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](https://github.com/Cysharp/R3).
.NET has [`ObservableCollection<T>`](https://docs.microsoft.com/en-us/dotnet/api/system.collections.objectmodel.observablecollection-1), 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 generics version of `NotifyCollectionChangedEventHandler` and `NotifyCollectionChangedEventArgs`, it using latest C# features(`in`, `readonly ref struct`, `ReadOnlySpan<T>`).
```csharp
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;
}
```
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.
```csharp
public interface IObservableCollection<T> : IReadOnlyCollection<T>
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.
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](https://github.com/Cysharp/R3):
Handling all `CollectionChanged` event manually is hard. We recommend to use `SynchronizedView` that transform element and handling all collection changed event for view synchronize.
The basic idea behind using ObservableCollections is to create a View. In order to automate this pipeline, the view can be sortable, filtered, and have side effects on the values when they are changed.
Once the R3 extension package is installed, you can subscribe to `ObserveAdd`, `ObserveRemove`, `ObserveReplace`, `ObserveMove`, and `ObserveReset` events as Rx, allowing you to compose events individually.
Since Blazor re-renders the whole thing by StateHasChanged, you may think that Observable collections are unnecessary. However, when you split it into Components, it is beneficial for Component confidence to detect the change and change its own State.
The View selector in ObservableCollections is also useful for converting data to a View that represents a Cell, for example, when creating something like a 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. `BindingOperations.EnableCollectionSynchronization` to work safely with change notifications from different threads.
In Unity projects, you can installing `ObservableCollections` with [NugetForUnity](https://github.com/GlitchEnzo/NuGetForUnity). If R3 integration is required, similarly install `ObservableCollections.R3` via NuGetForUnity.
It is also possible to manage Order by managing indexes inserted from eventArgs, but it is very difficult with many caveats. If you don't have major performance issues, you can foreach the View itself on CollectionStateChanged (like Blazor) and reorder the transforms. If you have such a architecture, you can also use SortedView.
> Notice: foreach ObservableCollections and Views are thread-safe but it uses lock at iterating. In other words, the obtained Enumerator must be Dispose. foreach and LINQ are guaranteed to be Dispose, but be careful when you extract the Enumerator by yourself.
public sealed partial class ObservableDictionary<TKey,TValue> : IDictionary<TKey,TValue>, IReadOnlyDictionary<TKey,TValue>, IObservableCollection<KeyValuePair<TKey,TValue>> where TKey : notnull
public sealed partial class ObservableFixedSizeRingBuffer<T> : IList<T>, IReadOnlyList<T>, IObservableCollection<T>
public sealed partial class ObservableHashSet<T> : IReadOnlySet<T>, IReadOnlyCollection<T>, IObservableCollection<T> where T : notnull
public sealed partial class ObservableHashSet<T> : IReadOnlySet<T>, IReadOnlyCollection<T>, IObservableCollection<T>
where T : notnull
public sealed partial class ObservableList<T> : IList<T>, IReadOnlyList<T>, IObservableCollection<T>
public sealed partial class ObservableQueue<T> : IReadOnlyCollection<T>, IObservableCollection<T>
public sealed partial class ObservableRingBuffer<T> : IList<T>, IReadOnlyList<T>, IObservableCollection<T>
public sealed partial class ObservableStack<T> : IReadOnlyCollection<T>, IObservableCollection<T>
public sealed class RingBuffer<T> : IList<T>, IReadOnlyList<T>
public sealed class FreezedList<T> : IReadOnlyList<T>, IFreezedCollection<T>
public sealed class FreezedDictionary<TKey,TValue> : IReadOnlyDictionary<TKey,TValue>, IFreezedCollection<KeyValuePair<TKey,TValue>> where TKey : notnull
