I have a class that inherits from ObservableCollection
and adds a few additional methods such as AddRange
and RemoveRange
My base method call is this:
public void AddRange(IEnumerable<T> collection)
{
foreach (var i in collection) Items.Add(i);
OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
}
My problem with this is that I want to access e.NewItems
or e.OldItems
in the CollectionChanged
event to perform an action on whatever item is in the collection, and the NotifyCollectionChangedAction.Reset
action does not pass in these values
void Instances_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
if (e.NewItems != null) // e.NewItems is always null
{
foreach (var item in e.NewItems)
{
if (item is EventInstanceModel)
((EventInstanceModel)item).ParentEvent = this;
}
}
}
So I thought I could just use the NotifyCollectionChangedAction.Add
instead of Reset
, however that throws a Range actions are not supported
exception
public void AddRange(IEnumerable<T> collection)
{
var addedItems = collection.ToList();
foreach (var i in addedItems) Items.Add(i);
OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, addedItems));
}
So my question is, how can I raise a CollectionChanged event, and pass it the new or old item list?
I've been looking into it and apparently the CollectionChanged
method cannot be raised with multiple items.
So I can call
OnCollectionChanged(new NotifyCollectionChangedEventArgs(
NotifyCollectionChangedAction.Add, singleItem));
but I can't call
OnCollectionChanged(new NotifyCollectionChangedEventArgs(
NotifyCollectionChangedAction.Add, listOfItems));
For now what I have done is simply raise the Add event for every item added, but I am still rather unhappy at this since it means I raise the CollectionChanged
event for every item in the AddRange
method instead of only once.
public void AddRange(IEnumerable<T> collection)
{
foreach (var i in collection)
{
Items.Add(i);
OnCollectionChanged(new NotifyCollectionChangedEventArgs(
NotifyCollectionChangedAction.Add, i));
}
}
This works fine for me "stand-alone". Meaning I'm not using an ObservableCollection
for data binding. So it's not an ObservableCollection
issue but rather a ListCollectionView
limitation.
Please read the following article, it's a very interesting read:
Your post says: My problem with this is that I want to access e.NewItems or e.OldItems in the CollectionChanged event to perform an action on whatever item is in the collection, and the NotifyCollectionChangedAction.Reset action does not pass in these values .
We're not "stuck" with that behavior, however, because (as many things OOP) if you don't like how something works, you can inherit it to make a custom event that behaves the way you want it to. Example : This custom class inherits from NotifyCollectionChangedEventArgs
and I've changed its behavior so that NotifyCollectionChangedAction.Reset
does pass in the changed values. (In this case I made a new list named RemovedItems
to avoid confusion but one is free to modify existing collections like NewItems
or OldItems
as well.) I use it in production code and it works well for me.
public class NotifyCollectionResetEventArgs : NotifyCollectionChangedEventArgs
{
public NotifyCollectionResetEventArgs(List<object> removedItems)
: base(action: NotifyCollectionChangedAction.Reset)
{
RemovedItems = removedItems.ToList();
}
public List<object> RemovedItems { get; }
}
The class that inherits ObservableCollection overrides the Clear method:
public class ObservableAttributeValueCollection : ObservableCollection<object>
{
List<object> _removedItems = null;
public new void Clear()
{
_removedItems = Items.ToList<object>(); // 1. Capture the items.
base.Clear(); // 2. Clear the items
_removedItems = null; // 3. Clean up
}
protected override void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
{
if(e.Action == NotifyCollectionChangedAction.Reset)
{
base.OnCollectionChanged(new NotifyCollectionResetEventArgs(_removedItems));
}
else
{
base.OnCollectionChanged(e);
}
}
}
Finally, consuming this event in a handler uses the 'is' pattern to assign the ePlus
variable.
void handleCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
switch (e.Action)
{
case NotifyCollectionChangedAction.Reset:
if (e is NotifyCollectionResetEventArgs ePlus)
{
foreach (var item in ePlus.RemovedItems)
{
// Do Something
}
}
break;
}
}
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