Add automatic keep-alive to Server Sent Events

[Kaliumhexacyanoferrat]

As the provider of an event stream, I would like the connection to be automatically be kept alive so that I do not have to manually send dummy messages or comments once in a while.

Example

If enabled, the handler would spawn an additional thread that frequently sends a comment or ping event to the connected client to prevent the connection from being terminated by a proxy and to check whether the client is still connected. If sending the comment fails, the handler should cancel the generator logic using a CancellationToken. Both threads would need to be carefully be synchronized to prevent them from writing to the connection at the same time.

var source = EventSource.Create()
                        .Generator(...)
                        .KeepAlive();

Acceptance criteria

• Keeping the connection alive is an optional feature
• If the keep alive functionality detects that the client is no longer active the generator logic can be cancelled
• Write access to the connection is synchronized
• The feature is documented on the website

[Matasx]

The problem fits to the producer/consumer pattern (some process writing to the connection is consumer and Ping / user logic are producers of events). In C# you can largely utilize BlockingCollection<>. If you want a bit more syntactic sugar around BlockingCollection, you can use 3rd party AsyncCollection.