The Google I/O 2018 video about Flutter explains how to use Dart streams to manage state in a Flutter application. The speaker talked about using Sink as input stream and Stream as output stream. What is the difference between Sink and Stream? I searched the documentation but it doesn't say too much thanks.

4 Answers

Sink and Stream both are parts of the StreamController. You add a data to the StreamController using Sink which can be listened via the Stream.

Example:

final _user = StreamController<User>();
Sink get updateUser => _user.sink;
Stream<User> get user => _user.stream;

Usage:

updateUser.add(yourUserObject); // This will add data to the stream.

Whenever a data is added to the stream via sink, it will be emitted which can be listened using the listen method.

user.listen((user) => print(user)); 

You can perform a various number of actions before the stream is emitted. transform method is an example which can be used to transform the input data before it gets emitted.

3

A StreamSink is a StreamConsumer, which means it can take several streams (added by addStream) and processes the events these streams emit.

If it is the StreamSink of a StreamController then all events from the added streams are emitted by the stream created by the StreamController.

This way you can pipe (forward) one or more streams into another one.

6

If you are looking for a very basic definitions on stream and sinks, then refer to this:

Stream - the conveyor belt is called as a stream

StreamController - this is what controls the stream

StreamTransformer - this is what processes the input data

StreamBuilder - it’s a method that takes stream as an input and provides us with a builder which rebuilds every time there is a new value of a stream

sink - the property which takes an input

stream - the property which gives the output out of the Stream

For more details, please refer to this article

0

Let us take a simple example of SINKS & STREAMS in Flutter. Please Reade the comments

 class LoginBloc { final _repository = Repository(); final _loginResponse = BehaviorSubject<bool>(); //---->> a simple Sink Stream<bool> get isSuccessful => _loginResponse.stream; //-----> Stream linked with above sink /* * Below is an async function which uses Repository class * to hit a login API and gets the result in a variable * isUserLoginSuccessful[true/false]. and then Add the result * into the sink. * now whenever something is added to the sink, a callback is given to * the stream linked to that Sink, which is managed by the framework itself * */ Future getLoginResponse() async { bool isUserLoginSuccessful = await _repository.processUserLogin(); _loginResponse.sink.add(isUserLoginSuccessful); } dispose() { _loginResponse.close(); } }

Now, I am using this LoginBloc in Login Screen.

 class Login extends StatelessWidget { final LoginBloc loginBloc; // ----> Here is the Object of LoginBloc Login(this.loginBloc); void _onClickLoginButton() async { // Hit login API // fetch Login API response data loginBloc.getLoginResponse(); //------> here is the function we are using in Login } @override Widget build(BuildContext context) { return StreamBuilder<bool>( // ----> You need to use a StreamBuilder Widget on the top Root, or according to the Business logic stream: loginBloc.isSuccessful, // ----> here is the stream which is triggered by Sink which is linked by this stream builder: (context, snapshot) { // DO WHATEVER YOU WANT AFTER CALLBACK TO STREAM
});

i hope this may make your stream & sink concept more clearer.

1

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