Added Before function to ResponseWriter.

This allows middleware to have a function get
called right before ResponseWriter has been written to.
It can be used to set headers or do other necessary things
before a response has been written.

response_writer.go

@@ -9,6 +9,8 @@ type (
 		http.ResponseWriter
 		Status() int
 		Written() bool
+		// Before allows for a function to be called before the ResponseWriter has been written to.
+		Before(BeforeFunc)
 
 		// private
 		setStatus(int)
@@ -16,15 +18,19 @@ type (
 
 	responseWriter struct {
 		http.ResponseWriter
-		status  int
+		status      int
-		written bool
+		written     bool
+		beforeFuncs []BeforeFunc
 	}
+
+	BeforeFunc func(ResponseWriter)
 )
 
 func (w *responseWriter) reset(writer http.ResponseWriter) {
 	w.ResponseWriter = writer
 	w.status = 0
 	w.written = false
+	w.beforeFuncs = w.beforeFuncs[:0]
 }
 
 func (w *responseWriter) setStatus(code int) {
@@ -32,6 +38,7 @@ func (w *responseWriter) setStatus(code int) {
 }
 
 func (w *responseWriter) WriteHeader(code int) {
+	w.callBefore()
 	w.status = code
 	w.written = true
 	w.ResponseWriter.WriteHeader(code)
@@ -44,3 +51,13 @@ func (w *responseWriter) Status() int {
 func (w *responseWriter) Written() bool {
 	return w.written
 }
+
+func (w *responseWriter) Before(before BeforeFunc) {
+	w.beforeFuncs = append(w.beforeFuncs, before)
+}
+
+func (w *responseWriter) callBefore() {
+	for i := len(w.beforeFuncs) - 1; i >= 0; i-- {
+		w.beforeFuncs[i](w)
+	}
+}