Understanding Streams: Everything you need to know

Introduction

Streams are very complex and hard to understand. In this blog, I'll try to explain this concept with some examples.
For beginners, this concept should be clear in their mind, because most of the time our application is dealing with asynchronous data, For example: Fetch data from API, DB, etc.
Before start coding and implementing Stream let's first understand what actually a Stream is.

What are Streams?

A streams are nothing but a continuous flow of Data.
It provides an asynchronous sequence of Data. And because of the asynchronous type, we have to use async and await.
The async and await are keywords that are used to define an asynchronous function.

someAsyncFunction () async{
 await getData();
}

There are two main ends associated with Steams: 1.Sink and 2.Source.
The Sink is the end from where we can add data inside the Streams and
The Source is the end from where we can get the data from the Streams.
If you've not understood then, you can think of a Streams like a water pipe, where the flow of water (Data) is continuously flowing from one end (Sink) and coming out from the second end (Source).
There are basically two types of Streams:
1. Single Subscription Stream and
2. Broadcast Stream
The Single Subscription Streams allow only one listener to listen to the stream. If you try to listen to the same stream again, you'll get an exception. The Broadcast Stream allows more than one listener to listen to the same stream.
A listener listens to the particular event of the stream. Whenever data flows in the Streams the listener will recognize that event and listens to it.
If there are no data flowing inside the Streams then it gets destroyed.
Now let's write some code to understand how actually we can use the Streams.

How to use Streams :

To use Streams we have to make the function async generator (async* ).
The async* is used when a function returns multiple future values one at a time.
When this function is called the Stream is created. Now to return the data from this function we have to use yield or yield*.
yield is like return but the difference is yield doesn't terminate the function immediately.
Let's take one simple example :

Stream<int> numberGenerator() async*{
for(int i=0; i<10; i++){
  await Future.delayed(Duration(milliseconds: 1000));
  yield i;
}
}

void main() {
final myStream = numberGenerator();  
final subscription= myStream.listen(                                
  (data)=>{
    print("Number: $data")
  }
);
}

The subscription is a type of StreamSubscription. When you listen on a Stream using Stream.listen, a StreamSubscription object is returned.
The subscription provides events to the listener, and holds the callbacks used to handle the events. The subscription can also be used to unsubscribe from the events, or to temporarily pause the events from the stream.
Here when subscription starts listening to the myStream, one by one all the integer values start flowing into the stream.
And then it will yield that int values one by one asynchronously.
listen() properties:
onData():
On each data event from this stream, onData handler is called. If onData is null, nothing happens.
Example :

void main() {
final myStream = numberGenerator();  
final subscription = myStream.listen(                                
(data)=>{                             // data handler function
  print("Number: $data")
}
);
}

onDone() :
Called when stream completes all its events.

void main() {
final myStream = timedCounter();
final subscription = myStream.listen(
  (data){
    print("Number: $data");
  },
  onDone: () {
    print("You've reached at the end");
  },
);
}

Stream<int> timedCounter() async* {
int i = 0;
while (i <= 5) {
  await Future.delayed(Duration(milliseconds: 1000));
  yield i++;
}
}

As you can see as soon as the stream completes its listening, onDone function is executed.
onError(error):
Subscription is automatically canceled when an error occurred.

void main() {
final myStream = timedCounter();
final subscription = myStream .listen(
  (data){
    print("Number: $data");
  },
  onDone: () {
    print("You've reached at the end");
  },
  onError: (e){
    print("Error: $e");
  },
);
}

Stream<int> timedCounter() async* {
int i = 0;
while (i <= 5) {
  await Future.delayed(Duration(milliseconds: 1000));
  if(i ==2){
    throw Exception("Error Occurred");
  }
  yield i++;
}
}

As soon as number 2 is encountered we've thrown an error and onError function executed.
cancelOnError :
If cancelOnError is true, the subscription is automatically canceled when the first error event is delivered. The default is false.
We can see in the above example that even if the error is encountered, the stream has not stopped its execution. It executed onDone too. To close the stream subscription as soon as the error is encountered we have to give true value to cancelOnError.

void main() {
final myStream = timedCounter();
final subscription = myStream.listen(
  (data){
    print("Number: $data");
  },
  onDone: () {
    print("You've reached at the end");
  },
  onError: (e){
    print("Error: $e");
  },
  cancelOnError: true
);
}

Stream<int> timedCounter() async* {
int i = 0;
while (i <= 5) {
  await Future.delayed(Duration(milliseconds: 1000));
  if(i ==2){
    throw Exception("Error Occurred");
  }
  yield i++;
}
}

Manipulating Streams :

We can manipulate the stream data on the fly.
We can chain up methods such as map, where, take and expand.
We can chain up all the methods that are available in the iterable(like, elementAt, cast, contains, any, etc) to manipulate the data of the stream
map() :

void main() {
final myStream = timedCounter()
  .map((data)=> 'Number : ${data*2}')
  .listen(print);
}

Stream<int> timedCounter() async* {
int i = 0;
while (i <= 5) {
  await Future.delayed(Duration(milliseconds: 1000));
  yield i++;
}
}

where() :

void main() {
final myStream = timedCounter()
  .where((data) => data % 2 == 0)
  .map((data)=> 'Number : $data')
  .listen(print);
}

Stream<int> timedCounter() async* {
int i = 0;
while (i <= 5) {
  await Future.delayed(Duration(milliseconds: 1000));
  yield i++;
}
}

Types of Stream :

Single Subscription Stream :
The Single Subscription Stream as the name suggests, only allows one listener to listen to the stream.
This type of stream is basically used in reading a file, receive a web request, etc. It's because we want a continuous flow of data in the correct order and without any errors.
You can listen to this stream by the listen() method as we've seen in the above examples.
This stream can listen only once. Listening again to this stream will cause an error.
Example :

void main() {
final myStream = timedCounter();
final subscription = myStream.listen(
    (data){
      print("Number: $data");
    },
 );
}

Stream<int> timedCounter() async* {
int i = 0;
while (i <= 5) {
  await Future.delayed(Duration(milliseconds: 1000));
  yield i++;
}
}

Listening with two subscribers will cause an error

void main() {
final myStream = timedCounter();
final subscriber1 = myStream.listen(
    (data){
      print("Sub 1 : Number: $data");
    },
 );
final subscriber2= myStream.listen(
      (data){
      print("Sub 2 : Number: $data");
    },
);
}

Stream<int> timedCounter() async* {
int i = 0;
while (i <= 5) {
  await Future.delayed(Duration(milliseconds: 1000));
  yield i++;
}
}

Broadcast Stream :
When more than one subscribers want to listen to one particular stream, we have to make that stream broadcast stream.
asBroadcastStream() is used to make any stream a Broadcast Stream.
Any subscriber can start listening to events as soon as they subscribe to it.
Example :

void main() {
final myStream = timedCounter().asBroadcastStream();
final subscriber = myStream.listen(
    (data){
      print("Sub 1 : Number: $data");
    },
 );
final subscriber2 = myStream.map(
    (data)=> 'Sub 2 : Number : ${data*2}'
   ).listen(print);
}

Stream<int> timedCounter() async* {
int i = 0;
while (i <= 5) {
  await Future.delayed(Duration(milliseconds: 1000));
  yield i++;
}
}

Creating your Own Stream

Most of the time we use the stream provided by the network libraries, file libraries, state management, etc.
But we can create our own stream by using StreamController.
StreamController gives you the ability to add your own events from anywhere.
To use StreamController you have to import dart:io package inside your application.

import 'dart:async';

void main() {
final _streamController = StreamController<int>();   // initialization of StreamController
int _number = 1;

addData() {
  Timer.periodic(Duration(seconds: 1),(_) {
    _streamController.sink.add(_number);    // adding data to stream
    _number++;
  });
}

addData();

Stream<int> myStream = _streamController.stream;  //creating a stream
final subscription = myStream.listen(                         // listening to the stream
  (data) => {
    print(data)
  },
);
}

Above stream will add data infinitely.
If you want to close the stream at a certain point, you can do something like this :

import 'dart:async';

void main() {
final _streamController = StreamController<int>();
int _number = 1;

addData() {
  Timer.periodic(Duration(seconds: 1),(_) {
    if(_number == 5){
      _streamController.close();      // closing a stream
      return;
    }
    _streamController.sink.add(_number);
    _number++;
  });
}

addData();

Stream<int> myStream = _streamController.stream;
final subscription = myStream.listen(
  (data) => {
    print(data)
  },
  onDone: (){
    print("All data received");    // called when stream is closed
  }
);
}

You can also listen to different events like onListen, onPause, onCancel, onResume using StreamController.

StreamController<int>(
    onListen:  ...,
    onPause: ...,
    onResume: ...,
    onCancel: ...
);

StreamBuilder in Flutter

We are often dealing with asynchronous data in our app.
Flutter provides StreamBuilder which listens to the event flowing from the stream.
For every new event it rebuilt the widget giving them the latest event to work with.
You can give your stream to StreamBuilder's stream property.

StreamBuilder(
  stream: myStream
  //...
)

The builder property is used to return a Widget that we want to display on the screen.
It has context and a snapshot as a parameter

StreamBuilder(
  stream: myStream,
  builder: (context, snapshot) {
      return Container()
  }
)

The initialValue property of the StreamBuilder is used to give the initial data to the widget while it's waiting for the first event.
You can check if the snapshot has data or not, has any error or not and also the connection state :

StreamBuilder(
  stream: myStream,
  builder: (context, snapshot) {
      if(!snapshot.hasData) return CircularProgressIndicator()   //checking is there any data
      if(snapshot.hasError) return Text("Something went wrong") //cheking for the error
      if(snapshot.connectionState == ConnectionState.done){} // or `waiting`,`none`, `active`
      return Container()
  }
)

Let's take one simple example :
In the below example, there is one container and a button underneath it.
When the button is pressed we are adding a new data(Color)to the stream.

class FlutterStreamBuilder extends StatefulWidget {
@override
_FlutterStreamBuilderState createState() => _FlutterStreamBuilderState();
}

class _FlutterStreamBuilderState extends State<FlutterStreamBuilder> {
final colorStream = StreamController<Color>();  

  // generate new Color randomly
 Color generateColor() {
  final random = Random();

  return Color.fromARGB(
    255,
    random.nextInt(255),
    random.nextInt(255),
    random.nextInt(255),
  );
}

// add Color to `colorStream`
addData(){
  colorStream.sink.add(generateColor());
}

 @override
void initState() {
  addData();
  super.initState();
}

@override                     
void dispose() {
  colorStream.close(); // To prevent memory leak, Make Sure you close the Stream.
  super.dispose();
}

@override
Widget build(BuildContext context) {
  return Scaffold(
    body: SizedBox.expand(
      child: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        crossAxisAlignment: CrossAxisAlignment.center,
        children: [
          StreamBuilder(
              stream: colorStream.stream,
              builder:
                  (BuildContext context, AsyncSnapshot<dynamic> snapshot) {
                if (!snapshot.hasData) {
                  return Center(child:CircularProgressIndicator());
                }

                if (snapshot.connectionState == ConnectionState.done) {}

                return Container(
                  height: 220,
                  width: 220,
                  color: snapshot.data,
                );
              }),
          SizedBox(height:30),
          ElevatedButton(onPressed: addData, child: Text("Click"))
        ],
      ),
    ),
  );
}
}

Wrapping Up

Thanks for reading. Hope you liked it.
Make sure you leave feedback in the comments 🙂.

Understanding Streams: Everything you need to know

Comments (6)

Flutter

Iterable & Iterator in Flutter & Dart

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Introduction

What are Streams?

How to use Streams :

`listen()` properties:

onData():

onDone() :

onError(error):

cancelOnError :

Manipulating Streams :

map() :

where() :

Types of Stream :

Single Subscription Stream :

Broadcast Stream :

Creating your Own Stream

StreamBuilder in Flutter

Wrapping Up

Command Palette

Comments (6)

Flutter

Iterable & Iterator in Flutter & Dart

More from this blog

Introduction

What are Streams?

How to use Streams :

listen() properties:

onData():

onDone() :

onError(error):

cancelOnError :

Manipulating Streams :

map() :

where() :

Types of Stream :

Single Subscription Stream :

Broadcast Stream :

Creating your Own Stream

StreamBuilder in Flutter

Wrapping Up

`listen()` properties: