<PackageReference Include="System.Reactive" Version="6.0.0-preview.13" />

IEventSource<T>

public interface IEventSource<T>
Represents a data stream signaling its elements by means of an event.
using System.Runtime.CompilerServices; namespace System.Reactive { [System.Runtime.CompilerServices.NullableContext(1)] public interface IEventSource<[System.Runtime.CompilerServices.Nullable(2)] out T> { event Action<T> OnNext; } }