better convert

projection/framing.go

@@ -1,5 +1,7 @@
 package projection
 
+import "encoding/json"
+
 // Bus framing contract (shared by producer + consumer).
 //
 // A projection stream carries two channels distinguished only by the event's
@@ -55,3 +57,24 @@ type Transform[W, T any] func(W) T
 // wire value W — the store applies the same Transform to it. Return (_, false)
 // for a genuine miss. May be nil.
 type Fallback[W any] func(key string) (W, bool)
+
+// JSONDecode is a ready-made Decoder for domains whose wire schema is just a
+// JSON array of Update[W] (the generic shape — Key/Value/Deleted). Catalog does
+// NOT use this (its wire is a flat Projection for back-compat); identity
+// consumers like tracking popularity can, pairing it with JSONEncode on the
+// emitter side. Marshal errors propagate so HandleFrame can NACK.
+func JSONDecode[W any](payload []byte) ([]Update[W], error) {
+	var out []Update[W]
+	if err := json.Unmarshal(payload, &out); err != nil {
+		return nil, err
+	}
+	return out, nil
+}
+
+// JSONEncode is the inverse of JSONDecode — the EncodeFunc an Emitter uses when
+// the wire schema is a JSON array of Update[W]. A nil/empty slice marshals to a
+// payload JSONDecode reads back as zero updates, which is what begin/end frames
+// need.
+func JSONEncode[W any](items []Update[W]) ([]byte, error) {
+	return json.Marshal(items)
+}