Implementing CollectionChanged

Question

I have added CollectionChanged eventhandler(onCollectionChanged) to one of the ObservableCollection property.

I have found out that onCollectionChanged method gets invoked only in case of add items or remove items to the collection, but not in the case of collection item gets edited.

I would like to know how to send the list/collection of newly added, removed and edited items in a single collection.

Thanks.

Solution 1

You have to add a PropertyChanged listener to each item (which must implement INotifyPropertyChanged) to get notification about editing objects in a observable list.

public ObservableCollection<Item> Names { get; set; }
public List<Item> ModifiedItems { get; set; }

public ViewModel()
{
   this.ModifiedItems = new List<Item>();

   this.Names = new ObservableCollection<Item>();
   this.Names.CollectionChanged += this.OnCollectionChanged;
}

void OnCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
    if (e.NewItems != null)
    {
        foreach(Item newItem in e.NewItems)
        {
            ModifiedItems.Add(newItem);

            //Add listener for each item on PropertyChanged event
            newItem.PropertyChanged += this.OnItemPropertyChanged;         
        }
    }

    if (e.OldItems != null)
    {
        foreach(Item oldItem in e.OldItems)
        {
            ModifiedItems.Add(oldItem);

            oldItem.PropertyChanged -= this.OnItemPropertyChanged;
        }
    }
}

void OnItemPropertyChanged(object sender, PropertyChangedEventArgs e)
{
    Item item = sender as Item;
    if(item != null)
       ModifiedItems.Add(item);
}

Maybe you have to check if some item is already in the ModifedItems-List (with List's method Contains(object obj)) and only add a new item if the result of that method is false.

The class Item must implement INotifyPropertyChanged. See this example to know how. As Robert Rossney said you can also make that with IEditableObject - if you have that requirement.

Solution 2

An ItemsControl listens to CollectionChanged to manage the display of the collection of items it's presenting on the screen. A ContentControl listens to PropertyChanged to manage the display of the specific item that it's presenting on the screen. It's pretty easy to keep the two concepts separate in your mind once you understand this.

Tracking whether or not an item is edited isn't something either of these interfaces does. Property changes aren't edits - that is, they don't necessarily represent some kind of user-initiated change to the state of the object. For instance, an object might have an ElapsedTime property that's being continuously updated by a timer; the UI needs to be notified of these property-change events, but they certainly don't represent changes in the object's underlying data.

The standard way to track whether or not an object is edited is to first make that object implement IEditableObject. You can then, internally to the object's class, decide what changes constitute an edit (i.e. require you to call BeginEdit) and what changes don't. You can then implement a boolean IsDirty property that gets set when BeginEdit is called and cleared when EndEdit or CancelEdit is called. (I really don't understand why that property isn't part of IEditableObject; I haven't yet implemented an editable object that didn't require it.)

Of course, there's no need to implement that second level of abstraction if you don't need it - you can certainly listen PropertyChanged event and just assume that the object has been edited if it gets raised. It really depends on your requirements.

Solution 3

My edit to 'this answer' is rejected! So I put my edit here:

void OnCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
if (e.NewItems != null)
{
    foreach(Item newItem in e.NewItems)
    {
        ModifiedItems.Add(newItem);

        //Add listener for each item on PropertyChanged event
        if (e.Action == NotifyCollectionChangedAction.Add)
            newItem.PropertyChanged += this.ListTagInfo_PropertyChanged;
        else if (e.Action == NotifyCollectionChangedAction.Remove)
            newItem.PropertyChanged -= this.ListTagInfo_PropertyChanged;
    }
}

// MSDN: OldItems:Gets the list of items affected by a Replace, Remove, or Move action.  
//if (e.OldItems != null) <--- removed
}

Solution 4

INotifyCollectionChanged is not one in the same with INotiftyPropertyChanged. Changing properties of underlying objects does not in any way suggest the collection has changed.

One way to achieve this behavior is to create a custom collection which will interrogate the object once added and register for the INotifyPropertyChanged.PropertyChanged event; it would then need to de-register appropriately when an item is removed.

One caveat with this approach is when your objects are nested N levels deep. To solve this you will need to essentially interrogate each property using reflection to determine if it is perhaps yet another collection implementing INotifyCollectionChanged or other container which will need to be traversed.

Here is a rudimentary un-tested example...

    public class ObservableCollectionExt<T> : ObservableCollection<T>
    {
        public override event System.Collections.Specialized.NotifyCollectionChangedEventHandler CollectionChanged;

        protected override void SetItem(int index, T item)
        {
            base.SetItem(index, item);

            if(item is INotifyPropertyChanged)
                (item as INotifyPropertyChanged).PropertyChanged += new PropertyChangedEventHandler(OnPropertyChanged);
        }

        protected override void ClearItems()
        {
            for (int i = 0; i < this.Items.Count; i++)
                DeRegisterINotifyPropertyChanged(this.IndexOf(this.Items[i]));

            base.ClearItems();
        }

        protected override void InsertItem(int index, T item)
        {
            base.InsertItem(index, item);
            RegisterINotifyPropertyChanged(item);
        }

        protected override void RemoveItem(int index)
        {
            base.RemoveItem(index);
            DeRegisterINotifyPropertyChanged(index);
        }

        private void RegisterINotifyPropertyChanged(T item)
        {
            if (item is INotifyPropertyChanged)
                (item as INotifyPropertyChanged).PropertyChanged += new PropertyChangedEventHandler(OnPropertyChanged);
        }

        private void DeRegisterINotifyPropertyChanged(int index)
        {
            if (this.Items[index] is INotifyPropertyChanged)
                (this.Items[index] as INotifyPropertyChanged).PropertyChanged -= OnPropertyChanged;
        }

        private void OnPropertyChanged(object sender, PropertyChangedEventArgs e)
        {
            T item = (T)sender;
            OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset, item)); 
        }
    }

Solution 5

I think that populating the ObservableCollection with items that implement INotifyPropertyChanged will cause the CollectionChanged event to fire when an item raises its PropertyChanged notification.

On second thought, I think you need to use BindingList<T> to get individual item changes to propagate in this way out-of-the-box.

Otherwise, you'll need to manually subscribe to each item's change notifications and raise the CollectionChanged event. Note that if you're creating your own, derived ObservableCollection<T>, you'll have to subscribe at instantiation and on Add() and Insert(), and unsubscribe on Remove(), RemoveAt() and Clear(). Otherwise, you can subscribe to the CollectionChanged event and use the added and removed items from the event args to subscribe/unsubscribe.

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Comments

[Aaron McIver]

That is correct in that you will NOT get change notifications when the items within the container change.

[Alain]

This will throw an exception on both Add and Remove operations unless you modify the code to check whether e.NewItems and e.OldItems is null before trying to foreach over them.

[v.g.]

Why do you need this ModifiedItems list, when you have Names collection?

[Riva]

1up for mentioning BindingList. It actually works well with WPF and is IMHO in many cases better then ObservableCollection.

[Riva]

Afaik BindingList actually works pretty well with WPF.

[tjmoore]

This will fail. Constructor of NotifyCollectionChangedEventArgs throws the following exception when you add an item as a parameter: System.ArgumentException: 'Reset action must be initialized with no changed items.'