AspNetCore.Docs/aspnetcore/signalr/streaming.md

---
title: Use streaming in ASP.NET Core SignalR
author: tdykstra
description: 
monikerRange: '>= aspnetcore-2.1'
ms.author: tdykstra
ms.custom: mvc
ms.date: 11/14/2018
uid: signalr/streaming
---

# Use streaming in ASP.NET Core SignalR

By [Brennan Conroy](https://github.com/BrennanConroy)

ASP.NET Core SignalR supports streaming return values of server methods. This is useful for scenarios where fragments of data will come in over time. When a return value is streamed to the client, each fragment is sent to the client as soon as it becomes available, rather than waiting for all the data to become available.

[View or download sample code](https://github.com/aspnet/Docs/tree/live/aspnetcore/signalr/streaming/sample) ([how to download](xref:index#how-to-download-a-sample))

## Set up the hub

A hub method automatically becomes a streaming hub method when it returns a `ChannelReader<T>` or a `Task<ChannelReader<T>>`. Below is a sample that shows the basics of streaming data to the client. Whenever an object is written to the `ChannelReader` that object is immediately sent to the client. At the end, the `ChannelReader` is completed to tell the client the stream is closed.

> [!NOTE]
> Write to the `ChannelReader` on a background thread and return the `ChannelReader` as soon as possible. Other hub invocations will be blocked until a `ChannelReader` is returned.

::: moniker range="= aspnetcore-2.1"

[!code-csharp[Streaming hub method](streaming/sample/Hubs/StreamHub.aspnetcore21.cs?range=12-36)]

::: moniker-end

::: moniker range=">= aspnetcore-2.2"

[!code-csharp[Streaming hub method](streaming/sample/Hubs/StreamHub.cs?range=11-35)]

> [!NOTE]
> In ASP.NET Core 2.2 or later, streaming Hub methods can accept a `CancellationToken` parameter that will be triggered when the client unsubscribes from the stream. Use this token to stop the server operation and release any resources if the client disconnects before the end of the stream.

::: moniker-end

## .NET client

The `StreamAsChannelAsync` method on `HubConnection` is used to invoke a streaming method. Pass the hub method name, and arguments defined in the hub method to `StreamAsChannelAsync`. The generic parameter on `StreamAsChannelAsync<T>` specifies the type of objects returned by the streaming method. A `ChannelReader<T>` is returned from the stream invocation, and represents the stream on the client. To read data, a common pattern is to loop over `WaitToReadAsync` and call `TryRead` when data is available. The loop will end when the stream has been closed by the server, or the cancellation token passed to `StreamAsChannelAsync` is canceled.

::: moniker range=">= aspnetcore-2.2"

```csharp
// Call "Cancel" on this CancellationTokenSource to send a cancellation message to 
// the server, which will trigger the corresponding token in the Hub method.
var cancellationTokenSource = new CancellationTokenSource();
var channel = await hubConnection.StreamAsChannelAsync<int>(
    "Counter", 10, 500, cancellationTokenSource.Token);

// Wait asynchronously for data to become available
while (await channel.WaitToReadAsync())
{
    // Read all currently available data synchronously, before waiting for more data
    while (channel.TryRead(out var count))
    {
        Console.WriteLine($"{count}");
    }
}

Console.WriteLine("Streaming completed");
```

::: moniker-end

::: moniker range="= aspnetcore-2.1"

```csharp
var channel = await hubConnection
    .StreamAsChannelAsync<int>("Counter", 10, 500, CancellationToken.None);

// Wait asynchronously for data to become available
while (await channel.WaitToReadAsync())
{
    // Read all currently available data synchronously, before waiting for more data
    while (channel.TryRead(out var count))
    {
        Console.WriteLine($"{count}");
    }
}

Console.WriteLine("Streaming completed");
```

::: moniker-end

## JavaScript client

JavaScript clients call streaming methods on hubs by using `connection.stream`. The `stream` method accepts two arguments:

* The name of the hub method. In the following example, the hub method name is `Counter`.
* Arguments defined in the hub method. In the following example, the arguments are: a count for the number of stream items to receive, and the delay between stream items.

`connection.stream` returns an `IStreamResult` which contains a `subscribe` method. Pass an `IStreamSubscriber` to `subscribe` and set the `next`, `error`, and `complete` callbacks to get notifications from the `stream` invocation.

[!code-javascript[Streaming javascript](streaming/sample/wwwroot/js/stream.js?range=19-36)]

::: moniker range="= aspnetcore-2.1"

To end the stream from the client, call the `dispose` method on the `ISubscription` that is returned from the `subscribe` method.

::: moniker-end

::: moniker range=">= aspnetcore-2.2"

To end the stream from the client, call the `dispose` method on the `ISubscription` that is returned from the `subscribe` method. Calling this method will cause the `CancellationToken` parameter of the Hub method (if you provided one) to be canceled.

::: moniker-end

## Related resources

* [Hubs](xref:signalr/hubs)
* [.NET client](xref:signalr/dotnet-client)
* [JavaScript client](xref:signalr/javascript-client)
* [Publish to Azure](xref:signalr/publish-to-azure-web-app)