implement mvc HandleError as requested at #1244

Former-commit-id: 58a69f9cffe67c3aa1bab5d9425c5df65e2367ed

hero/func_result.go

@@ -20,6 +20,23 @@ type Result interface {
 	Dispatch(ctx context.Context)
 }
 
+type (
+	// ErrorHandler is the optional interface to handle errors per hero func,
+	// see `mvc/Application#HandleError` for MVC application-level error handler registration too.
+	ErrorHandler interface {
+		HandleError(ctx context.Context, err error)
+	}
+
+	// ErrorHandlerFunc implements the `ErrorHandler`.
+	// It describes the type defnition for an error handler.
+	ErrorHandlerFunc func(ctx context.Context, err error)
+)
+
+// HandleError fires when the `DispatchFuncResult` returns a non-nil error.
+func (fn ErrorHandlerFunc) HandleError(ctx context.Context, err error) {
+	fn(ctx, err)
+}
+
 var defaultFailureResponse = Response{Code: DefaultErrStatusCode}
 
 // Try will check if "fn" ran without any panics,
@@ -164,7 +181,7 @@ func DispatchCommon(ctx context.Context,
 // Result or (Result, error) and so on...
 //
 // where Get is an HTTP METHOD.
-func DispatchFuncResult(ctx context.Context, values []reflect.Value) {
+func DispatchFuncResult(ctx context.Context, errorHandler ErrorHandler, values []reflect.Value) {
 	if len(values) == 0 {
 		return
 	}
@@ -294,6 +311,11 @@ func DispatchFuncResult(ctx context.Context, values []reflect.Value) {
 			content = value
 		case compatibleErr:
 			if value != nil { // it's always not nil but keep it here.
+				if errorHandler != nil {
+					errorHandler.HandleError(ctx, value)
+					break
+				}
+
 				err = value
 				if statusCode < 400 {
 					statusCode = DefaultErrStatusCode