Stream / Iterator

What is Stream? In short: call once, return multiple times; like Iterators.

Flutter's Stream is a powerful abstraction. When using it as the return value of Rust function, we can allow the scenario that we call function once, and then return multiple times.

For example, your Rust function may run computationally heavy algorithms, and for every hundreds of milliseconds, it finds out a new piece of the full solution. In this case, it can immediately give that piece to Flutter, then Flutter can render it to UI immediately. Therefore, users do not need to wait for the full algorithm to finish before he can see some partial results on the user interface.

As for the details, a Rust function with signature like fn f(sink: StreamSink<T>, ..) -> Result<()> is translated to a Dart function Stream<T> f(..).

Notice that, you can hold that StreamSink forever, and use it freely even after the Rust function itself returns. The logger example below also demonstrates this (the create_log_stream returns almost immediately, while you can use the StreamSink after, say, an hour).

The StreamSink can be placed at any location. For example, fn f(a: i32, b: StreamSink<String>) and fn f(a: StreamSink<String>, b: i32) are both valid.


See logging examples which uses streams extensively.

What about streaming from Dart/Flutter to Rust?

This is not currently supported. As a workaround, consider iterating through your Dart stream and calling a normal Rust function for each item.