取消可观察集合上的集合更改事件

Question

How can i cancel the collection changed event on an observable collection? When my collection changes it invokes methods on a third party dll.These methods may or may not fail.

If they fail, i want dont want the item to be added to or removed from the collection. Looking at the name, it looks like the collection changed event is fired after something has been added or deleted, but how could i achieve my functionality?

Answer 1

Too late but it might help someone else :

class ExtendedObservableCollection<T> : ObservableCollection<T>
{
    private bool _suppressNotification = false;

    public bool AllowNotifications { get { return _suppressNotification; } }

    protected override void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
    {
        if (!_suppressNotification)
            base.OnCollectionChanged(e);
    }

    protected override void OnPropertyChanged(PropertyChangedEventArgs e)
    {
        if (!_suppressNotification)
            base.OnPropertyChanged(e);
    }

    public void ActivateNotifications()
    {
        _suppressNotification = false;
    }

    public void DesactivateNotifications()
    {
        _suppressNotification = true;
    }

    public void AddRange(IEnumerable<T> list)
    {
        if (list == null)
            throw new ArgumentNullException("list");

        _suppressNotification = true;

        foreach (T item in list)
        {
            Add(item);
        }
        _suppressNotification = false;
        OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add));
    }


}

Answer 2

I'll share the homely hack I came up with for my WPF/MVVM solution. In the methods I call for e.Action == NotifyCollectionChangedAction.Remove or e.Action == NotifyCollectionChangedAction.Add I check for failure and take any actions required to undo the change and set a boolean member _updateObservable to true. Since I can't modify the ObservableCollection during the change event, I have to defer it. Setting the boolean seemed like the easiest thing.

Then in the view model I have a property used for binding the selected item on the observable. I added to that property get method if (_updateObservable) UpdateObservable(); It appears the selected item bound property always fires its getter even if the item added or deleted does not directly affect the selected item. UpdateObservable() adds back or removes any items required from the collection and sets the flag to false.

Answer 3

您可以使用new命令简单地覆盖该特定方法。

Answer 4

If you can manage to handle the event in your code, NotifyCollectionChangedEventArgs.NewItems returns an IList of the new items involved in the change. You could then remove these items from the collection if the methods in the third party dll failed.