http request is blocked by Cors policy for flutter web

The same problems hit me two weeks ago. For me, the following error is giving me a wrong direction to checking the problem:

Access to XMLHttpRequest at 'https://example.com/api' from origin 'http://localhost:49168' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.

It says something about the request, but turns out it had nothing to do with the request. And it also had nothing to do with the web server (apache or nginx in my case).

The solution is by adding header to the response (yes, response) from your backend. I don't know the solution for php code, but I use the following code in my golang backend to add header to the response:

// Adding CORS header to response.
func (server *WebUploadServer) addCorsHeader(res http.ResponseWriter) {
    headers := res.Header()
    // use your domain or ip address instead of "*".
    // Using "*" is allowing access from every one
    // only for development.
    headers.Add("Access-Control-Allow-Origin", "*")
    headers.Add("Vary", "Origin")
    headers.Add("Vary", "Access-Control-Request-Method")
    headers.Add("Vary", "Access-Control-Request-Headers")
    headers.Add("Access-Control-Allow-Headers", "Content-Type, Origin, Accept, token")
    headers.Add("Access-Control-Allow-Methods", "GET, POST, OPTIONS")
}


// Here we handle the web request.
func (server *WebUploadServer) webHandler(w http.ResponseWriter, r *http.Request) {
    fmt.Println("Endpoint hit")

    server.addCorsHeader(w) // Here, we add the header to the response

    switch r.Method {
    case "POST":
        // do something here.
    case "OPTIONS":
        w.WriteHeader(http.StatusOK)
    case "GET":
        fallthrough
    default:
        fmt.Fprintf(w, "Sorry, only  POST method is supported.")
    }
}