GDPR, Cookies consent banner Flutter web


I’m building my website with Flutter but programming for web is very new to me and I’m not quite sure I understand exactly how Cookies work.
I still need to understand what cookies are to be written where, and where do I take those cookies from.
Building the banner to manage the should be easy, and if I’m not wrong it should be the first thing that pops up in the home page.
For example Medium banner is just a dismissible banner swing the message
To make Medium work, we log user data. By using Medium, you agree to our Privacy Policy.
with a link the Privacy Policy, but it doesn’t have any opt-in so it doesn’t look GDPR compliant..

Here shows how cookies are written and read in flutter web


html.window.localStorage['local_value'] = localStorage;
html.window.sessionStorage['session_value'] = sessionStorage;
html.window.document.cookie= "username=${cookies}; expires=Thu, 18 Dec 2020 12:00:00 UTC";

As far as I understood cookies are of these types.

To track user behavior( page visits , number of users etc..) and as I use google Analytics I do need to ask consent for these.
here Google Analytics, Flutter, cookies and the General Data Protection Regulation (GDPR) is shown how to activate/deactivate it, so I shouldn’t store anything myself if I’m not wrong.

These would be from a YouTube linked video on my Home page for example, so I need to ask consent for these too.
Haven’t checked it yet but I guess it should be similar to Google Analytics

Session cookies:
These should be used to remember Items in a shopping basket.
I shouldn’t be needing these..

Persistent cookies:
These should be for keeping user’s logged in.
One of the web pages is the Retailer access, which is retailer’s app (the supply side of the marketplace ).
I’m using Google signing to log in users, so I should be needing these, as the login form it always presented to users when navigating to Retailer access even after they logged in.

Secure cookies:
These are for https only and used for check-out/payment pages.
In my web the retailer’s app only creates products, manages workshop bookings, and handles communication with customers..
The mobile app (the demand side of the marketplace) is where all the payments are made using Stripe, so I shouldn’t have to store anything on web..right?

Sorry for the long question, I hope it’s clear enough.
Thank you for your help.


I basically had the problem since I am also using third-party scripts (firebase, stripe, …) and I need the user’s consent before any of those scripts are run.

I build my solution around Yett (, which blocks scripts that are part of a previously defined blacklist. You could even implement this functionality by yourself, the author has written an interesting medium article.

In my case I only have "essential" scripts, so I built a solution where the flutter app loads only if the user consented to the all necessary scripts. But it shouldn’t be too difficult to adjust this solution if one needs more fined grained control about the user’s cookie settings and I added a second entry for "Analytics" as a possible starting point.

I store the user’s settings in the localStorage and retrieve them directly on app start to create the blacklist and to decide whether the cookie banner should be shown.

This is my index.html.

It references the following scripts: get_consent.js, set_consent.js, init_firebase.js and load_app.js (more info on them below).

<!DOCTYPE html>
    If you are serving your web app in a path other than the root, change the
    href value below to reflect the base path you are serving from.

    The path provided below has to start and end with a slash "/" in order for
    it to work correctly.

    For more details:
  <base href="/">

  <meta charset="UTF-8">
  <meta content="IE=Edge" http-equiv="X-UA-Compatible">
  <meta name="description" content="A new Flutter project.">

  <!-- iOS meta tags & icons -->
  <meta name="apple-mobile-web-app-capable" content="yes">
  <meta name="apple-mobile-web-app-status-bar-style" content="black">
  <meta name="apple-mobile-web-app-title" content="flutter_utils">
  <link rel="apple-touch-icon" href="icons/Icon-192.png">
  <!-- Assigns blacklist of urls based on localStorage (must be placed before yett script) -->
  <script src="get_consent.js"></script>
  <!-- Yett is used to block all third-party scripts that are part of the blacklist (must be placed before all other (third-party) scripts) -->
  <script src=""></script>
  <script src=""></script>

  <link rel="manifest" href="manifest.json">
  <link rel="stylesheet" type="text/css" href="style.css">
  <!-- The standard consent popup (hidden by default) -->
  <div id="consent-popup" class="hidden consent-div">
    <h2>We use cookies and other technologies</h2>
    <p>This website uses cookies and similar functions to process end device information and personal data. The processing serves the integration of content, external services and elements of third parties, statistical analysis/measurement, personalized advertising and the integration of social media. Depending on the function, data may be passed on to third parties within the EU in the process. Your consent is always voluntary, not required for the use of our website and can be rejected or revoked at any time via the icon at the bottom right.
      <button id="accept-btn" class="btn inline">Accept</button>
      <button id="reject-btn" class="btn inline">Reject</button>
      <button id="info-btn" class="btn inline">More info</button>    
  <!-- Detailed consent popup allows the user to control scripts by their category -->
  <div id="consent-popup-details" class="hidden consent-div">
    <h2>Choose what to accept</h2>
      <div class="row-div">
        <label class="switch">
          <!-- Essentials must always be checked -->
          <input id="essential-cb" type="checkbox" checked disabled=true>
          <span class="slider round"></span>
      Here you can find all technically necessary scripts, cookies and other elements that are necessary for the operation of the website.
      <div class="row-div">
        <label class="switch">
          <input id ="analytics-cb" type="checkbox">
          <span class="slider round"></span>
      For the site, visitors, web page views and diveerse other data are stored anonymously.
      <button id="save-btn" class="btn inline">Save</button>
      <button id="cancel-btn" class="btn inline">Cancel</button>   
  <!-- Updates localStorage with user's cookie settings -->
  <script src="set_consent.js"></script>
  <script src=""></script>
  <script src=""></script>
  <script src=""></script>
  <script src=""></script>
  <!-- Initializes firebase (if user gave consent) -->
  <script src="init_firebase.js"></script>
  <!-- Loads flutter app (if user gave consent) -->
  <script src="load_app.js"></script>     

The get_consent.js is the first script, retrieves the user’s settings from localStorage and also defines the yett blacklist:

const essentialCookies = ["", ""];
const analyticsCookies = [""];
const allCookies = [...essentialCookies, ...analyticsCookies];
const consentPropertyName = "cookie_consent";

const retrieveConsentSettings = () => {
  const consentJsonString = localStorage.getItem(consentPropertyName);
  return JSON.parse(consentJsonString);

const checkConsentIsMissing = () => {
  const consentObj = retrieveConsentSettings();
  if (!consentObj || consentObj.length == 0) {
    return true;
  return false;

const consentIsMissing = checkConsentIsMissing();

var blacklist;
if (consentIsMissing) {
  blacklist = allCookies;
} else {
  const acceptedCookies = retrieveConsentSettings();
  // Remove all script urls from blacklist that the user accepts (if all are accepted the blacklist will be empty)
  blacklist = allCookies.filter( ( el ) => !acceptedCookies.includes( el ) );

// Yett blacklist expects list of RegExp objects
var blacklistRegEx = [];
for (let index = 0; index < blacklist.length; index++) {
  const regExp = new RegExp(blacklist[index]);

YETT_BLACKLIST = blacklistRegEx;

set_consent.js is responsible for updating localStorage with the user’s settings and also hides/shows the respective divs for the cookie consent. Usually, one could simply call window.yett.unblock() to unblock the scripts, but since their order matters I decided to simply reload the window after the localStorage was updated:

const saveToStorage = (acceptedCookies) => {
  const jsonString = JSON.stringify(acceptedCookies);
  localStorage.setItem(consentPropertyName, jsonString);

window.onload = () => {
  const consentPopup = document.getElementById("consent-popup");
  const consentPopupDetails = document.getElementById("consent-popup-details");
  const acceptBtn = document.getElementById("accept-btn");
  const moreInfoBtn = document.getElementById("info-btn");
  const saveBtn = document.getElementById("save-btn");
  const cancelBtn = document.getElementById("cancel-btn");
  const rejectBtn = document.getElementById("reject-btn");

  const acceptFn = (event) => {
    const cookiesTmp = [...essentialCookies, ...analyticsCookies];
    // Reload window after localStorage was updated.
    // The blacklist will then only contain items the user has not yet consented to.

  const cancelFn = (event) => {

  const rejectFn = (event) => {
    // Possible To-Do: Show placeholder content if even essential scripts are rejected.

  const showDetailsFn = () => {

  const saveFn = (event) => {
    const analyticsChecked = document.getElementById("analytics-cb").checked;
    var cookiesTmp = [...essentialCookies];
    if (analyticsChecked) {
    // Reload window after localStorage was updated.
    // The blacklist will then only contain items the user has not yet consented to.

  acceptBtn.addEventListener("click", acceptFn);
  moreInfoBtn.addEventListener("click", showDetailsFn);
  saveBtn.addEventListener("click", saveFn);
  cancelBtn.addEventListener("click", cancelFn);
  rejectBtn.addEventListener("click", rejectFn);

  if (consentIsMissing) {

init_firebase.js is the usual script for initializing the service, but I only inizialize if consent was obtained:

var firebaseConfig = {
  // your standard config

// Initialize Firebase only if user consented
if (!consentIsMissing) {

The same logic is applied to the script load_app.js. The Flutter app is only loaded if the user consented.

One might therefore add some fallback content to the index.html which would be shown if the user rejects the necessary scripts. Depending on your use case it might also be an option load the app anyway and then differentiate within the app by accessing the user’s settings from localStorage.

var serviceWorkerVersion = null;
var scriptLoaded = false;
function loadMainDartJs() {
  if (scriptLoaded) {
  scriptLoaded = true;
  var scriptTag = document.createElement("script");
  scriptTag.src = "main.dart.js";
  scriptTag.type = "application/javascript";

// Load app only if user consented
if (!consentIsMissing) {
  if ("serviceWorker" in navigator) {
    // Service workers are supported. Use them.
    window.addEventListener("load", function () {
      // Wait for registration to finish before dropping the <script> tag.
      // Otherwise, the browser will load the script multiple times,
      // potentially different versions.
      var serviceWorkerUrl =
        "flutter_service_worker.js?v=" + serviceWorkerVersion;
      navigator.serviceWorker.register(serviceWorkerUrl).then((reg) => {
        function waitForActivation(serviceWorker) {
          serviceWorker.addEventListener("statechange", () => {
            if (serviceWorker.state == "activated") {
              console.log("Installed new service worker.");
        if (! && (reg.installing || reg.waiting)) {
          // No active web worker and we have installed or are installing
          // one for the first time. Simply wait for it to activate.
          waitForActivation(reg.installing ?? reg.waiting);
        } else if (! {
          // When the app updates the serviceWorkerVersion changes, so we
          // need to ask the service worker to update.
          console.log("New service worker available.");
        } else {
          // Existing service worker is still good.
          console.log("Loading app from service worker.");

      // If service worker doesn't succeed in a reasonable amount of time,
      // fallback to plaint <script> tag.
      setTimeout(() => {
        if (!scriptLoaded) {
            "Failed to load app from service worker. Falling back to plain <script> tag."
      }, 4000);
  } else {
    // Service workers not supported. Just drop the <script> tag.

And here is my style.css:

body {
  height: 100%;
  width: 100%;
  background-color: #2d2d2d;
  font-family: Arial, Helvetica, sans-serif;

.hidden {
  display: none;
  visibility: hidden;

.consent-div {
  position: fixed;
  bottom: 40px;
  left: 10%;
  right: 10%;
  width: 80%;
  padding: 14px 14px;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: space-between;
  background-color: #eee;
  border-radius: 5px;
  box-shadow: 0 0 5px 5px rgba(0, 0, 0, 0.2);

.row-div {
  display: flex;
  justify-content: space-between;
  align-items: center;

#save-btn {
  background-color: #103900;

#cancel-btn {
  background-color: #ff0000;

.btn {
  height: 25px;
  width: 140px;
  background-color: #777;
  border: none;
  color: white;
  border-radius: 5px;
  cursor: pointer;

.inline {
  display: inline-block;
  margin-right: 5px;

h2 {
  margin-block-start: 0.5em;
  margin-block-end: 0em;

h3 {
  margin-block-start: 0.5em;
  margin-block-end: 0em;

/* The switch - the box around the slider */
.switch {
  position: relative;
  display: inline-block;
  width: 50px;
  height: 25px;

/* Hide default HTML checkbox */
.switch input {
  opacity: 0;
  width: 0;
  height: 0;

/* The slider */
.slider {
  position: absolute;
  cursor: pointer;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  background-color: #ccc;

.slider:before {
  position: absolute;
  content: "";
  height: 18px;
  width: 18px;
  left: 4px;
  bottom: 4px;
  background-color: white;

input:checked + .slider {
  background-color: #2196f3;

input:focus + .slider {
  box-shadow: 0 0 1px #2196f3;

input:checked + .slider:before {
  -webkit-transform: translateX(24px);
  -ms-transform: translateX(24px);
  transform: translateX(24px);

/* Rounded sliders */
.slider.round {
  border-radius: 34px;

.slider.round:before {
  border-radius: 50%;

Answered By – hnnngwdlch

Answer Checked By – Terry (FlutterFixes Volunteer)

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