readme: improve compression example

README.md

@@ -358,7 +358,7 @@ Extension][rfc-pmce].
 
 It provides minimalistic I/O wrappers to be used in conjunction with any
 deflate implementation (for example, the standard library's
-[compress/flate][compress/flate].
+[compress/flate][compress/flate]).
 
 It is also compatible with `wsutil`'s reader and writer by providing
 `wsflate.MessageState` type, which implements `wsutil.SendExtension` and
@@ -453,6 +453,84 @@ func main() {
 }
 ```
 
+You can use compression with `wsutil` package this way:
+
+```go
+	// Upgrade somehow and negotiate compression to get the conn...
+
+	// Initialize flate reader. We are using nil as a source io.Reader because
+	// we will Reset() it in the message i/o loop below.
+	fr := wsflate.NewReader(nil, func(r io.Reader) wsflate.Decompressor {
+		return flate.NewReader(r)
+	})
+	// Initialize flate writer. We are using nil as a destination io.Writer
+	// because we will Reset() it in the message i/o loop below.
+	fw := wsflate.NewWriter(nil, func(w io.Writer) wsflate.Compressor {
+		f, _ := flate.NewWriter(w, 9)
+		return f
+	})
+
+	// Declare compression message state variable.
+	//
+	// It has two goals:
+	// - Allow users to check whether received message is compressed or not.
+	// - Help wsutil.Reader and wsutil.Writer to set/unset appropriate
+	//   WebSocket header bits while writing next frame to the wire (it
+	//   implements wsutil.RecvExtension and wsutil.SendExtension).
+	var msg wsflate.MessageState
+
+	// Initialize WebSocket reader as previously. 
+	// Please note the use of Reader.Extensions field as well as
+	// of ws.StateExtended flag.
+	rd := &wsutil.Reader{
+		Source:     conn,
+		State:      ws.StateServerSide | ws.StateExtended,
+		Extensions: []wsutil.RecvExtension{
+			&msg, 
+		},
+	}
+
+	// Initialize WebSocket writer with ws.StateExtended flag as well.
+	wr := wsutil.NewWriter(conn, ws.StateServerSide|ws.StateExtended, 0)
+	// Use the message state as wsutil.SendExtension.
+	wr.SetExtensions(&msg)
+
+	for {
+		h, err := rd.NextFrame()
+		if err != nil {
+			// handle error.
+		}
+		if h.OpCode.IsControl() {
+			// handle control frame.
+		}
+		if !msg.IsCompressed() {
+			// handle uncompressed frame (skipped for the sake of example
+			// simplicity).
+		}
+
+		// Reset the writer to echo same op code.
+		wr.Reset(h.OpCode)
+
+		// Reset both flate reader and writer to start the new round of i/o.
+		fr.Reset(rd)
+		fw.Reset(wr)
+
+		// Copy whole message from reader to writer decompressing it and
+		// compressing again.
+		if _, err := io.Copy(fw, fr); err != nil {
+			// handle error.
+		}
+		// Flush any remaining buffers from flate writer to WebSocket writer.
+		if err := fw.Close(); err != nil {
+			// handle error.
+		}
+		// Flush the whole WebSocket message to the wire.
+		if err := wr.Flush(); err != nil {
+			// handle error.
+		}
+	}
+```
+
 
 [rfc-url]: https://tools.ietf.org/html/rfc6455
 [rfc-pmce]: https://tools.ietf.org/html/rfc7692#section-7