Adjust Handler interface and support middleware

handler.go

@@ -0,0 +1,38 @@
+package jsonrpc2
+
+// Handler handles JSON-RPC requests and notifications.
+type Handler interface {
+	// Handle is called to handle a request. No other requests are handled
+	// until it returns. If you do not require strict ordering behavior
+	// of received RPCs, it is suggested to wrap your handler in
+	// AsyncHandler.
+	Handle(*Conn, *Request)
+}
+
+type HandlerFunc func(*Conn, *Request)
+
+func (f HandlerFunc) Handle(conn *Conn, req *Request) {
+	f(conn, req)
+}
+
+type Middleware func(Handler) Handler
+
+type chain struct {
+	ms []Middleware
+}
+
+func Chain(middleware ...Middleware) chain {
+	return chain{ms: append([]Middleware(nil), middleware...)}
+}
+
+func (c chain) Then(h Handler) Handler {
+	if h == nil {
+		panic("nil handler")
+	}
+
+	for i := range c.ms {
+		h = c.ms[len(c.ms)-1-i](h)
+	}
+
+	return h
+}