Comments on: PropertyDescriptor AddValueChanged Alternative

By: jon

jon — Mon, 23 Feb 2009 15:18:11 +0000

If you are just replacing the AddValueChanged to use this, remember you will need a reference to the notifier, the previous way (DependencyPropertyDescriptor) did as it has an internal hash map. maybe save someone a few minutes

By: jon

jon — Mon, 23 Feb 2009 15:15:36 +0000

If you change this line

if (null != notifier.ValueChanged)
notifier.ValueChanged(notifier, EventArgs.Empty);

to this, you will get the correct sender in the event

if (null != notifier.ValueChanged)
notifier.ValueChanged(notifier.PropertySource, EventArgs.Empty);

By: Bob

Bob — Wed, 03 Dec 2008 20:57:14 +0000

Hey Andrew, I had been looking all day for a replacement for WPF’s DependencyPropertyDescriptor and I came across this article. Unfortunately it looks like they removed the Bindable() attribute from the final Silverlight release – do you have an update to this code that works in the final bits?
Thanks,
Bob

By: agsmith

agsmith — Tue, 26 Aug 2008 19:00:38 +0000

Thanks. I’m glad it helped.
If you look at the internal ValueChangedEventManager class in the PresentationFramework assembly (which is what MS is using for this) you can see that they are using a custom class (ValueChangedRecord) to add a handler for the property changed of the PD and that class has a reference back to the WeakEventManager (and the listenerlist) and uses the manager’s DeliverEventToList method instead of the DeliverEvent method.

By: Neelima

Neelima — Tue, 26 Aug 2008 18:21:51 +0000

Correction to my above comment

The reason being, the way you attach the event in weakeventmanager is

source.Event += DeliverEvent;

By: Neelima

Neelima — Tue, 26 Aug 2008 18:21:15 +0000

Hi Andrew,

Thanks for this great article! It really helped a lot with cleaning up the code I am working on currently.

I have been using WeakEventManager class with some other events but I have not been able to come up with a way to use it with DependencyPropertyDescriptor.

The reason being, the way you attach the event in weakeventmanager is

source.<> = DeliverEvent;

I am not able to fit DependencyPropertyDescripter into that structure.

Please let me know if you have any suggestions.

By: Josh Smith

Josh Smith — Wed, 09 Apr 2008 17:49:29 +0000

Thanks. That makes sense.

By: agsmith

agsmith — Wed, 09 Apr 2008 17:44:42 +0000

No, it won’t leak memory if you remove the handler. When the last handler is removed, the item is removed from the hashtable. But if you’re relying on say an item being removed from the collection before you remove the handler, that may not happen – e.g. if you just close the form while the list is populated. In that case, you might be able to also unhook within the Unloaded event but then you have to know to re-hook again should your element get reloaded (e.g. if the element is in a tab control and you switch to another tab and back).

By: Josh Smith

Josh Smith — Wed, 09 Apr 2008 17:22:35 +0000

Nice solution, Andrew. I like how you use Binding’s internal weakref in this case. Very cool!

One question for you…does the memory leak still exist if you call RemoveValueChanged on the property descriptor? Does that remove the entry from the internal hashtable?

Thanks,
Josh

By: Karl Shifflett

Karl Shifflett — Wed, 09 Apr 2008 10:55:35 +0000

Andrew,

Very nice! Thank you for sharing this solution.

Cheers,

Karl