export pathvar for user-defined routers (#911)

* refactor

* export pathvar for user-defined routers

rest/pathvar/params.go

@@ -0,0 +1,29 @@
+package pathvar
+
+import (
+	"context"
+	"net/http"
+)
+
+var pathVars = contextKey("pathVars")
+
+// Vars parses path variables and returns a map.
+func Vars(r *http.Request) map[string]string {
+	vars, ok := r.Context().Value(pathVars).(map[string]string)
+	if ok {
+		return vars
+	}
+
+	return nil
+}
+
+// WithVars writes params into given r and returns a new http.Request.
+func WithVars(r *http.Request, params map[string]string) *http.Request {
+	return r.WithContext(context.WithValue(r.Context(), pathVars, params))
+}
+
+type contextKey string
+
+func (c contextKey) String() string {
+	return "rest/pathvar/context key: " + string(c)
+}