Framed
Marks a @ProtocolMessage class (typically a sealed parent) with a framing length prefix that is computed from — and bounds — the body's wire size.
The framework owns framing: the encode path emits the prefix carrying the encoded body's byte count, the decode path reads the prefix, narrows buffer.limit() to bound the body, and asserts strict consumption. @FramedBy is structural and handles both fixed-size and variable-size bodies uniformly.
Sealed-parent composition: when applied to a @ProtocolMessage sealed parent, every variant inherits the framing rule. There is no per-variant override — protocols whose framing varies by variant are out of scope for this annotation. Validator enforces that every variant of a @FramedBy parent has the named after field as a FixedSize discriminator.
Parameters
A KClass referencing a Kotlin object that implements BoundingLengthCodec<UInt>. The codec drives prefix wire format (var-byte-int, fixed-width, etc.) and provides maxWireSize so the emitter can size the slack region for the slicing scheme.
Names a sibling constructor field that the prefix sits immediately after on the wire. Empty (default) means the prefix is at offset 0. The named field must exist and — when the class carries @PacketType — be the discriminator. It must either have Exact wire width (fixed-width scalars / value classes wrapping them) or be a varint value class (inner scalar carrying @UseCodec(<VariableLengthCodec>), e.g. an HTTP/3 frame type): the emit then measures the header's width per value via the codec instead of a compile-time constant.
@ProtocolMessage
@FramedBy(MqttRemainingLengthCodec::class, after = "header")
sealed interface MqttPacket {
// Each variant declares its own header field; the framework writes
// the prefix between header and body, computed from body bytes.
}