sse

A small, allocation-light server implementation of the WHATWG Server-Sent Events protocol.

Message, Comment, and Send are safe to call from multiple goroutines and the hot path does no heap allocations.

Keyed data lines

Some formats layered on SSE key each data line and repeat the key across a multi-line value. Datastar writes an element patch as:

event: datastar-patch-elements
data: selector #foo
data: mode inner
data: elements <div>
data: elements   Hello world!
data: elements </div>

Build one with NewMessage, writing each key's value with its prefix. PrefixString takes a value you already have; PrefixWriter hands back an io.Writer for one you want to stream, so a template can render straight into the message:

m := sse.NewMessage(sse.WithEvent("datastar-patch-elements"))
m.PrefixString("selector ", "#foo")
m.PrefixString("mode ", "inner")
if err := tmpl.Execute(m.PrefixWriter("elements "), page); err != nil {
	return err
}
return stream.Send(m)

Both repeat the prefix on every line of a multi-line value, and both terminate the line the previous one left open, so every value starts on a fresh data line. PrefixWriter writers stay usable afterwards — writing to an earlier one resumes its prefix on a new line — and one that is never written to contributes nothing, where PrefixString always writes its line.

PrefixString returns nothing to check: writing to a buffer cannot fail, and a prefix containing CR or LF is a mistake in the calling code, reported by Send the way an invalid id or event already is.

*Message is itself an io.Writer for unkeyed data, and the writers from PrefixWriter implement io.StringWriter.