OptionalonMessage: (msg: EventSourceMessage) => voidA function that will be called on each message.
OptionalonId: (id: string) => voidA function that will be called on each id field.
OptionalonRetry: (retry: number) => voidA function that will be called on each retry field.
A function that should be called for each incoming line buffer.
Parses line buffers into EventSourceMessages.