SseHandler constructor
_uri is the URL under which the server is listening for
incoming bi-directional SSE connections.
If keepAlive is supplied, connections will remain active for this
period after a disconnect and can be reconnected transparently. If there
is no reconnect within that period, the connection will be closed
normally.
If keepAlive is not supplied, connections will be closed immediately
after a disconnect.
Implementation
SseHandler(this._uri, {Duration? keepAlive}) : _keepAlive = keepAlive;