Issues with post request in Flutter/Dart frontend with Go backend

post go dart flutter restful-authentication

1,225

In your code

    http.post(url,body:json.encode(body)).then((http.Response response){
      cookie = response.headers['session'];
    });
    debugPrint(cookie);

the line

debugPrint(cookie);

is executed before

cookie = response.headers['session'];

so it is expected that cookie is null when you print it. For some weird reason debugPrint(null) throws an exception about "slit on null"

Try instead

    http.post(url,body:json.encode(body)).then((http.Response response){
      cookie = response.headers['session'];
      debugPrint(cookie);   
    });

1,225

Author by

DanT29

Updated on December 05, 2022

Comments

DanT29 6 months

I am new to Flutter/Dart, Go and mobile development in general. I am currently coding a login authentication page in flutter that connects to a backend written in go. To check if everything worked I wanted to print out the cookie that my backend sends back in the console. Unfortunately I am getting errors not sure what to do.

EDIT: I have been able to read the cookie, should I save the cookie in this format "session=UUID" or just the "UUID"? I want to send this cookie back in the header for future get requests. My Go code will check for the cookie name "session",but I'm not sure if sending it back in that format is correct.

Flutter code (I read on another stackoverflow post to use 10.0.2.2 as localhost when using an android emulator):

EDIT: After playing around with the code I was able to read the cookie in the header but it has the name as well not just the UUID.

import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;
import 'dart:convert';
class Login extends StatefulWidget {
  @override
  State<StatefulWidget> createState(){
    return new LoginState();
  }
}
class LoginState extends State <Login> {
  final formKey = GlobalKey<FormState>();
  String _email, _password;
  String cookie;
  void loginApi(){
    var form = formKey.currentState;
    var url = "http://10.0.2.2:8080";
      if (form.validate()) {
        form.save();
        var body = { "Email":_email, "Pass":_password};
        http.post(url,body:json.encode(body)).then((response){
          print(response.headers['set-cookie']);
        });
      }
  }
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text("Login"),
        backgroundColor: Colors.white,
      ),
      body: new Container(
        alignment: Alignment.center,
        child: Form(
            key: formKey,
            child: Column(
              children: <Widget>[
                TextFormField(
                  autocorrect: false,
                  decoration: InputDecoration(
                    labelText: "Email:",
                  ),
                  validator: (str) =>
                  !str.contains('@') ? "Not a Valid Email!" : null,
                  onSaved: (str) =>  _email = str,
                ),
                TextFormField(
                  autocorrect: false,
                  decoration: InputDecoration(
                    labelText: "Password:",
                  ),
                  validator: (str) =>
                  str.length <= 7 ? "Not a Valid Password!" : null,
                  onSaved: (str) => _password = str,
                  obscureText: false,
                ),
                RaisedButton(
                  child: Text("Submit"),
                  onPressed:loginApi,
                ),
              ],
            )
        ),
      ),
    );
  }
}

Golang code (EDIT: Inserted a missing curly bracket after the panic error and deleted the curly bracket at the end of the code):

package main
import (
    "encoding/json"
    "html/template"
    "net/http"
    uuid "github.com/satori/go.uuid"
    "golang.org/x/crypto/bcrypt"
)
type user struct {
    UserName string //Same as email
    Password []byte
}
type logindata struct {
    Email string
    Pass  string
}
var tpl *template.Template
var dbUsers = map[string]user{}
var dbSess = map[string]string{}
func init() {
    bs, _ := bcrypt.GenerateFromPassword([]byte("password"), bcrypt.MinCost)
    dbUsers["[email protected]"] = user{"[email protected]", bs}
}
func main() {
    http.HandleFunc("/", login)
    http.Handle("/favicon.ico", http.NotFoundHandler()) //Don't have favicon
    http.ListenAndServe(":8080", nil)
}
func login(w http.ResponseWriter, req *http.Request) {
    var data logindata
    decoder := json.NewDecoder(req.Body)
    err_decode := decoder.Decode(&data)
    if err_decode != nil {
        panic(err_decode)
    }
    u, ok := dbUsers[data.Email]
    if !ok {
        http.Error(w, "Username and/or password do not match", http.StatusForbidden)
        return
    }
    err_compare := bcrypt.CompareHashAndPassword(u.Password, []byte(data.Pass))
    if err_compare != nil {
        http.Error(w, "Username and/or password do not match", http.StatusForbidden)
        return
    }
    // create session
    sID, _ := uuid.NewV4()
    c := &http.Cookie{
        Name:  "session",
        Value: sID.String(),
    }
    http.SetCookie(w, c)
    dbSess[c.Value] = data.Email
    return
}

Output from console for flutter code (after I put in username and password and click submit, username is "[email protected]" password is "password"):

Performing hot reload...
Reloaded 0 of 489 libraries in 544ms.
I/flutter ( 3778): session=db3690d6-db6e-4658-8b5b-5f2d3f908a65

I also went to localhost:8080 on my browser expecting to show a blank page but it displayed "page not working" and my terminal outputted the following error I guess because I was accessing it with a browser and I wasn't sending in JSON data, not sure:

2018/05/24 23:29:38 http: panic serving [::1]:40010: EOF
goroutine 6 [running]:
net/http.(*conn).serve.func1(0xc82001a280)
    /usr/lib/go-1.6/src/net/http/server.go:1389 +0xc1
panic(0x76c6c0, 0xc82000a160)
    /usr/lib/go-1.6/src/runtime/panic.go:443 +0x4e9
main.login(0x7f96c9a848b8, 0xc82012c000, 0xc8201121c0)
    /home/daniel/Desktop/Workspace/Genesis/main.go:43 +0x176
net/http.HandlerFunc.ServeHTTP(0x8a8688, 0x7f96c9a848b8, 0xc82012c000, 0xc8201121c0)
    /usr/lib/go-1.6/src/net/http/server.go:1618 +0x3a
net/http.(*ServeMux).ServeHTTP(0xc820013020, 0x7f96c9a848b8, 0xc82012c000, 0xc8201121c0)
    /usr/lib/go-1.6/src/net/http/server.go:1910 +0x17d
net/http.serverHandler.ServeHTTP(0xc82001a100, 0x7f96c9a848b8, 0xc82012c000, 0xc8201121c0)
    /usr/lib/go-1.6/src/net/http/server.go:2081 +0x19e
net/http.(*conn).serve(0xc82001a280)
    /usr/lib/go-1.6/src/net/http/server.go:1472 +0xf2e
created by net/http.(*Server).Serve
    /usr/lib/go-1.6/src/net/http/server.go:2137 +0x44e
2018/05/24 23:29:38 http: panic serving [::1]:40012: EOF
goroutine 18 [running]:
net/http.(*conn).serve.func1(0xc82011c080)
    /usr/lib/go-1.6/src/net/http/server.go:1389 +0xc1
panic(0x76c6c0, 0xc82000a160)
    /usr/lib/go-1.6/src/runtime/panic.go:443 +0x4e9
main.login(0x7f96c9a848b8, 0xc8201161a0, 0xc820154000)
    /home/daniel/Desktop/Workspace/Genesis/main.go:43 +0x176
net/http.HandlerFunc.ServeHTTP(0x8a8688, 0x7f96c9a848b8, 0xc8201161a0, 0xc820154000)
    /usr/lib/go-1.6/src/net/http/server.go:1618 +0x3a
net/http.(*ServeMux).ServeHTTP(0xc820013020, 0x7f96c9a848b8, 0xc8201161a0, 0xc820154000)
    /usr/lib/go-1.6/src/net/http/server.go:1910 +0x17d
net/http.serverHandler.ServeHTTP(0xc82001a100, 0x7f96c9a848b8, 0xc8201161a0, 0xc820154000)
    /usr/lib/go-1.6/src/net/http/server.go:2081 +0x19e
net/http.(*conn).serve(0xc82011c080)
    /usr/lib/go-1.6/src/net/http/server.go:1472 +0xf2e
created by net/http.(*Server).Serve
    /usr/lib/go-1.6/src/net/http/server.go:2137 +0x44e
2018/05/24 23:29:43 http: panic serving [::1]:40016: EOF
goroutine 7 [running]:
net/http.(*conn).serve.func1(0xc82001a380)
    /usr/lib/go-1.6/src/net/http/server.go:1389 +0xc1
panic(0x76c6c0, 0xc82000a160)
    /usr/lib/go-1.6/src/runtime/panic.go:443 +0x4e9
main.login(0x7f96c9a848b8, 0xc82012c1a0, 0xc8201122a0)
    /home/daniel/Desktop/Workspace/Genesis/main.go:43 +0x176
net/http.HandlerFunc.ServeHTTP(0x8a8688, 0x7f96c9a848b8, 0xc82012c1a0, 0xc8201122a0)
    /usr/lib/go-1.6/src/net/http/server.go:1618 +0x3a
net/http.(*ServeMux).ServeHTTP(0xc820013020, 0x7f96c9a848b8, 0xc82012c1a0, 0xc8201122a0)
    /usr/lib/go-1.6/src/net/http/server.go:1910 +0x17d
net/http.serverHandler.ServeHTTP(0xc82001a100, 0x7f96c9a848b8, 0xc82012c1a0, 0xc8201122a0)
    /usr/lib/go-1.6/src/net/http/server.go:2081 +0x19e
net/http.(*conn).serve(0xc82001a380)
    /usr/lib/go-1.6/src/net/http/server.go:1472 +0xf2e
created by net/http.(*Server).Serve
    /usr/lib/go-1.6/src/net/http/server.go:2137 +0x44e

Günter Zöchbauer almost 5 years

What were you tapping on to get this error?
DanT29 almost 5 years

Yes I tapped on the submit button
Günter Zöchbauer almost 5 years

What line is Login.dart:32:19?
DanT29 almost 5 years

debugPrint(cookie); part of the loginApi function I made for the onPressed field for the raised button
Günter Zöchbauer almost 5 years

Can you try print(cookie) instead of debugPrint(cookie) or perhaps print(response.headers);? I get the impression there is something fishy going on with the headers.
DanT29 almost 5 years

it is showing this as the error: I/flutter ( 5490): null for print(cookie), I will try the other one now
Günter Zöchbauer almost 5 years

That doesn't look like an error.
DanT29 almost 5 years

In my loginApi function I changed my debugPrint(cookie) to print(cookie), the output of the console just showed me this after putting in the username and password: Performing full restart... Restarted app in 2,049ms. W/IInputConnectionWrapper( 5490): getCursorCapsMode on inactive InputConnection I/flutter ( 5490): null
Günter Zöchbauer almost 5 years

That doesn't look like an error, just that response.headers['session'] returns null.
DanT29 almost 5 years

I can't seem to code print(response.headers) it is underlining response saying "undefined name response"
Anzel almost 5 years

@DanT29, under your go code, are you sure you have put the closing } parentheses at the right place? You didn't close it after the panic(error) line seems very odd to me. The remaining code is unreachable
DanT29 almost 5 years

@Anzel good call! my post requests are working on postman when i use raw JSON now. I'm still getting errors but will update my question details.

DanT29 almost 5 years

I still seem to be getting the same error, could this be an issue on my back-end? I think I'm retrieving the headers correctly.
Günter Zöchbauer almost 5 years

Same error as which one? "The method 'split' was called on null."? Then you don't get headers back. Perhaps you should check the responseCode
DanT29 almost 5 years

Yes it showed me this : E/flutter ( 5490): [ERROR:topaz/lib/tonic/logging/dart_error.cc(16)] Unhandled exception: E/flutter ( 5490): NoSuchMethodError: The method 'split' was called on null. E/flutter ( 5490): Receiver: null E/flutter ( 5490): Tried calling: split("\n") ...followed by a bunch of other errors. I will try to look over my backend code...I'm guessing from what you see my post request is correct?