How can I implement an exhaustMap handler in Rx.Net?

问题

I am looking for something similar to the exhaustMap operator from rxjs, but RX.NET does not seem to have such an operator.

What I need to achieve is that, upon every element of the source stream, I need to start an async handler, and until it finishes, I would like to drop any elements from the source. As soon as the handler finishes, resume taking elements.

What I don't want is to start an async handler upon every element - while the handler runs, I want to drop source elements.

I also suspect I need to cleverly use the defer operator here?

Thank you!

回答1:

Here is an implementation of the ExhaustMap operator. The source observable is projected to an IObservable<Task<TResult>>, where each subsequent task is either the previous one if it's still running, or otherwise a new task associated with the current item. Repeated occurrences of the same task are then removed with the DistinctUntilChanged operator, and finally the observable is flattened with the Concat operator.

public static IObservable<TResult> ExhaustMap<TSource, TResult>(
    this IObservable<TSource> source,
    Func<TSource, Task<TResult>> function)
{
    return source
        .Scan(Task.FromResult<TResult>(default), (previousTask, item) =>
        {
            return !previousTask.IsCompleted ? previousTask : HideIdentity(function(item));
        })
        .DistinctUntilChanged()
        .Concat();

    async Task<TResult> HideIdentity(Task<TResult> task) => await task;
}

The tasks returned by the function are not guaranteed to be distinct, hence the need for the HideIdentity local function that returns distinct wrappers of the tasks.

Usage example:

Observable
    .Interval(TimeSpan.FromMilliseconds(200))
    .Select(x => (int)x + 1)
    .Take(10)
    .Do(x => Console.WriteLine($"Input: {x}"))
    .ExhaustMap(async x => { await Task.Delay(x % 3 == 0 ? 500 : 100); return x; })
    .Do(x => Console.WriteLine($"Result: {x}"))
    .Wait();

Output:

Input: 1
Result: 1
Input: 2
Result: 2
Input: 3
Input: 4
Input: 5
Result: 3
Input: 6
Input: 7
Input: 8
Result: 6
Input: 9
Input: 10
Result: 9

来源：https://stackoverflow.com/questions/64353907/how-can-i-implement-an-exhaustmap-handler-in-rx-net

标签

.net

system.reactive

rx.net