core/play-java/src/main/java/play/libs/Comet.java
public static Flow<ByteString, ByteString, NotUsed> flow(String callbackName) {
    ByteString cb = ByteString.fromString(callbackName);
    return Flow.of(ByteString.class)
        .map(
            (msg) -> {
              return formatted(cb, msg);
            }
public static Flow<ByteString, ByteString, NotUsed> flow(String callbackName) {
    ByteString cb = ByteString.fromString(callbackName);
    return Flow.of(ByteString.class)
        .map(
            (msg) -> {
              return formatted(cb, msg);
            }
Produces a flow of ByteString with a prepended block and a script wrapper.

@param callbackName the javascript callback method.
@return a flow of ByteString elements.