gin-writer-override.go

// instrumentedWriter is a wrapper around gin.ResponseWriter that calls
// BeforeWriteHeaderCallback before response headers is written.
type instrumentedWriter struct {
	gin.ResponseWriter
	BeforeWriteHeaderCallback func()
}

func (w *instrumentedWriter) Write(data []byte) (int, error) {
	w.runBeforeWriteHeaderCallbackOnce()
	return w.ResponseWriter.Write(data)
}

func (w *instrumentedWriter) WriteString(s string) (int, error) {
	w.runBeforeWriteHeaderCallbackOnce()
	return w.ResponseWriter.WriteString(s)
}

func (w *instrumentedWriter) Flush() {
	w.runBeforeWriteHeaderCallbackOnce()
	w.ResponseWriter.Flush()
}

func (w *instrumentedWriter) runBeforeWriteHeaderCallbackOnce() {
	if w.BeforeWriteHeaderCallback == nil {
		return
	}
	w.BeforeWriteHeaderCallback()
	w.BeforeWriteHeaderCallback = nil
}

// how to use

g.POST("/path", function (c gin.Context) {
	c.Writer = &instrumentedWriter{
		ResponseWriter: c.Writer,
		BeforeWriteHeaderCallback: func() {
			// your code to be called before headers is sent
		},
	}

	// your request processing code
}