Add BuildVarsFunc to allow modifying route variables before generating a route's URL

route.go

@@ -31,6 +31,8 @@ type Route struct {
 	name string
 	// Error resulted from building a route.
 	err error
+
+	buildVarsFunc BuildVarsFunc
 }
 
 // Match matches the route against the request.
@@ -336,6 +338,19 @@ func (r *Route) Schemes(schemes ...string) *Route {
 	return r.addMatcher(schemeMatcher(schemes))
 }
 
+// BuildVarsFunc --------------------------------------------------------------
+
+// BuildVarsFunc is the function signature used by custom build variable
+// functions (which can modify route variables before a route's URL is built).
+type BuildVarsFunc func(map[string]string) map[string]string
+
+// BuildVarsFunc adds a custom function to be used to modify build variables
+// before a route's URL is built.
+func (r *Route) BuildVarsFunc(f BuildVarsFunc) *Route {
+	r.buildVarsFunc = f
+	return r
+}
+
 // Subrouter ------------------------------------------------------------------
 
 // Subrouter creates a subrouter for the route.
@@ -398,17 +413,20 @@ func (r *Route) URL(pairs ...string) (*url.URL, error) {
 	if r.regexp == nil {
 		return nil, errors.New("mux: route doesn't have a host or path")
 	}
+	values, err := r.buildVars(pairs...)
+	if err != nil {
+		return nil, err
+	}
 	var scheme, host, path string
-	var err error
 	if r.regexp.host != nil {
 		// Set a default scheme.
 		scheme = "http"
-		if host, err = r.regexp.host.url(pairs...); err != nil {
+		if host, err = r.regexp.host.url(values); err != nil {
 			return nil, err
 		}
 	}
 	if r.regexp.path != nil {
-		if path, err = r.regexp.path.url(pairs...); err != nil {
+		if path, err = r.regexp.path.url(values); err != nil {
 			return nil, err
 		}
 	}
@@ -429,7 +447,11 @@ func (r *Route) URLHost(pairs ...string) (*url.URL, error) {
 	if r.regexp == nil || r.regexp.host == nil {
 		return nil, errors.New("mux: route doesn't have a host")
 	}
-	host, err := r.regexp.host.url(pairs...)
+	values, err := r.buildVars(pairs...)
+	if err != nil {
+		return nil, err
+	}
+	host, err := r.regexp.host.url(values)
 	if err != nil {
 		return nil, err
 	}
@@ -449,7 +471,11 @@ func (r *Route) URLPath(pairs ...string) (*url.URL, error) {
 	if r.regexp == nil || r.regexp.path == nil {
 		return nil, errors.New("mux: route doesn't have a path")
 	}
-	path, err := r.regexp.path.url(pairs...)
+	values, err := r.buildVars(pairs...)
+	if err != nil {
+		return nil, err
+	}
+	path, err := r.regexp.path.url(values)
 	if err != nil {
 		return nil, err
 	}
@@ -458,6 +484,19 @@ func (r *Route) URLPath(pairs ...string) (*url.URL, error) {
 	}, nil
 }
 
+// buildVars converts the route variable pairs into a map. If the route has a
+// BuildVarsFunc, it is invoked.
+func (r *Route) buildVars(pairs ...string) (map[string]string, error) {
+	m, err := mapFromPairs(pairs...)
+	if err != nil {
+		return nil, err
+	}
+	if r.buildVarsFunc != nil {
+		m = r.buildVarsFunc(m)
+	}
+	return m, nil
+}
+
 // ----------------------------------------------------------------------------
 // parentRoute
 // ----------------------------------------------------------------------------