Adding feature to implement custom rest errors

The previous version only had the error string as customizable, so I included the ability to modify the whole reponse. See issue ant0ine#193

rest/response.go

@@ -11,7 +11,6 @@ import (
 // Note, the responseWriter object instantiated by the framework also implements many other interfaces
 // accessible by type assertion: http.ResponseWriter, http.Flusher, http.CloseNotifier, http.Hijacker.
 type ResponseWriter interface {
-
 	// Identical to the http.ResponseWriter interface
 	Header() http.Header
 
@@ -34,12 +33,34 @@ type ResponseWriter interface {
 // eg: rest.ErrorFieldName = "errorMessage"
 var ErrorFieldName = "Error"
 
+// This allows to customize the error messages used in the error response payload.
+// It defaults to the ErrorFieldName method for compatibility reasons (only recieves error string and http code),
+// but can be changed before starting the server.
+//
+// Sends a json payload of the struct given. Be sure to define the json keys in the struct.
+// eg: rest.CustomErrorStruct = &MyCustomStruct{}
+var ErrorFunc func(*Request, string, int) interface{}
+
 // Error produces an error response in JSON with the following structure, '{"Error":"My error message"}'
 // The standard plain text net/http Error helper can still be called like this:
 // http.Error(w, "error message", code)
 func Error(w ResponseWriter, error string, code int) {
+	// Call new method to support backwards compat
+	ErrorWithRequest(nil, w, error, code)
+}
+
+// Error produces an error response in JSON with context of the request
+func ErrorWithRequest(r *Request, w ResponseWriter, error string, code int) {
 	w.WriteHeader(code)
-	err := w.WriteJson(map[string]string{ErrorFieldName: error})
+
+	var errPayload interface{}
+	if ErrorFunc != nil {
+		errPayload = ErrorFunc(r, error, code)
+	} else {
+		errPayload = map[string]string{ErrorFieldName: error}
+	}
+
+	err := w.WriteJson(errPayload)
 	if err != nil {
 		panic(err)
 	}

rest/response_test.go

@@ -6,6 +6,29 @@ import (
 	"github.com/ant0ine/go-json-rest/rest/test"
 )
 
+func CustomError(r *Request, error string, code int) interface{} {
+	// r = nil when using test requests
+	var header string
+	switch code {
+	case 400:
+		header = "Bad Input"
+		break
+	case 404:
+		header = "Not Found"
+		break
+	default:
+		header = "API Error"
+	}
+
+	return map[string]interface{}{
+		"error": map[string]interface{}{
+			"header":  header,
+			"code":    code,
+			"message": error,
+		},
+	}
+}
+
 func TestResponseNotIndent(t *testing.T) {
 
 	writer := responseWriter{
@@ -24,7 +47,7 @@ func TestResponseNotIndent(t *testing.T) {
 	}
 }
 
-// The following tests could instantiate only the reponseWriter,
+// The following tests could instantiate only the responseWriter,
 // but using the Api object allows to use the rest/test utilities,
 // and make the tests easier to write.
 
@@ -54,6 +77,24 @@ func TestErrorResponse(t *testing.T) {
 	recorded.BodyIs("{\"Error\":\"test\"}")
 }
 
+func TestCustomErrorResponse(t *testing.T) {
+
+	api := NewApi()
+	ErrorFunc = CustomError
+
+	api.SetApp(AppSimple(func(w ResponseWriter, r *Request) {
+		Error(w, "test", 500)
+	}))
+
+	recorded := test.RunRequest(t, api.MakeHandler(), test.MakeSimpleRequest("GET", "http://localhost/", nil))
+	recorded.CodeIs(500)
+	recorded.ContentTypeIsJson()
+	recorded.BodyIs(`{"error":{"code":500,"header":"API Error","message":"test"}}`)
+
+	// reset the package variable to not effect other tests
+	ErrorFunc = nil
+}
+
 func TestNotFoundResponse(t *testing.T) {
 
 	api := NewApi()
@@ -66,3 +107,21 @@ func TestNotFoundResponse(t *testing.T) {
 	recorded.ContentTypeIsJson()
 	recorded.BodyIs("{\"Error\":\"Resource not found\"}")
 }
+
+func TestCustomNotFoundResponse(t *testing.T) {
+
+	api := NewApi()
+	ErrorFunc = CustomError
+
+	api.SetApp(AppSimple(func(w ResponseWriter, r *Request) {
+		NotFound(w, r)
+	}))
+
+	recorded := test.RunRequest(t, api.MakeHandler(), test.MakeSimpleRequest("GET", "http://localhost/", nil))
+	recorded.CodeIs(404)
+	recorded.ContentTypeIsJson()
+	recorded.BodyIs(`{"error":{"code":404,"header":"Not Found","message":"Resource not found"}}`)
+
+	// reset the package variable to not effect other tests
+	ErrorFunc = nil
+}