that didn't work either, it still
came out as 3.0 somehow and the Convert was never called again so the
3.0 isn't being invalidated.

I'll try your idea since the collection will be a property of a
UserControl once I get it working.

[Chris Jobson]

>I am trying to bind a WPF dependency property to an aggregate of a

collection. I've implemented a wrapper around a Dictionary that
implements INotifyCollecti onChanged to fire the event when items are
added or removed.

Debugging through it I notice that there is nothing listening to the
event after I bind to it which I expected the SetBinding to do as I
don't know what delegate to attach to the event. It's the binding that
needs updating after all.

I've tried a test with just an ObservableColle ction to see if I was
missing something but that doesn't work either.

How do I wire up the event to the binding or do whatever it takes to
get my test to pass?

I think the problem is that only bindings which expect to bind to a
collection (like ListBox.ItemsSo urce) check for and subscribe to
INotifyCollecti onChanged (though I don't know exactly how they do it).

What you could do is make your collection a property of your class, bind to
the property, implement INotifyProperty Changed, then subscribe to the
collection's INotifyCollecti onChanged yourself and raise a PropertyChanged
event. This seems rather clumsy and I'm sure there ought to be a better
way - maybe someone more knowledgeable will point it out.

Chris Jobson

[Some Bloke]

After looking into this a bit more I found
http://www.drwpf.com/Blog/Default.as...=36&EntryID=18 that told
me that it is a CollectionView that does the subscribing to
CollectionChang ed in ListBox and the like.

Naturally I tried wrapping my collection in a CollectionView and
setting that as the source but now the Max property doesn't get
evaluated at all (my converter is never called) so thebinding is
treating the view differently to any other plain IEnumerable, like it
thinks it doesn't need to do anything if it already has a
CollectionView.

This seems like one of those times when I'm digging deeper than I need
to and all this clever Binding and CollectionView stuff does what I
need if only I knew how to use it properly.

[Chris Jobson]

After looking into this a bit more I found

http://www.drwpf.com/Blog/Default.as...=36&EntryID=18 that told
me that it is a CollectionView that does the subscribing to
CollectionChang ed in ListBox and the like.

Thanks for that link. I'd read it some time ago but had forgotten the bit
about CollectionView.

Naturally I tried wrapping my collection in a CollectionView and
setting that as the source but now the Max property doesn't get
evaluated at all (my converter is never called) so thebinding is
treating the view differently to any other plain IEnumerable, like it
thinks it doesn't need to do anything if it already has a
CollectionView.

I *think* (but could well be wrong - I find it all a bit confusing!) it's
still the same problem - binding Max to a CollectionView is more or less the
same as binding it to an ObservableColle ction, and in both cases a change to
the collection won't re-evaluate Max because it isn't interested in
INotifyCollecti onChanged. AIUI the special feature of CollectionView is that
it if your property is of type CollectionView then you can bind a collection
to it and the property will re-evaluate when the collection changes.

(By the way, I'm surprised your idea of InvalidatePrope rty(MaxProperty )
didn't work. I've never used InvalidatePrope rty myself, but from the
description in MSDN it sounds ideal.)

I've just come across what might be another solution to your problem -
BindableLINQ (http://www.codeplex.com/bindablelinq/). In particular this
post (http://www.paulstovell.com/blog/bind...ble-aggregates)
seems to describe something very similar to your requirement. It's only a
beta, and I've no experience of it myself, but it might be worth
investigating.

Chris