Toby Rogers

Toby Rogers


Flutter Stream Tutorial for Beginners

Originally published by Dane Mackier at

What is a Stream

There are many comparisons of how to visualise a Stream so I’ll use a common one. A stream is like a pipe, you put a value on the one end and if there’s a listener on the other end that listener will receive that value. A Stream can have multiple listeners and all of those listeners will receive the same value when it’s put in the pipeline. The way you put values on a stream is by using a StreamController.

Full Video can be seen here

How to create a Stream

If you want to create a stream where you can put a value on you start with a StreamController.

StreamController<double> controller = StreamController<double>();

This will construct a controller that you can then use to manipulate the stream the controller manages. The controllers stream can be accessed through the stream property

Stream stream =;

How to use a stream

The next thing to do is to be able to get the values from a stream. This is commonly referred to as subscribing or listening to a stream. When you subscribe to a stream you will only get the values that are emitted (put onto the stream) after the subscription. You subscribe to the stream by calling the listen function and supplying it with a Function to call back to when there's a new value available, commonly referred to as a callback function, or just a callback.

stream.listen((value) {

  print(‘Value from controller: $value’);


Emit / Add a value onto the stream

When you have subscriptions to the stream it means that there’s a function waiting to be executed somewhere. The way you emit a value over a stream is by calling add on the streams controller.


When you call that function, the callback supplied in the section above will execute. Which will print out

Value from controller: 12

That’s all the basics of a stream in terms of using it. Now lets go on to managing the stream.

Managing the stream

The listen call returns a StreamSubscription of the type of your stream. This can be used to manage the stream subscription. The most common usage of the subscription is cancelling the listening when you’re no longer required to receive the data. Basically making sure there are no memory leaks. A subscription to a stream will stay active until the entire memory is destroyed, usually the entire lifecycle of your app. This is perfectly fine in some cases and not fine in others.

When you subscribe to a Stream and you have to cancel it afterwards you can store it in a SteamSubscription

StreamSubscription<double> streamSubscription = stream.listen((value) {

  print(‘Value from controller: $value’);


This will give you the subscription object for the registered callback.

Cancel a stream

Lets go over when you want to do this. In flutter, streams are usually used with the StreamBuilder which manages and unsubscribes from a stream for you internally once the widget is destroyed. A good rule to follow is when you subscribe to a stream, keep the Subscription and write the code in the dispose method to call cancel. If your stream needs to be alive for the entire duration of the application then you don’t have to cancel the stream on dispose or when it’s not needed.


Common Stream Errors

One thing that’s very common to see when devs use Streams in dart is the “Stream already subscribed to” message. A lot of people think that this is because there’s an active subscription and cancelling that would get rid of the error, but that’s not true. Lets look at how we can create this exception ourselves, then we’ll figure out how to fix it.

stream.listen((value) {

  print(‘1st Sub: $value’);


stream.listen((value) {

  print(‘2nd Sub: $value’);


This will throw the error “Bad state: Stream has already been listened to”. Now even if you cancel the first subscription and subscribe again you’ll still get this error and that is by design.

streamSubscription = stream.listen((value) {

     print(‘1st Sub: $value’);

  });  await streamSubscription.cancel();  stream.listen((value) {

    print(‘2nd Sub: $value’);


The code above will still throw the “Bad state” stream error. The reason for that is because there are two types of Streams:

Single Subscription Stream: For use with a sequence of events that are parts of a larger whole. Things like reading a file or a web request. To ensure the subscriber that subscribed first gets all the correct information in the correct order there’s a limitation allowing you to only subscribe once for the lifecycle of the streams existence.

Broadcast Stream: This kind of stream is for use with individual emissions that can be handled one at a time without the context or knowledge of the previous events.

You can use both for individual events, like I do, but just be weary of the subscription policy on the first one. When using a StreamBuilder in Flutter you’ll most likely always get the exception because the Stream will be subscribed to multiple times during build function calls (which happen a lot).

Fixing the Bad State stream error

To fix this you’ll have to specifically create a broadcast StreamController so that the underlying stream is constructed and managed as a Broadcast stream that allows multiple subscriptions.

StreamController<double> controller = StreamController<double>.broadcast();

IMPORTANT NOTE — Using either of these stream types does not mean you don’t have to manage your subscription. If you manually subscribe to a stream you HAVE to clean it up (cancel) if there’s a change that you might subscribe to it again. Multiple subscriptions cause memory leaks, make sure when your code goes out of scope or out of view you dispose, then re-subscribe if you have to.

Manual Streams

Another way and also a common method of creating streams is through an async * function. This is a Function that will run asynchronously and return (yield) a value whenever there’s a new one, but it won’t stop the execution of that function. To make more sense lets look at it like this. Below is a Future that will return a random value after waiting 1 second.

Future<double> getRandomValue() async {

  var random = Random(2);

  await Future.delayed(Duration(seconds: 1));

  return random.nextDouble();


This code can be used and you’ll get a random value back once off and the function will be finished executing. This means if you want another random value you’ll have to call and await the function again. Like below.

var value1 = await getRandomValue();

var value2 = await getRandomValue();

What if you wanted to call the function once and continuously get random values from that function without stopping it’s execution? That’s where async* and yield comes in. Lets make a function that returns a stream and every second will emit a new random value.

Stream<double> getRandomValues() async* {

    var random = Random(2);    while (true) {

      await Future.delayed(Duration(seconds: 1));

      yield random.nextDouble();



This is called a generator function. It looks similar to the previous one, but lets look at the differences.

  • The first thing to notice is that we now return a Stream and not a Future. That means instead of awaiting on the value we’ll have to subscribe to the stream.
  • The second difference is that async* instead of async. This tells the runtime that this code should be run asynchronously but execution will continue even after “returning” a value.
  • The last difference is the replacement of return with yield. This is basically a return function but it doesn’t exit the function. Instead it continues executing the rest of the code after yield.

So how do you use this Stream (Generator Function)? The same as above.

getRandomValues().listen((value) {

    print(‘1st: $value’);


This will print out something like below where each line is printed after every second of delay.

1st: 0.000783592309359204

1st: 0.232325923093592045

1st: 0.456078359230935920

1st: 0.565783592309359204

Streams created in this manner are broadcast by default and allows for multiple subscriptions. In terms of basics for usage that’s all, you have to know about streams. In terms of basics, that’s all you have to know about streams, how to use them and effectively manage them without causing bugs in your code. Once you understand Streams and how they work and require more functionality you can look at RxDart.

Thanks for reading

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Further reading

Flutter: Adding Bluetooth Functionality

Let’s Develop a Mobile App in Flutter

Flutter Todos Tutorial with “flutter_bloc”

#flutter #dart #mobile-apps #ios

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Buddha Community

Flutter Stream Tutorial for Beginners

Google's Flutter 1.20 stable announced with new features - Navoki

Flutter Google cross-platform UI framework has released a new version 1.20 stable.

Flutter is Google’s UI framework to make apps for Android, iOS, Web, Windows, Mac, Linux, and Fuchsia OS. Since the last 2 years, the flutter Framework has already achieved popularity among mobile developers to develop Android and iOS apps. In the last few releases, Flutter also added the support of making web applications and desktop applications.

Last month they introduced the support of the Linux desktop app that can be distributed through Canonical Snap Store(Snapcraft), this enables the developers to publish there Linux desktop app for their users and publish on Snap Store.  If you want to learn how to Publish Flutter Desktop app in Snap Store that here is the tutorial.

Flutter 1.20 Framework is built on Google’s made Dart programming language that is a cross-platform language providing native performance, new UI widgets, and other more features for the developer usage.

Here are the few key points of this release:

Performance improvements for Flutter and Dart

In this release, they have got multiple performance improvements in the Dart language itself. A new improvement is to reduce the app size in the release versions of the app. Another performance improvement is to reduce junk in the display of app animation by using the warm-up phase.


If your app is junk information during the first run then the Skia Shading Language shader provides for pre-compilation as part of your app’s build. This can speed it up by more than 2x.

Added a better support of mouse cursors for web and desktop flutter app,. Now many widgets will show cursor on top of them or you can specify the type of supported cursor you want.

Autofill for mobile text fields

Autofill was already supported in native applications now its been added to the Flutter SDK. Now prefilled information stored by your OS can be used for autofill in the application. This feature will be available soon on the flutter web.


A new widget for interaction

InteractiveViewer is a new widget design for common interactions in your app like pan, zoom drag and drop for resizing the widget. Informations on this you can check more on this API documentation where you can try this widget on the DartPad. In this release, drag-drop has more features added like you can know precisely where the drop happened and get the position.

Updated Material Slider, RangeSlider, TimePicker, and DatePicker

In this new release, there are many pre-existing widgets that were updated to match the latest material guidelines, these updates include better interaction with Slider and RangeSliderDatePicker with support for date range and time picker with the new style.


New pubspec.yaml format

Other than these widget updates there is some update within the project also like in pubspec.yaml file format. If you are a flutter plugin publisher then your old pubspec.yaml  is no longer supported to publish a plugin as the older format does not specify for which platform plugin you are making. All existing plugin will continue to work with flutter apps but you should make a plugin update as soon as possible.

Preview of embedded Dart DevTools in Visual Studio Code

Visual Studio code flutter extension got an update in this release. You get a preview of new features where you can analyze that Dev tools in your coding workspace. Enable this feature in your vs code by _dart.previewEmbeddedDevTools_setting. Dart DevTools menu you can choose your favorite page embed on your code workspace.

Network tracking

The updated the Dev tools comes with the network page that enables network profiling. You can track the timings and other information like status and content type of your** network calls** within your app. You can also monitor gRPC traffic.

Generate type-safe platform channels for platform interop

Pigeon is a command-line tool that will generate types of safe platform channels without adding additional dependencies. With this instead of manually matching method strings on platform channel and serializing arguments, you can invoke native class and pass nonprimitive data objects by directly calling the Dartmethod.

There is still a long list of updates in the new version of Flutter 1.2 that we cannot cover in this blog. You can get more details you can visit the official site to know more. Also, you can subscribe to the Navoki newsletter to get updates on these features and upcoming new updates and lessons. In upcoming new versions, we might see more new features and improvements.

You can get more free Flutter tutorials you can follow these courses:

#dart #developers #flutter #app developed #dart devtools in visual studio code #firebase local emulator suite in flutter #flutter autofill #flutter date picker #flutter desktop linux app build and publish on snapcraft store #flutter pigeon #flutter range slider #flutter slider #flutter time picker #flutter tutorial #flutter widget #google flutter #linux #navoki #pubspec format #setup flutter desktop on windows

Terry  Tremblay

Terry Tremblay


What is Flutter and why you should learn it?

Flutter is an open-source UI toolkit for mobile developers, so they can use it to build native-looking** Android and iOS** applications from the same code base for both platforms. Flutter is also working to make Flutter apps for Web, PWA (progressive Web-App) and Desktop platform (Windows,macOS,Linux).


Flutter was officially released in December 2018. Since then, it has gone a much stronger flutter community.

There has been much increase in flutter developers, flutter packages, youtube tutorials, blogs, flutter examples apps, official and private events, and more. Flutter is now on top software repos based and trending on GitHub.

Flutter meaning?

What is Flutter? this question comes to many new developer’s mind.


Flutter means flying wings quickly, and lightly but obviously, this doesn’t apply in our SDK.

So Flutter was one of the companies that were acquired by **Google **for around $40 million. That company was based on providing gesture detection and recognition from a standard webcam. But later when the Flutter was going to release in alpha version for developer it’s name was Sky, but since Google already owned Flutter name, so they rename it to Flutter.

Where Flutter is used?

Flutter is used in many startup companies nowadays, and even some MNCs are also adopting Flutter as a mobile development framework. Many top famous companies are using their apps in Flutter. Some of them here are





Reflectly app

Reflectly app

Abbey Road Studios

Abbey Road Studios

and many more other apps. Mobile development companies also adopted Flutter as a service for their clients. Even I was one of them who developed flutter apps as a freelancer and later as an IT company for mobile apps.

Flutter as a service

#dart #flutter #uncategorized #flutter framework #flutter jobs #flutter language #flutter meaning #flutter meaning in hindi #google flutter #how does flutter work #what is flutter

Adobe XD plugin for Flutter with CodePen Tutorial

Recently Adobe XD releases a new version of the plugin that you can use to export designs directly into flutter widgets or screens. Yes, you read it right, now you can make and export your favorite design in Adobe XD and export all the design in the widget form or as a full-screen design, this can save you a lot of time required in designing.

What we will do?
I will make a simple design of a dialogue box with a card design with text over it as shown below. After you complete this exercise you can experiment with the UI. You can make your own components or import UI kits available with the Adobe XD.

#developers #flutter #adobe xd design export to flutter #adobe xd flutter code #adobe xd flutter code generator - plugin #adobe xd flutter plugin #adobe xd flutter plugin tutorial #adobe xd plugins #adobe xd to flutter #adobe xd tutorial #codepen for flutter.

Punith Raaj


The Ultimate Guide To Tik Tok Clone App With Firebase - Ep 2

The Ultimate Guide To Tik Tok Clone App With Firebase - Ep 2
In this video, I'm going to show you how to make a Cool Tik Tok App a new Instagram using Flutter,firebase and visual studio code.

In this tutorial, you will learn how to Upload a Profile Pic to Firestore Data Storage.

🚀 Nice, clean and modern TikTok Clone #App #UI made in #Flutter⚠️

Starter Project :

► Timestamps 
0:00 Intro 0:20 
Upload Profile Screen 
16:35 Image Picker
20:06 Image Cropper 
24:25 Firestore Data Storage Configuration.

⚠️ IMPORTANT: If you want to learn, I strongly advise you to watch the video at a slow speed and try to follow the code and understand what is done, without having to copy the code, and then download it from GitHub.

► Social Media 

► Previous Episode :
► Playlist:

I hope you liked it, and don't forget to like,comment, subscribe, share this video with your friends, and star the repository on GitHub!
⭐️ Thanks for watching the video and for more updates don't forget to click on the notification. 
⭐️Please comment your suggestion for my improvement. 
⭐️Remember to like, subscribe, share this video, and star the repo on Github :)

Hope you enjoyed this video!
If you loved it, you can Buy me a coffee :

LIKE & SHARE & ACTIVATE THE BELL Thanks For Watching :-)

#flutter tutorial - tiktok clone with firebase #flutter challenge @tiktokclone #fluttertutorial firebase #flutter firebase #flutter pageview #morioh #flutter

Punith Raaj


Flutter Hotel Booking UI - Book your Stay At A New Hotel With Flutter - Ep1
Flutter Hotel Booking UI - Book your Stay At A New Hotel With Flutter - Ep1
#flutter #fluttertravelapp #hotelbookingui #flutter ui design 

In this video, I'm going to show you how to make a Cool Hotel Booking App using Flutter and visual studio code. 

In this tutorial, you will learn how to create a Splash Screen and Introduction Screen, how to implement a SmoothPageIndicator in Flutter. 

🚀 Nice, clean and modern Hotel Booking #App #UI made in #Flutter 

⚠️ IMPORTANT: If you want to learn, I strongly advise you to watch the video at a slow speed and try to follow the code and understand what is done, without having to copy the code, and then download it from GitHub. 

► Social Media 





I hope you liked it, and don't forget to like,comment, subscribe, share this video with your friends, and star the repository on GitHub! 

⭐️ Thanks for watching the video and for more updates don't forget to click on the notification.⭐️Please comment your suggestion for my improvement. ⭐️Remember to like, subscribe, share this video, and star the repo on Github :)Hope you enjoyed this video! If you loved it, you can Buy me a coffee :

#flutter riverpod #flutter travel app #appointment app flutter #morioh