Question

If I have stream with a long running operation, something like:

inputStream.Select(n => Task.Run(() =>
{
    // Long running operation
    Thread.Sleep(TimeSpan.FromSeconds(5));

    return n * n;
}).ToObservable())
.Switch()
.Subscribe(result => 
{
    // Use result in some way
    Console.WriteLine(result);
});

How can I get a CancellationToken inside of the Task.Run call so that when Switch disposes the subscription of an in-flight calculation, it sets the CancellationToken as cancelled so I know to abort the calculation.

Answer 1

You can use the Observable.StartAsync method, e.g.

inputStream.Select(n => Observable.StartAsync((token => Task.Run(() =>
{
    if (token.IsCancellationRequested)
    {
        // .. don't need to do anything
        return 0;
    }
    else
    {
        Thread.Sleep(TimeSpan.FromSeconds(1));
        return n * n;

    }                   
}))))
.Switch()
.Subscribe(Console.WriteLine);

Alternatively, if you’ll be producing multiple values you can use the Observable.Create overload that works with Task to get a CancellationToken. E.g.

inputStream.Select(n => Observable.Create<int>((observer, token) => Task.Run(() =>
{
    while (!token.IsCancellationRequested)
    {
        Thread.Sleep(TimeSpan.FromSeconds(1));
        observer.OnNext(n * n);
    }

    observer.OnCompleted();
})))
.Switch()
.Subscribe(Console.WriteLine);

Inside of your task, you need to call OnNext to produce values. The return value of the task, if any, is ignored.