How can I instantly return values from an async loop?

Question

I've got an async discover function which discovers devices in the local network which I call on a button click. I'm sending a broadcast message to all devices and listen for responses for 5 seconds using a CancellationTokenSource. After the token has expired I'm returning an IEnumerable of parsed responses to my WPF model.

I'd like return incoming responses directly (and stop listening after 5 seconds) so that I can show discovered devices instantly in the UI instead of showing them all after 5 seconds.

This is my code:

public async Task<IEnumerable<IDevice>> Discover()
{
    var client = new MyClient();
    var responseData = await GetResponseData(client);
    return this.ParseResponseData(responseData);
}

private IEnumerable<IDevice> ParseResponseData(List<DeviceResponseData> responseData)
{
    foreach (var data in responseData)
    {
        yield return DeviceFactory.Create(data);
    }
}

private static async Task<List<DeviceResponseData>> GetResponseData(MyClient client,
    int timeout = 5000)
{
    var cancellationToken = new CancellationTokenSource(timeout);
    var data = new List<DeviceResponseData>();

    // ... prepare message and send it
    await client.SendAsync(message, new CancellationToken());

    try
    {
        while (!cancellationToken.IsCancellationRequested)
        {
            // Wait indefinitely until any message is received.
            var response = await client.ReceiveAsync(cancellationToken.Token);

            data.Add(new DeviceResponseData(/* ... */ response));
        }
    }
    catch (TaskCanceledException e)
    {

    }

    return data;
}

Answer 1

If you want to show results as they come in, there are many ways of achieving this, like decoupled messages, events, etc.

However, you could just use a simple Action

private static async Task<List<DeviceResponseData>> GetResponseData(MyClient client, Action<DeviceResponseData> update, int timeout = 5000)
{
   var cancellationToken = new CancellationTokenSource(timeout);
   ...
   while (!cancellationToken.IsCancellationRequested)
   {
      // Wait indefinitely until any message is received.
      var response = await client.ReceiveAsync(cancellationToken.Token);

      var result = new DeviceResponseData( /* ... */ response);

      data.Add(result);
      update(result);

   }
   ...
}

usage

var allResults =  await GetResponseData(client,data => UdpateUI(data), timeout);

_{Note : because this the Async Await Pattern, you wont have to marshal the result form the Action back to the UI Context, if that's where this was called from}.