DependentObservable depending on ObservableCollections

[mai...@gmail.com]

Hi.

Are DependentObservable depending on ObservableCollections being re-evaluated on collectionChanged() collection event (e.g. when one add or remove any element)?

Thanks!

[Andy Tsui]

No. DependentObservable observes only IObservables on simple set() operation. Individual item change event is not observed. 

[mai...@gmail.com]

Ok, thanks.

Nevertheless it could be useful to parametrize such behavior, since having a bound view whose visual state is related to specific criteria applied to a collection is quite a common case...am I wrong?

At this time I'm compelled to use a dummy booleanobservable which dependant depends on,  toggled whenever I update collection...

[Andy Tsui]

Collection Change Event consists of different states, it can either be Reset/Add/Delete/Change which is over complicated for the general DependentObservable to handle.. but it is easy to create your own, take your boolean obs as example, you can implement it with CollectionObserver, and toggle whenever the collection changed. 

[Andy Tsui]

http://andytsui.wordpress.com/2013/03/28/example-in-collection-observer-collection-size-converter/

On Mon, Jan 14, 2013 at 8:05 PM, <mai...@gmail.com> wrote:

[Lorenzo Maiorfi]

COuld you please post a micro-snippet of a simple use case concerning for example a viewmodel method being invoked whenever a member ArrayListObservable changes? I'm a little confused about how to use CollectionObserver interface and how to "inject" it while creating a DependantObservable.

Thank you very much!

[Andy Tsui]

I think the example in the blog post should explain that, any particular problem you have on the example?

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[Lorenzo Maiorfi]

I was looking for something like that (now that I have understood how to get it done it seems much, much simpler :) ):

public class BasicViewModel

{

    [...]

    public final ArrayListObservable<BasicItemViewModel> Items = new ArrayListObservable<BasicItemViewModel>(BasicItemViewModel.class);

    CollectionObserver _itemsCollectionObserver;

    [...]

   _itemsCollectionObserver=new CollectionObserver()

   {

    @Override

    public void onCollectionChanged(IObservableCollection<?> arg0, CollectionChangedEventArg arg1, Collection<Object> arg2)

    {

    BasicViewModel.this.ItemsStatus.set(String.format("%d items",arg0.size()));

    }

    };

[...]