Question

I’m writing simple WP7 app to study ReactiveUI.
I want to show a collection on the screen, items should be selectable, and I want to have a Command which removes all selected items from the collection. Also the command must be executable only when there is at least one selected item.
I’ve got a collection defined in this way:

Persons = model.Persons
                .CreateDerivedCollection(x => new PersonViewModel(x));

In PersonViewModel I have a property:

private bool _isSelected;
public bool IsSelected
{
    get { return _isSelected; }
    set { this.RaiseAndSetIfChanged(x => x.IsSelected, ref _isSelected, value); }
}

There is no information about selection state in model, just in viewModel.
In Page ViewModel I have this code:

Persons = model.Persons.CreateDerivedCollection(x => new PersonViewModel(x));
Persons.ChangeTrackingEnabled = true;

var deleteSelectedCanExecute = Persons.ItemChanged
                                  .Select(_ => Persons .Any(p => p.IsSelected));

DeleteSelectedCommand = new ReactiveCommand
                (
                       deleteSelectedCanExecute
                );
DeleteSelectedCommand.Subscribe(
                x => RemoveSelected()
                );

and a method:

   private void RemoveSelected()
    {
        var res = Persons.Where(p => p.IsSelected)
             .Select(x => x.Model).ToList();
        foreach (var person in res)
        {
            _model.Persons.Remove(person);
        }
    }

first question (not so important, I think I can find the solution by myself):
when I run an app the DeleteSelected button is Active.DeleteSelectedCommand.CanExecute does not fires. However after selecting/deselection any item – button states works fine.

and the main problem:

After I run DeleteSelectedCommand it removes all selected items (I see it in the debugger). And then I got “NotSupportedException” with following stack trace:

   at System.Threading.Interlocked.Decrement(Int64& location)
   at ReactiveUI.RefcountDisposeWrapper.Release()
   at ReactiveUI.ReactiveCollection`1.removeItemFromPropertyTracking(PersonViewModel toUntrack)
   at ReactiveUI.ReactiveCollection`1.<setupRx>b__18(PersonViewModelx)
   at System.Reactive.AnonymousObserver`1.Next(PersonViewModelvalue)
   at System.Reactive.AbstractObserver`1.OnNext(PersonViewModelvalue)
   at System.Reactive.AutoDetachObserver`1.Next(PersonViewModelvalue)
   at System.Reactive.AbstractObserver`1.OnNext(PersonViewModelvalue)
   at System.Reactive.ScheduledObserver`1.<>c__DisplayClass4.<Next>b__2()
   at System.Reactive.ScheduledObserver`1.<EnsureActive>b__0(Action self)
   at System.Reactive.Concurrency.Scheduler.<Schedule>b__0(Action`1 _action, Action`1 self)
   at System.Reactive.Concurrency.Scheduler.<>c__DisplayClass9`1.<InvokeRec1>b__6(Action`1 state1)
   at System.Reactive.Concurrency.Scheduler.InvokeRec1[TState](IScheduler scheduler, Pair`2 pair)
   at System.Reactive.Concurrency.DispatcherScheduler.<>c__DisplayClass1`1.<Schedule>b__0()
   at System.Reflection.RuntimeMethodInfo.InternalInvoke(RuntimeMethodInfo rtmi, Object obj, BindingFlags invokeAttr, Binder binder, Object parameters, CultureInfo culture, Boolean isBinderDefault, Assembly caller, Boolean verifyAccess, StackCrawlMark& stackMark)
   at System.Reflection.RuntimeMethodInfo.InternalInvoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, StackCrawlMark& stackMark)
   at System.Reflection.MethodBase.Invoke(Object obj, Object[] parameters)
   at System.Delegate.DynamicInvokeOne(Object[] args)
   at System.MulticastDelegate.DynamicInvokeImpl(Object[] args)
   at System.Delegate.DynamicInvoke(Object[] args)
   at System.Windows.Threading.DispatcherOperation.Invoke()
   at System.Windows.Threading.Dispatcher.Dispatch(DispatcherPriority priority)
   at System.Windows.Threading.Dispatcher.OnInvoke(Object context)
   at System.Windows.Hosting.CallbackCookie.Invoke(Object[] args)
   at System.Windows.Hosting.DelegateWrapper.InternalInvoke(Object[] args)
   at System.Windows.RuntimeHost.ManagedHost.InvokeDelegate(IntPtr pHandle, Int32 nParamCount, ScriptParam[] pParams, ScriptParam& pResult)

So I’m doing it wrong, but what’s the problem? I can’t understand from the ST. What’s the right way to implement this behaviour. It’s so common isn’t it?

upd

If I remove all code about deleteSelectedCanExecute and run program – it crashes. If i remove Participants.ChangeTrackingEnabled = true; – it works as I expect.

Answer 1

Anton, you are the proud owner of a new ReactiveUI commit – build from source and your crash should go away.

As to your question about the selection, this is one scenario that is a bit tricky to do at the moment if the collection can change sizes. Code somewhere must not only subscribe to each item in the collection, it must also keep a list of which items are being added or removed (i.e. there are two ways to no longer be selected, Item.IsSelected going from true => false, or Item being removed).

If you don’t have a list that changes rapidly, you can do this in a somewhat inefficient though far easier way:

var cmd = new ReactiveCommand(
    Persons.ItemsCountChanged.Select(_ => 
        Persons.Any(x => x.IsSelected)));

Incidentally, this solution also doesn’t require ChangeTrackingEnabled, so you don’t need to work around the bug I just fixed.