TsgcSSEClient

TsgcSSEClient connects to a URL that returns a text/event-stream, reads the stream on a background thread and fires an event for every message the server pushes. When the stream ends or the network fails, the client reconnects automatically and sends the Last-Event-ID header, so the server can resume the stream from the last event received. A retry: field sent by the server sets the reconnection delay for the rest of the session. The component is available in all editions and is registered in the SGC HTTP palette tab.

 

Follow the steps below to configure this component:

 

1. Create a new instance of TsgcSSEClient component and set the URL of the event stream.

 

2. Handle the OnEvent event and call Open. Example:

 

uses
  sgcHTTP_SSE_Client;

oSSE := TsgcSSEClient.Create(nil);
oSSE.URL := 'https://www.example.com/events';
oSSE.OnEvent := OnSSEEvent;
oSSE.Open;

procedure TForm1.OnSSEEvent(Sender: TObject; const aEvent: TsgcSSEEvent);
begin
  ShowMessage(aEvent.EventType + ': ' + aEvent.Data);
end;

 

How the client reacts to the server response:

• 200 with text/event-stream: the stream opens, OnOpen fires and every event fires OnEvent.
• 3xx redirect: the redirect is followed.
• 204 No Content: the server asks the client to stop. The client closes without an error and does not reconnect.
• Any other status, or a wrong content type: OnError fires, then OnClose, and the client does not reconnect.
• Network error or read timeout: OnError fires and the client reconnects using ReconnectOptions.
• End of the stream: the client reconnects using ReconnectOptions.

 

ReconnectOptions.MaxAttempts counts consecutive failures, and the counter restarts every time a connection opens. Use Headers to add request headers such as a bearer token, and OnBeforeConnect to configure TLS, a proxy or authentication on the underlying TsgcHTTP1Client.