Description
The ReactiveUI project uses Rx's ability to wrap a Task<T> as an IObservable<T>, but there's a capability they have wanted for a long time that Rx currently does not offer: the ability both to cancel the underlying task and to observe its progress after cancellation.
There's no built in way to do this, because the mechanism by which you cancel a Task<T> wrapped in an IObservable<T> is to unsubscribe (i.e. to call Dispose on the IDisposable returned by Subscribe). The problem with this is that unsubscribing tells Rx that you don't want to receive any more notifications.
reactiveui/ReactiveUI#2153 describes a problem in ReactiveUI that can be attributed to this missing Rx capability.
Although this is not insurmountable, as the PR at reactiveui/ReactiveUI#3556 shows, it would would be useful if Rx provided an out-of-the-box solution to this. We could do this by providing something like the FromAsyncWithPostCancelNotifications extension method defined by that PR:
As @ChrisPulman points out at reactiveui/ReactiveUI#3556 (comment) we would need a set of overloads to cover all the variations.
FromAsyncWithPostCancelNotifications might not be the ideal name for this, but we can give that some thought later.