mazrean / iwrapper

A tool to assist in creating wrappers for Go interfaces.

iwrapper

日本語

A tool to assist in creating wrappers for Go interfaces.

Motivation

Consider creating a wrapper for http.ResponseWriter. The simplest approach is to use embedding as shown below:

type MyResponseWriter struct {
  http.ResponseWriter
}

func WrapResponseWriter(rw http.ResponseWriter) http.ResponseWriter {
  return &MyResponseWriter{rw}
}

However, with this method, the result of type assertion changes after wrapping.

// ResponseWriter implemented with hijacker
var rw http.ResponseWriter

// Before wrap: ok => true
_, ok = rw.(http.Hijacker)

// After wrap: ok => false
_, ok = WrapResponseWriter(rw).(http.Hijacker)

Such type assertions, often performed in standard libraries to maintain backward compatibility, can lead to unexpected behavioral changes.

By using the function (ResponseWriterWrapper) generated with iwrapper, this problem can be addressed as shown below:

// 1. Implement http.Hijacker in MyResponseWriter
func (w *MyResponseWriter) Hijack() (net.Conn, *bufio.ReadWriter, error) {
	return w.ResponseWriter.(http.Hijacker).Hijack()
}

// 2. Use ResponseWriterWrapper for wrapping
func WrapResponseWriterWithIWrapper(rw http.ResponseWriter) http.ResponseWriter {
	return ResponseWriterWrapper(rw, func(rw http.ResponseWriter) ResponseWriter {
		return &MyResponseWriter{rw}
	})
}

// After wrapping using iwrapper: ok => true
_, ok = WrapResponseWriterWithIWrapper(rw).(http.Hijacker)

Usage

Consider wrapping http.ResponseWriter so that the results of type assertions for http.Hijacker, http.CloseNotifier, and http.Flusher remain unchanged:

1. Create the following Go file as the configuration for iwrapper:

   package testdata
   
   //go:generate go run github.com/mazrean/iwrapper -src=$GOFILE -dst=iwrapper_$GOFILE
   
   import (
     "net/http"
   )
   
   //iwrapper:target
   type ResponseWriter interface {
     //iwrapper:require
     http.ResponseWriter
     http.Hijacker
     http.CloseNotifier
     http.Flusher
   }

   • You can customize the generated function name with iwrapper:target func:"ResponseWriterWrapFunc".
2. Execute go generate.
   • This produces the wrapping function (ResponseWriterWrapper) in iwrapper_<configuration filename>.go.

Detailed generated code and its usage examples are available in /example/.

License

MIT