How can I raise a CollectionChanged event on an ObservableCollection, and pass it the changed items?
Solution 1
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));
}
}
Solution 2
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:
Solution 3
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;
}
}
Related videos on Youtube
Rachel
"The three chief virtues of a programmer are: Laziness, Impatience and Hubris." - Larry Wall Laziness: I'm too lazy to do the same task repeatedly so write scripts to do that task for me. This makes people think I am intelligent. Impatience: I'm too impatient to wait for my code to run so rewrite the code to improve performance. This makes people think I am a good programmer. Hubris: When someone asks if I can do something I just say Yes, then go find out how to do it (Google!). This makes people think I can do anything. Ultimately, it means I can make a career out of being Lazy, Impatient, and Hubristic.
Updated on February 23, 2022Comments
-
Rachel about 2 years
I have a class that inherits from
ObservableCollection
and adds a few additional methods such asAddRange
andRemoveRange
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
ore.OldItems
in theCollectionChanged
event to perform an action on whatever item is in the collection, and theNotifyCollectionChangedAction.Reset
action does not pass in these valuesvoid 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 ofReset
, however that throws aRange actions are not supported
exceptionpublic 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?
-
Rachel over 12 yearsI did see that link, however he also points out at the bottom in the
Not Quite Done
section that his version ofAddRange
doesn't work if more than one item gets added due to the limitations onListCollectionView
. You're right that's it's a limitation ofListCollectionView
and notObservableCollection
though -
Rachel over 12 years@Rick I checked into something like that, but it doesn't pass the
e.NewItems
/e.OldItems
into theCollectionChanged
event -
Wobbles about 9 yearsWhy not just put the event call outside the foreach loop right after it, then it would only be called once.
-
Rachel about 9 yearsThis question was about how to raise a
CollectionChanged
event with the parameter of every item that had been added. The answer is that we have to raise the event individually per item, since it does not support being raised with a collection of items.