ObservableCollections is a high performance observable collections(ObservableList<T>, ObservableDictionary<TKey, TValue>, ObservableHashSet<T>, ObservableQueue<T>, ObservableStack<T>, ObservableRingBuffer<T>, ObservableFixedSizeRingBuffer<T>) with synchronized views and Observe Extension for R3.
.NET has ObservableCollection<T>, however it has many lacks of features. It based INotifyCollectionChanged, NotifyCollectionChangedEventHandler and NotifyCollectionChangedEventArgs. There are no generics so everything boxed, allocate memory every time. Also NotifyCollectionChangedEventArgs holds all values to IList even if it is single value, this also causes allocations. ObservableCollection<T> has no Range feature so a lot of wastage occurs when adding multiple values, because it is a single value notification. Also, it is not thread-safe is hard to do linkage with the notifier.
ObservableCollections introduces there generics version, NotifyCollectionChangedEventHandler<T> and NotifyCollectionChangedEventArgs<T>, it using latest C# features(in, readonly ref struct, ReadOnlySpan<T>). Also, Sort and Reverse will now be notified.
public delegate void NotifyCollectionChangedEventHandler<T>(in NotifyCollectionChangedEventArgs<T> e);
public readonly ref struct NotifyCollectionChangedEventArgs<T>
{
public readonly NotifyCollectionChangedAction Action;
public readonly bool IsSingleItem;
public readonly T NewItem;
public readonly T OldItem;
public readonly ReadOnlySpan<T> NewItems;
public readonly ReadOnlySpan<T> OldItems;
public readonly int NewStartingIndex;
public readonly int OldStartingIndex;
public readonly SortOperation<T> SortOperation;
}Also, use the interface IObservableCollection<T> instead of INotifyCollectionChanged. This is guaranteed to be thread-safe and can produce a View that is fully synchronized with the collection.
public interface IObservableCollection<T> : IReadOnlyCollection<T>
{
event NotifyCollectionChangedEventHandler<T>? CollectionChanged;
object SyncRoot { get; }
ISynchronizedView<T, TView> CreateView<TView>(Func<T, TView> transform);
}SynchronizedView helps to separate between Model and View (ViewModel). We will use ObservableCollections as the Model and generate SynchronizedView as the View (ViewModel). This architecture can be applied not only to WPF, but also to Blazor, Unity, etc.
The View retains the transformed values. The transform function is called only once during Add, so costly objects that are linked can also be instantiated. Additionally, it has a feature to dynamically show or hide values using filters.
Observable Collections themselves do not implement INotifyCollectionChanged, so they cannot be bound on XAML platforms and the like. However, they can be converted to collections that implement INotifyCollectionChanged using ToNotifyCollectionChanged(), making them suitable for binding.
ObservableCollections has not just a simple list, there are many more data structures. ObservableList<T>, ObservableDictionary<TKey, TValue>, ObservableHashSet<T>, ObservableQueue<T>, ObservableStack<T>, ObservableRingBuffer<T>, ObservableFixedSizeRingBuffer<T>. RingBuffer, especially FixedSizeRingBuffer, can be achieved with efficient performance when there is rotation (e.g., displaying up to 1000 logs, where old ones are deleted when new ones are added). Of course, the AddRange allows for efficient batch processing of large numbers of additions.
If you want to handle each change event with Rx, you can monitor it with the following method by combining it with R3:
Observable<CollectionChangedEvent<T>> IObservableCollection<T>.ObserveChanged()
Observable<CollectionAddEvent<T>> IObservableCollection<T>.ObserveAdd()
Observable<CollectionRemoveEvent<T>> IObservableCollection<T>.ObserveRemove()
Observable<CollectionReplaceEvent<T>> IObservableCollection<T>.ObserveReplace()
Observable<CollectionMoveEvent<T>> IObservableCollection<T>.ObserveMove()
Observable<CollectionResetEvent<T>> IObservableCollection<T>.ObserveReset()
Observable<Unit> IObservableCollection<T>.ObserveClear<T>()
Observable<(int Index, int Count)> IObservableCollection<T>.ObserveReverse<T>()
Observable<(int Index, int Count, IComparer<T>? Comparer)> IObservableCollection<T>.ObserveSort<T>()
Observable<int> IObservableCollection<T>.ObserveCountChanged<T>()For .NET, use NuGet. For Unity, please read Unity section.
dotnet add package ObservableCollections
create new ObservableList<T>, ObservableDictionary<TKey, TValue>, ObservableHashSet<T>, ObservableQueue<T>, ObservableStack<T>, ObservableRingBuffer<T>, ObservableFixedSizeRingBuffer<T>.
// Basic sample, use like ObservableCollection<T>.
// CollectionChanged observes all collection modification
var list = new ObservableList<int>();
list.CollectionChanged += List_CollectionChanged;
list.Add(10);
list.Add(20);
list.AddRange(new[] { 10, 20, 30 });
static void List_CollectionChanged(in NotifyCollectionChangedEventArgs<int> e)
{
switch (e.Action)
{
case NotifyCollectionChangedAction.Add:
if (e.IsSingleItem)
{
Console.WriteLine(e.NewItem);
}
else
{
foreach (var item in e.NewItems)
{
Console.WriteLine(item);
}
}
break;
// Remove, Replace, Move, Reset
default:
break;
}
}While it is possible to manually handle the CollectionChanged event as shown in the example above, you can also create a SynchronizedView as a collection that holds a separate synchronized value.
var list = new ObservableList<int>();
var view = list.CreateView(x => x.ToString() + "$");
list.Add(10);
list.Add(20);
list.AddRange(new[] { 30, 40, 50 });
list[1] = 60;
list.RemoveAt(3);
foreach (var v in view)
{
// 10$, 60$, 30$, 50$
Console.WriteLine(v);
}
// Dispose view is unsubscribe collection changed event.
view.Dispose();The view can modify the objects being enumerated by attaching a Filter.
var list = new ObservableList<int>();
using var view = list.CreateView(x => x.ToString() + "$");
list.Add(1);
list.Add(20);
list.AddRange(new[] { 30, 31, 32 });
// attach filter
view.AttachFilter(x => x % 2 == 0);
foreach (var v in view)
{
// 20$, 30$, 32$
Console.WriteLine(v);
}
// attach other filter(removed previous filter)
view.AttachFilter(x => x % 2 == 1);
foreach (var v in view)
{
// 1$, 31$
Console.WriteLine(v);
}
// Count shows filtered length
Console.WriteLine(view.Count); // 2The View only allows iteration and Count; it cannot be accessed via an indexer. If indexer access is required, you need to convert it using ToViewList(). Additionally, ToNotifyCollectionChanged() converts it to a synchronized view that implements INotifyCollectionChanged, which is necessary for XAML binding, in addition to providing indexer access.
// Queue <-> List Synchronization
var queue = new ObservableQueue<int>();
queue.Enqueue(1);
queue.Enqueue(10);
queue.Enqueue(100);
queue.Enqueue(1000);
queue.Enqueue(10000);
using var view = queue.CreateView(x => x.ToString() + "$");
using var viewList = view.ToViewList();
Console.WriteLine(viewList[2]); // 100$In the case of ObservableList, calls to Sort and Reverse can also be synchronized with the view.
var list = new ObservableList<int> { 1, 301, 20, 50001, 4000 };
using var view = list.CreateView(x => x.ToString() + "$");
view.AttachFilter(x => x % 2 == 0);
foreach (var v in view)
{
// 20$, 4000$
Console.WriteLine(v);
}
// Reverse operations on the list will affect the view
list.Reverse();
foreach (var v in view)
{
// 4000$, 20$
Console.WriteLine(v);
}
// remove filter
view.ResetFilter();
// The reverse operation is also reflected in the values hidden by the filter
foreach (var v in view)
{
// 4000$, 50001$, 20$, 301$, 1$
Console.WriteLine(v);
}
// also affect Sort Operations
list.Sort();
foreach (var v in view)
{
// 1$, 20$, 301$, 4000$, 50001$
Console.WriteLine(v);
}
// you can use custom comparer
list.Sort(new DescendantComaprer());
foreach (var v in view)
{
// 50001$, 4000$, 301$, 20$, 1$
Console.WriteLine(v);
}
class DescendantComaprer : IComparer<int>
{
public int Compare(int x, int y)
{
return y.CompareTo(x);
}
}Once the R3 extension package is installed, you can subscribe to ObserveChanged, ObserveAdd, ObserveRemove, ObserveReplace, ObserveMove, ObserveReset, ObserveClear, ObserveReverse, ObserveSort, ObserveCounteChanged events as Rx, allowing you to compose events individually.
dotnet add package ObservableCollections.R3
using R3;
using ObservableCollections;
var list = new ObservableList<int>();
list.ObserveAdd()
.Subscribe(x =>
{
Console.WriteLine(x);
});
list.Add(10);
list.Add(20);
list.AddRange(new[] { 10, 20, 30 });Note that ObserveReset is used to subscribe to Clear, Reverse, and Sort operations in bulk.
In addition to IObservableCollection<T>, there is also a subscription event for ISynchronizedView<T, TView>. In the case of View, ObserveRejected is also added.
Since it is not supported by dotnet/reactive, please use the Rx library R3.
In the case of Blazor, StateHasChanged is called and re-enumeration occurs in response to changes in the collection. It's advisable to use the CollectionStateChanged event for this purpose.
public partial class Index : IDisposable
{
ObservableList<int> list;
public ISynchronizedView<int, int> ItemsView { get; set; }
int count = 0;
protected override void OnInitialized()
{
list = new ObservableList<int>();
ItemsView = list.CreateView(x => x);
ItemsView.CollectionStateChanged += action =>
{
InvokeAsync(StateHasChanged);
};
}
void OnClick()
{
list.Add(count++);
}
public void Dispose()
{
ItemsView.Dispose();
}
}
// .razor, iterate view
@page "/"
<button @onclick=OnClick>button</button>
<table>
@foreach (var item in ItemsView)
{
<tr>
<td>@item</td>
</tr>
}
</table>Because of data binding in WPF, it is important that the collection is Observable. ObservableCollections high-performance IObservableCollection<T> cannot be bind to WPF. Call ToNotifyCollectionChanged() to convert it to INotifyCollectionChanged. Also, although ObservableCollections and Views are thread-safe, the WPF UI does not support change notifications from different threads. ToToNotifyCollectionChanged(IColllectionEventDispatcher) allows multi thread changed.
// WPF simple sample.
ObservableList<int> list;
public NotifyCollectionChangedSynchronizedViewList<int> ItemsView { get; set; }
public MainWindow()
{
InitializeComponent();
this.DataContext = this;
list = new ObservableList<int>();
// for ui synchronization safety of viewmodel
ItemsView = list.ToNotifyCollectionChanged(SynchronizationContextCollectionEventDispatcher.Current);
// if collection is changed only from ui-thread, can use this overload
// ItemsView = list.ToNotifyCollectionChanged();
}
protected override void OnClosed(EventArgs e)
{
ItemsView.Dispose();
}SynchronizationContextCollectionEventDispatcher.Current is default implementation of IColllectionEventDispatcher, it is used SynchronizationContext.Current for dispatche ui thread. You can create custom ICollectionEventDispatcher to use custom dispatcher object. For example use WPF Dispatcher:
public class WpfDispatcherCollection(Dispatcher dispatcher) : ICollectionEventDispatcher
{
public void Post(CollectionEventDispatcherEventArgs ev)
{
dispatcher.InvokeAsync(() =>
{
// notify in dispatcher
ev.Invoke();
});
}
}ToNotifyCollectionChanged() can also be called without going through a View. In this case, it's guaranteed that no filters will be applied, making it faster. If you want to apply filters, please generate a View before calling it. Additionally, ObservableList has a variation called ToNotifyCollectionChangedSlim(). This option doesn't generate a list for the View and shares the actual data, making it the fastest and most memory-efficient option. However, range operations such as AddRange, InsertRange and RemoveRange are not supported by WPF (or Avalonia), so they will throw runtime exceptions.
Views and ToNotifyCollectionChanged are internally connected by events, so they need to be Dispose to release those connections.
Standard Views are readonly. If you want to reflect the results of binding back to the original collection, use CreateWritableView to generate an IWritableSynchronizedView, and then use ToWritableNotifyCollectionChanged to create an INotifyCollectionChanged collection from it.
public delegate T WritableViewChangedEventHandler<T, TView>(TView newView, T originalValue, ref bool setValue);
public interface IWritableSynchronizedView<T, TView> : ISynchronizedView<T, TView>
{
NotifyCollectionChangedSynchronizedViewList<TView> ToWritableNotifyCollectionChanged(WritableViewChangedEventHandler<T, TView> converter);
NotifyCollectionChangedSynchronizedViewList<TView> ToWritableNotifyCollectionChanged(WritableViewChangedEventHandler<T, TView> converter, ICollectionEventDispatcher? collectionEventDispatcher);
}ToWritableNotifyCollectionChanged accepts a delegate called WritableViewChangedEventHandler. newView receives the newly bound value. If setValue is true, it sets a new value to the original collection, triggering notification propagation. The View is also regenerated. If T originalValue is a reference type, you can prevent such propagation by setting setValue to false.
var list = new ObservableList<Person>()
{
new (){ Age = 10, Name = "John" },
new (){ Age = 22, Name = "Jeyne" },
new (){ Age = 30, Name = "Mike" },
};
var view = list.CreateWritableView(x => x.Name);
view.AttachFilter(x => x.Age >= 20);
IList<string?> bindable = view.ToWritableNotifyCollectionChanged((string? newView, Person original, ref bool setValue) =>
{
if (setValue)
{
// default setValue == true is Set operation
original.Name = newView;
// You can modify setValue to false, it does not set original collection to new value.
// For mutable reference types, when there is only a single,
// bound View and to avoid recreating the View, setting false is effective.
// Otherwise, keeping it true will set the value in the original collection as well,
// and change notifications will be sent to lower-level Views(the delegate for View generation will also be called anew).
setValue = false;
return original;
}
else
{
// default setValue == false is Add operation
return new Person { Age = null, Name = newView };
}
});
bindable[1] = "Bob"; // change Mike(filtered view's [1]) to Bob.
bindable.Add("Ken");
// Show Views
foreach (var item in view)
{
Console.WriteLine(item);
}
Console.WriteLine("---");
// Show Originals
foreach (var item in list)
{
Console.WriteLine((item.Age, item.Name));
}
public class Person
{
public int? Age { get; set; }
public string? Name { get; set; }
}In Unity projects, you can installing ObservableCollections with NugetForUnity. If R3 integration is required, similarly install ObservableCollections.R3 via NuGetForUnity.
In Unity, ObservableCollections and Views are useful as CollectionManagers, since they need to convert T to Prefab for display. Since View objects are generated only once, it's possible to complement GameObjects tied to the collection.
public class SampleScript : MonoBehaviour
{
public Button prefab;
public GameObject root;
ObservableRingBuffer<int> collection;
ISynchronizedView<int, GameObject> view;
void Start()
{
collection = new ObservableRingBuffer<int>();
view = collection.CreateView(x =>
{
var item = GameObject.Instantiate(prefab);
item.GetComponentInChildren<Text>().text = x.ToString();
// add to root
item.transform.SetParent(root.transform);
return item.gameObject;
});
view.ViewChanged += View_ViewChanged;
}
void View_ViewChanged(in SynchronizedViewChangedEventArgs<int, GameObject> eventArgs)
{
// hook remove event
if (eventArgs.Action == NotifyCollectionChangedAction.Remove)
{
GameObject.Destroy(eventArgs.OldItem.View);
}
// hook for Filter attached, clear, etc...
// if (NotifyCollectionChangedAction.Reset) { }
}
void OnDestroy()
{
view.Dispose();
}
}ObservableCollections provides these collections.
class ObservableList<T> : IList<T>, IReadOnlyList<T>, IObservableCollection<T>, IReadOnlyObservableList<T>
class ObservableDictionary<TKey, TValue> : IDictionary<TKey, TValue>, IReadOnlyDictionary<TKey, TValue>, IObservableCollection<KeyValuePair<TKey, TValue>>, IReadOnlyObservableDictionary<TKey, TValue> where TKey : notnull
class ObservableHashSet<T> : IReadOnlySet<T>, IReadOnlyCollection<T>, IObservableCollection<T> where T : notnull
class ObservableQueue<T> : IReadOnlyCollection<T>, IObservableCollection<T>
class ObservableStack<T> : IReadOnlyCollection<T>, IObservableCollection<T>
class ObservableRingBuffer<T> : IList<T>, IReadOnlyList<T>, IObservableCollection<T>
class RingBuffer<T> : IList<T>, IReadOnlyList<T>
class ObservableFixedSizeRingBuffer<T> : IList<T>, IReadOnlyList<T>, IObservableCollection<T>
class AlternateIndexList<T> : IEnumerable<T>The IObservableCollection<T> is the base interface for all, containing the CollectionChanged event and the CreateView method.
public delegate void NotifyCollectionChangedEventHandler<T>(in NotifyCollectionChangedEventArgs<T> e);
public interface IObservableCollection<T> : IReadOnlyCollection<T>
{
object SyncRoot { get; }
event NotifyCollectionChangedEventHandler<T>? CollectionChanged;
ISynchronizedView<T, TView> CreateView<TView>(Func<T, TView> transform);
}The notification event NotifyCollectionChangedEventArgs<T> has the following definition:
/// <summary>
/// Contract:
/// IsSingleItem ? (NewItem, OldItem) : (NewItems, OldItems)
/// Action.Add
/// NewItem, NewItems, NewStartingIndex
/// Action.Remove
/// OldItem, OldItems, OldStartingIndex
/// Action.Replace
/// NewItem, NewItems, OldItem, OldItems, (NewStartingIndex, OldStartingIndex = samevalue)
/// Action.Move
/// NewStartingIndex, OldStartingIndex
/// Action.Reset
/// SortOperation(IsClear, IsReverse, IsSort)
/// </summary>
[StructLayout(LayoutKind.Auto)]
public readonly ref struct NotifyCollectionChangedEventArgs<T>
{
public readonly NotifyCollectionChangedAction Action;
public readonly bool IsSingleItem;
public readonly T NewItem;
public readonly T OldItem;
public readonly ReadOnlySpan<T> NewItems;
public readonly ReadOnlySpan<T> OldItems;
public readonly int NewStartingIndex;
public readonly int OldStartingIndex;
public readonly SortOperation<T> SortOperation;
}This is the interface for View:
public delegate void NotifyViewChangedEventHandler<T, TView>(in SynchronizedViewChangedEventArgs<T, TView> e);
public enum RejectedViewChangedAction
{
Add, Remove, Move
}
public interface ISynchronizedView<T, TView> : IReadOnlyCollection<TView>, IDisposable
{
object SyncRoot { get; }
ISynchronizedViewFilter<T, TView> Filter { get; }
IEnumerable<(T Value, TView View)> Filtered { get; }
IEnumerable<(T Value, TView View)> Unfiltered { get; }
int UnfilteredCount { get; }
event NotifyViewChangedEventHandler<T, TView>? ViewChanged;
event Action<RejectedViewChangedAction, int, int>? RejectedViewChanged; // int index, int oldIndex(when RejectedViewChangedAction is Move)
event Action<NotifyCollectionChangedAction>? CollectionStateChanged;
void AttachFilter(ISynchronizedViewFilter<T, TView> filter);
void ResetFilter();
ISynchronizedViewList<TView> ToViewList();
NotifyCollectionChangedSynchronizedViewList<TView> ToNotifyCollectionChanged();
NotifyCollectionChangedSynchronizedViewList<TView> ToNotifyCollectionChanged(ICollectionEventDispatcher? collectionEventDispatcher);
}The Count of the View returns the filtered value, but if you need the unfiltered value, use UnfilteredCount. Also, normal enumeration returns only TView, but if you need T or want to enumerate pre-filtered values, you can get them with Filtered and Unfiltered.
The View's notification event SynchronizedViewChangedEventArgs<T> has the following definition:
public readonly ref struct SynchronizedViewChangedEventArgs<T, TView>
{
public readonly NotifyCollectionChangedAction Action;
public readonly bool IsSingleItem;
public readonly (T Value, TView View) NewItem;
public readonly (T Value, TView View) OldItem;
public readonly ReadOnlySpan<T> NewValues;
public readonly ReadOnlySpan<TView> NewViews;
public readonly ReadOnlySpan<T> OldValues;
public readonly ReadOnlySpan<TView> OldViews;
public readonly int NewStartingIndex;
public readonly int OldStartingIndex;
public readonly SortOperation<T> SortOperation;
}When NotifyCollectionChangedAction is Reset, additional determination can be made with SortOperation<T>.
public readonly struct SortOperation<T>
{
public readonly int Index;
public readonly int Count;
public readonly IComparer<T>? Comparer;
public bool IsReverse { get; }
public bool IsClear { get; }
public bool IsSort { get; }
}When IsReverse is true, you need to use Index and Count. When IsSort is true, you need to use Index, Count, and Comparer values.
For Filter, you can either create one that implements this interface or generate one from a lambda expression using extension methods.
public interface ISynchronizedViewFilter<T, TView>
{
bool IsMatch(T value, TView view);
}
public static class SynchronizedViewExtensions
{
public static void AttachFilter<T, TView>(this ISynchronizedView<T, TView> source, Func<T, bool> filter)
{
}
public static void AttachFilter<T, TView>(this ISynchronizedView<T, TView> source, Func<T, TView, bool> filter)
{
}
}ObservableList<T> has writable view.
public sealed partial class ObservableList<T>
{
public IWritableSynchronizedView<T, TView> CreateWritableView<TView>(Func<T, TView> transform);
public NotifyCollectionChangedSynchronizedViewList<T> ToWritableNotifyCollectionChanged();
public NotifyCollectionChangedSynchronizedViewList<T> ToWritableNotifyCollectionChanged(ICollectionEventDispatcher? collectionEventDispatcher);
public NotifyCollectionChangedSynchronizedViewList<TView> ToWritableNotifyCollectionChanged<TView>(Func<T, TView> transform, WritableViewChangedEventHandler<T, TView>? converter);
public NotifyCollectionChangedSynchronizedViewList<TView> ToWritableNotifyCollectionChanged<TView>(Func<T, TView> transform, ICollectionEventDispatcher? collectionEventDispatcher, WritableViewChangedEventHandler<T, TView>? converter);
}
public delegate T WritableViewChangedEventHandler<T, TView>(TView newView, T originalValue, ref bool setValue);
public interface IWritableSynchronizedView<T, TView> : ISynchronizedView<T, TView>
{
(T Value, TView View) GetAt(int index);
void SetViewAt(int index, TView view);
void SetToSourceCollection(int index, T value);
void AddToSourceCollection(T value);
void InsertIntoSourceCollection(int index, T value);
bool RemoveFromSourceCollection(T value);
void RemoveAtSourceCollection(int index);
void ClearSourceCollection();
IWritableSynchronizedViewList<TView> ToWritableViewList(WritableViewChangedEventHandler<T, TView> converter);
INotifyCollectionChangedSynchronizedViewList<TView> ToWritableNotifyCollectionChanged(WritableViewChangedEventHandler<T, TView> converter);
INotifyCollectionChangedSynchronizedViewList<TView> ToWritableNotifyCollectionChanged(WritableViewChangedEventHandler<T, TView> converter, ICollectionEventDispatcher? collectionEventDispatcher);
}
public interface IWritableSynchronizedViewList<TView> : ISynchronizedViewList<TView>
{
new TView this[int index] { get; set; }
}Here are definitions for other collections:
public interface IReadOnlyObservableList<T> :
IReadOnlyList<T>, IObservableCollection<T>
{
}
public interface IReadOnlyObservableDictionary<TKey, TValue> :
IReadOnlyDictionary<TKey, TValue>, IObservableCollection<KeyValuePair<TKey, TValue>>
{
}
public interface ISynchronizedViewList<out TView> : IReadOnlyList<TView>, IDisposable
{
}
// Obsolete for public use
public interface INotifyCollectionChangedSynchronizedViewList<TView> : IList<TView>, IList, ISynchronizedViewList<TView>, INotifyCollectionChanged, INotifyPropertyChanged
{
}
public abstract class NotifyCollectionChangedSynchronizedViewList<TView> :
INotifyCollectionChangedSynchronizedViewList<TView>,
IWritableSynchronizedViewList<TView>,
IList<TView>,
IList
{
}This library is licensed under the MIT License.