SubRouter middleware

This middleware makes it much easier to write sub-routes, allowing you
to make the sub-router ignorant of its parent router's matched prefix.

The example app has also been modified to use this functionality for its
admin pages.

Fixes #65.

example/main.go

@@ -18,6 +18,7 @@ import (
 	"github.com/goji/param"
 	"github.com/zenazn/goji"
 	"github.com/zenazn/goji/web"
+	"github.com/zenazn/goji/web/middleware"
 )
 
 // Note: the code below cuts a lot of corners to make the example app simple.
@@ -40,22 +41,28 @@ func main() {
 	goji.Use(PlainText)
 
 	// If the patterns ends with "/*", the path is treated as a prefix, and
-	// can be used to implement sub-routes. Sub-routes can be used to set
-	// custom middleware on sub-applications. Goji's interfaces are
-	// completely composable.
+	// can be used to implement sub-routes.
 	admin := web.New()
 	goji.Handle("/admin/*", admin)
+
+	// The standard SubRouter middleware helps make writing sub-routers
+	// easy. Ordinarily, Goji does not manipulate the request's URL.Path,
+	// meaning you'd have to repeat "/admin/" in each of the following
+	// routes. This middleware allows you to cut down on the repetition by
+	// eliminating the shared, already-matched prefix.
+	admin.Use(middleware.SubRouter)
+	// You can also easily attach extra middleware to sub-routers that are
+	// not present on the parent router. This one, for instance, presents a
+	// password prompt to users of the admin endpoints.
 	admin.Use(SuperSecure)
 
+	admin.Get("/", AdminRoot)
+	admin.Get("/finances", AdminFinances)
+
 	// Goji's routing, like Sinatra's, is exact: no effort is made to
 	// normalize trailing slashes.
 	goji.Get("/admin", http.RedirectHandler("/admin/", 301))
 
-	// Set up admin routes. Note that sub-routes do *not* mutate the path in
-	// any way, so we need to supply full ("/admin/" prefixed) paths.
-	admin.Get("/admin/", AdminRoot)
-	admin.Get("/admin/finances", AdminFinances)
-
 	// Use a custom 404 handler
 	goji.NotFound(NotFound)
 

web/middleware/subrouter.go

@@ -0,0 +1,36 @@
+package middleware
+
+import (
+	"net/http"
+
+	"github.com/zenazn/goji/web"
+)
+
+type subrouter struct {
+	c *web.C
+	h http.Handler
+}
+
+func (s subrouter) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	if s.c.URLParams != nil {
+		if path, ok := s.c.URLParams["*"]; ok {
+			oldpath := r.URL.Path
+			r.URL.Path = path
+			defer func() {
+				r.URL.Path = oldpath
+			}()
+		}
+	}
+	s.h.ServeHTTP(w, r)
+}
+
+// SubRouter is a helper middleware that makes writing sub-routers easier.
+//
+// If you register a sub-router under a key like "/admin/*", Goji's router will
+// automatically set c.URLParams["*"] to the unmatched path suffix. This
+// middleware will help you set the request URL's Path to this unmatched suffix,
+// allowing you to write sub-routers with no knowledge of what routes the parent
+// router matches.
+func SubRouter(c *web.C, h http.Handler) http.Handler {
+	return subrouter{c, h}
+}