[API Proposal]: NotifyCollectionChangedAction for item updated / item property changed

[paulober]

Background and motivation

I currently have the problem of the ObservableCollection not issuing an CollectionChanged event when a property of an element changes. To solve this i implemented a custom "AdvancedObservableCollection" extending the ObservableCollection and only taking INotifyPropertyChanged classes. No every time a property of an element changes i trigger an OnCollectionChanged with NotifyCollectionChangedEventArgs.
The reason i've done all this, is to make a GridView control in WinUI 3 recognizing my changes to an element in the ItemsSource (the ObservableCollection).
Now all currently existent options like NotifyCollectionChangedAction.Reset or NotifyCollectionChangedAction.Replace causes to much updates in the client side and are not suitalbe for this type of usecase.

So i would like to suggest a new NotifyCollectionChangedAction like NotifyCollectionChangedAction.Update for the usecase of informing for Element/Item updates inside a collection that don't affect the structure of the collection only data inside one or more elements.

(Sorry for my english i'm not a native speaker.)

API Proposal

new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Updated, item));

API Usage

public class AdvancedObservableCollection<T>: ObservableCollection<T> where T : INotifyPropertyChanged
{
    public AdvancedObservableCollection()
    {
        CollectionChanged += CollectionChanged_Handler;
    }

    private void CollectionChanged_Handler(object? sender, NotifyCollectionChangedEventArgs e)
    {
        // unsubscribe all old objects
        if (e.OldItems != null)
        {
            foreach (T element in e.OldItems)
            {
                element.PropertyChanged -= Item_Changed;
            }
        }

        // subscribe all new objects
        if (e.NewItems != null)
        {
            foreach (T element in e.NewItems)
            {
                element.PropertyChanged += Item_Changed;
            }
        }
    }

    private void Item_Changed(object? sender, PropertyChangedEventArgs e)
    {
        if (sender is T item)
        {
            OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Updated, item));
        }
    }
}

Alternative Designs

No response

Risks

No response

Tagging subscribers to this area: @dotnet/area-system-collections
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Issue Details

---

Background and motivation

I currently have the problem of the ObservableCollection not issuing an CollectionChanged event when a property of an element changes. To solve this i implemented a custom "AdvancedObservableCollection" extending the ObservableCollection and only taking INotifyPropertyChanged classes. No every time a property of an element changes i trigger an OnCollectionChanged with NotifyCollectionChangedEventArgs.
The reason i've done all this, is to make a GridView control in WinUI 3 recognizing my changes to an element in the ItemsSource (the ObservableCollection).
Now all currently existent options like NotifyCollectionChangedAction.Reset or NotifyCollectionChangedAction.Replace causes to much updates in the client side and are not suitalbe for this type of usecase.

So i would like to suggest a new NotifyCollectionChangedAction like NotifyCollectionChangedAction.Update for the usecase of informing for Element/Item updates inside a collection that don't affect the structure of the collection only data inside one or more elements.

(Sorry for my english i'm not a native speaker.)

API Proposal

new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Updated, item));

API Usage

public class AdvancedObservableCollection<T>: ObservableCollection<T> where T : INotifyPropertyChanged
{
    public AdvancedObservableCollection()
    {
        CollectionChanged += CollectionChanged_Handler;
    }

    private void CollectionChanged_Handler(object? sender, NotifyCollectionChangedEventArgs e)
    {
        // unsubscribe all old objects
        if (e.OldItems != null)
        {
            foreach (T element in e.OldItems)
            {
                element.PropertyChanged -= Item_Changed;
            }
        }

        // subscribe all new objects
        if (e.NewItems != null)
        {
            foreach (T element in e.NewItems)
            {
                element.PropertyChanged += Item_Changed;
            }
        }
    }

    private void Item_Changed(object? sender, PropertyChangedEventArgs e)
    {
        if (sender is T item)
        {
            OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Updated, item));
        }
    }
}

Alternative Designs

No response

Risks

No response

Author:	paulober
Assignees:	-
Labels:	api-suggestion, area-System.Collections
Milestone:	-

[Symbai]

ObservableCollection not issuing an CollectionChanged event when a property of an element change

This sounds pretty fine to me. Because when a property of an element changes, the collection itself still hasn't changed. It only changes if you remove, add or move an element. To get notified of property changes you have to subscribe to each element itself, this is by design. Also changing the behavior will be a breaking change, because not every existing code checks the value of NotifyCollectionChangedAction.

[eiriktsarpalis]

Adding a new action type would most likely break observers that haven't been updated to handle it. We have been blocked in the past from updating observable collection (for example #18087, which doesn't even attempt to change the schema of NotifyCollectionChangedEventArgs)

[jbe2277]

My library System.Waf.Core provides an implemention of the INotifyCollectionItemChanged interface. The interface defines a new event CollectionItemChanged for this. It is implemented by the ObservableList<T> class. This reference implementation might be helping to create an API proposal.

Extract of INotifyCollectionItemChanged.cs

/// <summary>Notifies listeners that a property of a collection item changed.</summary>
public interface INotifyCollectionItemChanged
{
    /// <summary>Occurs when a collection item property changed. The sender argument of this event is the item and not the collection.</summary>
    event PropertyChangedEventHandler? CollectionItemChanged;
}

Extract of ObservableList.cs

/// <summary>Represents a dynamic data collection that provides notifications when items get added or removed, or when the whole list is refreshed.
/// It extends the <see cref="ObservableCollection{T}"/> by implementing the <see cref="INotifyCollectionChanging"/> and the <see cref="INotifyCollectionItemChanged"/> interface.</summary>
/// <typeparam name="T">The type of elements in the collection.</typeparam>
public class ObservableList<T> : ObservableCollection<T>, IReadOnlyObservableList<T>
{
    ...